The Rainbow Books The Rainbow BooksNATIONAL COMPUTER SECURITY CENTER
FORT GEORGE G. MEADE, MARYLAND 20755-6000
NCSC-TG-020-A
Library No. S-232,508
TRUSIX is not intended to be a standards body. nor does it intend to produce a de facto standard to compete against POSIX. Additionally, the TRUSIX documents are not to be construed as supplementary requirements to the TCSEC. The TCSEC is the only metric against which the trustworthiness of an operating system will be evaluated.
This document, "Rationale for Selecting Access Control List (ACL) Features for the UNIX System," is the first in a series of companion documents being produced by TRUSIX. The guidelines described in this document provide alternative methods for implementing ACLs in the UNIX system.
Recommendations for revision to this guideline are encouraged and will be reviewed periodically by the NCSC. Address all proposals for revision through appropriate channels to:
_________________________________ 18 August 1989
Patrick R. Gallagher, Jr.
Director
National Computer Security Center
* UNIX is a registered trademark of AT&T
Acknowledgment is also extended to the members of the POSIX P1003.6 Security Subcommittee and to those members of the computer security community who contributed their time and expertise by actively participating in the review of this document.
The recommendations of TRUSIX with regard to ACLs are as follows:
The DAC mechanism employed in the current UNIX System was designed for efficiency, flexibility. and ease of use. This mechanism allows and encourages the sharing of information, but at a very coarse granularity, via the use of file permission bits. File permission bits are associated with three classes: owner (sometimes referred to as "user"), group. and other. Access for each class is represented by a three-bit field allowing for read, write, and execute-permissions.
Several methods exist for allowing discretionary access control on objects. These methods include capabilities, profiles. access control lists (ACLs), protection bits, and password DAC mechanisms. The intent was to select a DAC mechanism with finer granularity than the current file permission bits, while maximizing the compatibility with both the current mechanism and POSIX P1003.1. Review of the methods described in A Guide to Understanding Discretionary Access Control in Trusted Systems(2), and of the desired outcome, point to the use of ACLs. It should be noted that ACLs can be considered a straightforward extension of the existing UNIX system protection bits, since the protection bits may be interpreted to be a limited form of an ACL, which always contains three entries.
It has been suggested that the fine granularity of control provided by ACLs may be simulated in UNIX systems by using the group mechanism. Groups are lists of users which may be used to specify who may access a file. In the worst case9 all possible combinations of users would have to be represented in order to fully implement these lists. This corresponds to (2**M-1) groups, where M is the number of bits in the group-ID. Since the number of possible combinations of users needed to implement this scheme for N users is (2"N-1), the maximum number of users which could effectively utilize such a system would be limited to the number of bits in the group-ID. This number (often 16 or 32) is an unreasonably small number for most UNIX systems and the management of the groups by users would be difficult. Also, this scheme does not allow for individual users in the lists to have different access rights. All users in the group would be forced to have the access rights given by the file group class permission bits. Some differences in access rights could be simulated by using the file other class permission bits, but not with the same functionality as provided by conventional ACLs.
The DAC features explored in this rationale are based on the DAC features requested by customers, the class B3 DAC requirements described in the DoD Trusted Computer Systems Evaluation Criteria [1] (TCSEC), and the DAC mechanisms used in existing trusted systems (e.g., Multics). Based on these inputs9 it has been determined that the current DAC mechanism in the UNIX System is adequate for most needs and that the only enhancement required is to allow reasonable, finer-grained control of objects. This provides the capability to share or deny access to individually specified users and/or groups and meets the class B3 requirements of the TCSEC.
The issues explored in this document will deal primarily with ACLs. Much of the terminology has been adopted from the P1003.1 document and the TCSEC; however, new terms will be defined when used. For most of the issues identified, alternative solutions are given along with a recommendation. Although an attempt was made to consider the' issues independently, it should be noted that sole of the issues are actually very dependent on each other and recommendations `made in some areas greatly influenced later recommendations.
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* UNIX is a registered trademark of AT&T
It is these classes of objects that will be protected by the discretionary access control alternatives described later in the paper. It should be pointed out, however, that discretionary access can not always be completely determined solely by the file permission bits and the ACL associated with the object. It is possible to have objects which have been administratively configured for a specific access and thus not completely affected by user DAC, e.g., a file system mounted read-only. There are other instances where discretionary access of objects may be time-dependent and thus not completely based on a current DAC setting. Examples of this would be the inability to write a shared-text file while it is being executed or trying to execute a file while it is open for writing. These situations are acknowledged special cases and will not be considered in the general discussion of determining effective discretionary access.
In this and subsequent sections, alternative implementations of a given topic are examined, followed by the TRUSIX recommendation.
| user1.projA | rw- |
| user2.projB | r-- |
| user3.* | rwx |
| *.projA | r-- |
| *.* | --- |
A database of tapes is maintained in /etc/tapedata. The database administrator (DBA) of the database wishes to produce a utility to control access to this database.To begin with, there are some rules for dealing with the database. Some users should have read and write access, others just read access, and still others should have no access to the database. Readers should only see data about their own tapes. In addition, since other database utilities have poor error handling, all updates to the database need to be made in the correct format.
The DBA has written a utility named tapedb which can read and update the database. /etc/tapedata and tapedb both have the group tape associated with them, and tapedb has the set-group-id bit on. The DBA has also created an ACL for /etc/tapedata which contains the following entries:
| user1.tape | r-- |
| user2.tape | r-- |
| user3.tape | rw- |
| user4.tape | rw- |
| *.* | --- |
While this example suggests a useful application of user and group ACL entries, there are other ways to implement the example which do not require this ACL entry type functionality. As described in the following section, the same effect can be achieved through ACLs containing user or group entries.
Additionally, identification by a user and group pair is not used in a UNIX System. In some systems, a user is identified by a user-ID, group-ID pair. In Multics, for example, a user is identified by a user-ID, project-ID pair. where a project-ID is equivalent to a group-ID on the UNIX system. User1 in projA. on a Multics system, is distinct from userl in projB. Since Multics users do not have the capability to change groups9 the only way for a user to be identified with another project would be to log in with another group-ID. In UNIX systems, however. a user is really only identified by the user-ID. Also, a user can easily change group-ID through the newgrp command or be associated with several groups at the same time if using a system with multiple groups. Thus, controlling access for a user while in a specific group is not as useful in a UNIX system.
| u: user1 | rw- |
| u: user2 | r-- |
| u: user3 | rw- |
| u: user4 | rw- |
| g: projA | r-- |
| g: projB | rw- |
| o: | rw- |
To address the protected subsystem implementation, consider again the tape database example described in the previous section. Rather than controlling access to the data, access can be controlled on two subprograms; one which reads data, the other which updates data. The ACL on the database, /etc/tapedata would be:
| g: tapereaders | r-- |
| g: tapewirters | rw- |
| o: | --- |
The user interface for access to the database is tapedb. The program tapedb is not setgid, however, it invokes two other programs, tapedb_read and tapedb_write, that are setgid. Only users allowed to read the database have execute permission on, tapedb_read, while only those allowed to update the database may execute tapedb_write. The ACL on tapedb_read would be:
| u: user1 | --x |
| u: user2 | --x |
| u: user3 | --x |
| u: user4 | --x |
| o: | --- |
The ACL on tapedb_write would be:
| u: user3 | --x |
| u: user4 | --x |
| o: | --- |
Thus, the same protected subsystem can be provided through ACLs of type user or group.
The main advantage of this scheme is that it provides more clarity for the user. This is considered to be a very important advantage as a user's understanding of such a mechanism is essential in promoting its correct usage. Additionally, this scheme removes the need for wildcard specifiers, thus eliminating the potential problems of picking an unused character as a specifier.
Consider the following example illustrating this scheme. Assume User1 and User2 are members of the group "GroupA" and User3 and User4 are not.
| File Owner/Group: | User2/GroupA |
| File Permission Bits: | rwxr-x--x |
| ACL Entries: | |
| User1 | rwx |
| User2 | r-- |
| User3 | rwx |
| User4 | --- |
In this example the file permission bits would have no effect on the access control decision. User3 is able to read, write and execute the file. User2 is able to read it, but not to execute or write to the file. The file permission bits are completely ignored.
The resulting pure ACL system does not have to worry about interactions between the ACL and the file permission bits, since the latter are not used for access control decisions. A single, well defined access policy is employed. Applications which should make use of DAC are forced to understand the new rules.
The major disadvantage of this scheme, however, is that compatibility is lost. Every DAC cognizant program, and that should be every program that manipulates the discretionary access control information on an object needs to be changed to understand ACLs.
Consider the two following examples which illustrate this approach. Once again assume User1 and User2 are members of the group "GroupA" and User3 and User4 are not.
Example A (ACL selected):
| File Owner/Group: | User2/GroupA |
| File Permission Bits: | rwxr-x--x |
| ACL Entries: | |
| User1 | rwx |
| User2 | r-- |
| User3 | rwx |
| User4 | --- |
Since there is an ACL on this file the access control is the same as in the previous example.
Example B (file permission bits selected):
| File Owner/Group: | User2/GroupA |
| File Permission Bits: | rwxr-x--x |
| ACL Entries: | NONE |
Since there are no ACL entries on this file the access control ~r determined by the permission bits. User2 (owner) has all access permissions to the file. User1 (a user in GroupA) is allowed read and execute access. User3 and User4 ("other" users) can only execute the file.
The resulting system behaves like a file permission bit based system if no one ever sets ACLs and like the pure ACL system if a default ACL mechanism is in use. Thus, either environment can be supported.
The compatibility issues raised in the previous section apply here as well. In addition, the programs have to determine which access control mechanism applies to each object created and set the DAC accordingly.
Consider the following example, which illustrates this approach. For this example, assume only User2 is in GroupA.
| File Owner/Group: | User2/GroupA |
| File Permission Bits: | rwxr-x--x |
| ACL Entries: | |
| User1 | rwx |
| User2 | r-- |
| User3 | rwx |
| User4 | --- |
In the example above, the file permission bits imply that User1 has execute permission, whereas the permissions specified in the ACL imply that User1 has full access. Without knowing which group User 1 is in, one cannot predict whether or not User1 can read the file. If User1 is in group GroupA, then User1 will have read and execute permissions. If User1 is not in group GroupA, then only execute permission will be granted. Similarly, without knowing User3's group, one cannot predict whether or not User3 has read access. User4 will have no possibility of access, due to no permissions specified in the ACL entry. As the example illustrates, there is no way to get a full ACL view with this scheme.
With this scheme, some compatibility is maintained. Calls to chmod have the desired effect from the restrictive point of view. ACL entries can further restrict access.
Making use of the ACL as the effective access control mechanism requires that the file permission bits be set wide-open (i.e., read, write, and execute bits are set for user, group and other). In situations where ACLs are not properly set, a new object will become generally accessible. Likewise, if the ACL is removed then the object will again be generally accessible. This scheme also allows for misleading status information given to programs which only use the existing mechanism.
Consider the following example illustrating this approach. Assume only User2 is in GroupA.
| File Owner/Group: | User2/GroupA |
| File Permission Bits: | rwxr-x--x |
| ACL Entries: | |
| User1 | rwx |
| User2 | r-- |
| User3 | rwx |
| User4 | --- |
User1, User2, and User3 have read, write, and execute access. User4 has execute access.
Again, some compatibility is maintained. Calls to chmod have the desired effect from the permissive point of view. The previous alternative's problem of leaving the permission bits wide-open is thus avoided.
The problem with this scheme, however, is that a chmod call which would deny all access (chmod("object", 0)) in a system without ACLs will not do so here.
In the following two examples assume the owner entry is evaluated before additional user entries, and the group entry is evaluated before additional gr1oup entries.
Example A:
| File Owner/Group: | User2/GroupA |
| File Permission Bits: | rwxr-x--x |
| ACL Entries: | |
| owner | rwx |
| User1 | rwx |
| User2 | r-- |
| User3 | rwx |
| User4 | --- |
| group | --- |
| other | --- |
In this example. it is not clear what permissions User2 is to be granted, since a particular method for determining owner access has not been specified for the case where an additional user entry also names the owner. User2 could be granted read, write, and execute access as the owner. read access only, as per the explicit entry for User2, or some combination of the two (e.g., the AND or OR of the two). User1, User3, and User4 get their access from their ACL entries.
Example B: (After a chmod("object", 0))
| File Owner/Group: | User2/GroupA |
| File Permission Bits: | --------- |
| ACL Entries: | |
| owner | --- |
| User1 | rwx |
| User2 | r-- |
| User3 | rwx |
| User4 | --- |
| group | --- |
| other | --- |
Changing the file permission bits to zero does not change the permissions granted to User1, User3, and User4, since their access is based on ACL entries. User2's access may change depending on how owner access is determined when additional user entries naming the owner also exist.
If no additional entries are added to the ACLs, this system looks like a system without ACLs. The literal meaning of the file permission bits is preserved in the ACL.
As in the previous alternative, however, a chmod call which would deny all access (chmod("object", 0)) in a system without ACLs will not do so here.
Example:
| File Owner/Group: | User2/GroupA |
| File Permission Bits: | rwxr-x--x |
| ACL Entries: | |
| User1 | rwx |
| User2 | r-- |
| User3 | rwx |
| User4 | --- |
Assume that the group file permission bits are chosen as the mask, i.e., all ACL entries will be ANDed against the file group class permission bits. User2, being the owner, gets read, write, and execute access to the file. User3 is allowed read and execute access. User1 is allowed read and execute access, the write access is disallowed by the file permission bits. User4 is not allowed any access to the file.
Calls to chmod have the desired effect from the restrictive point of view but not necessarily from the permissive point of view. Since the bits of the masked field will most likely be set wide-open, the literal meaning of the field chosen for the mask appears to be lost. The POSIX standard, however, allows for the extended meaning of the group class permission. bits.
To summarize the approaches identified in this section:
The ACL Masked By File Permission Bits approach is a compromise for both security and compatibility.
The Independent ACL And File Permission Bits (AND) approach suffers from the serious flaw that the file permission bits must be set very permissively in order to allow the ACL entries to predominate in the discretionary access calculation. A simple mistake in setting the ACL could grant object access to significantly more users than was intended.
The Independent ACL And File Permission Bits (OR) approach may require that both ACL and the file permission bits be changed in order to deny a particular access. Thus, existing programs could believe that they had prevented access when they, in fact, had not. Similarly, in the File Permission Bits Contained Within ACL approach, removing "other" permission might not have the desired effect9 since, the owner, group, and other entries may not be the only ones in the ACL. In neither case does a call to chmod with a zero argument unequivocally revoke access from all users as might be expected.
Whichever DAC scheme is ultimately selected, an appropriate balance must be struck between the mutually conflicting concerns of compatibility and security. In a DAC scheme where chmod cooperates with ACLs, chmod must not grant inappropriate access or require unreasonable (i.e., permissive public access) defaults.
Barring compatibility, the alternatives of ACLs replacing file permission bits (Pure ACLs and On Demand) would be the most elegant way of enhancing DAC for UNIX systems. By abandoning file permission bits, however, these schemes have been rendered incompatible with existing systems. Thus, they are not considered for a POSIX-compliant UNIX system DAC scheme.
Another concern with the use of multiple concurrent groups is the possibility of violating the least privilege principle. With multiple concurrent groups if a user is in several groups at once, he/she is granted access to all of those groups at all times rather than to just the ones he/she needs at any given time. This could be contrary to the idea of a user having a minimal set of privileges necessary to perform a particular function at any given time.
It can be argued, however, that the least privilege requirement in the TCSEC only applies to TCB architecture, making this issue irrelevant for DAC. On the other hand there may be a problem with a system which implements privileged roles through the group mechanism. The TCSEC class B3 Trusted Facility Management requirement states that separate roles must be assigned to operator and administrator functions and that each role be restricted to performing only those functions necessary for that role. Given a system, therefore, which uses the group mechanism to assign roles and grant access based on role identity to parts of the system which would otherwise be inaccessible, it is clear that least privilege could be violated through the use of multiple concurrent groups. The violation would occur if the user who was a member of the group assigned to a privileged role could also be a member of one or more additional groups. Proper administration of these privileged groups, however. could still allow for the use of multiple groups. but a subsetting capability, as described in the next subsection, would then be required.
Improperly controlled multiple concurrent groups with groups representing privileged roles could therefore be a violation of the least privilege principle. This would result in a failure to meet the class B3 requirements. This is only one specific implementation, however, and it is certainly conceivable that multiple concurrent groups could be implemented in such a way as to not be a violation of least privilege. The multiple concurrent group scheme is currently a feature in some UNIX systems and is thought to be an extremely useful and necessary feature to those who use it. Multiple concurrent groups would also be compatible with the POSIX standard
There are several ways of approaching this problem; any of these methods would be a possible and acceptable means of resolving this problem. First, it is necessary to consider whether a user should be able to add or delete groups from his/her group set and if so, with what restrictions. A user should certainly not be allowed to add groups for which he/she is not authorized. Therefore each user should have an "allowable group set" which consists of all groups that user has been given authorization to be a member of. Adding groups other than those which appear in this allowable group set would be unacceptable.
There are at least two ways to allow a user to work with a subset of his/her allowable group set. The first would be to keep the current scheme where a user becomes a member of all of his/her groups at login, but provide the user with a means (through some system call or command) to drop specific groups if desired and work as a member of some subset of his/her allowable group set. A command would allow a user the capability but require an explicit action to do so. A system call, on the other hand, would provide the means for restriction through a program which could be set up to run automatically for the user. This would mean, however, that the set of groups would either be hardcoded into the program or be set through some type of configuration file. Another possible approach would be to provide a mechanism that would cause a program's groups to be restricted when that program is executed. Although this eliminates the user having to remember to restrict his/her groups or having to hardcode a group set into a program, it would add further complexity to the system.
Two levels of ordering must actually be considered when deriving an ACL evaluation scheme; the ordering of the classes (user, group, other), and then the ordering of the entries within each class.
The method of evaluating an ACL in a most-to-least specific manner can be described as follows. The owner identity of the object is first checked against the effective identity of the subject. If there is a match the search stops. Next, a check is made against the owning group identity of the object and the effective group of the subject. If there is a match and the subject does not have multiple groups, the search stops. Otherwise the rest of the group entries are searched next. If the subject has multiple groups, the group entries are evaluated as presented in the Multiple Group Evaluation section, otherwise they are searched in order as the user entries are. Finally, if no user or group entries were found to match the effective identity of the subject, access is determined based on the other entry.
For the following discussion on the ordering of ACL entries9 it will be assumed that the classes will be ordered and follow this most-to-least specific regime.
As long as the "user, then group, then other" order is followed, the only security relevant problem with this method occurs when evaluating group entries with multiple groups. If a user is a member of multiple groups and matches more than one of the group entries, the resulting access may be dependent upon the ordering of the group entries. See the Multiple Group Evaluation section for various possibilities. Unless all matching group entries are considered when determining access, the burden is placed on the user to correctly order the group entries.
This method may appear to be more convenient for users, however, it may require the user to have extensive knowledge of group membership. Additionally, it does not allow for very efficient access evaluation as discussed in the following section.
The system may use any of a variety of ordering methods, two of which are alphabetical ordering by user or group name and numeric ordering by user or group ID. An ordering of lowest to highest UID or GID, or vice-versa, is recommended as it provides an efficient way to check for redundant entries. Redundant entries should not be allowed in an ACL.
It is important to mention that actual sorting need not be done by the kernel itself as long as the kernel enforces the specified ordering. In other words, the sorting can be achieved through the use of library routines. The ACL commands would automatically use the library sorting routines and users would also be encouraged to do so when writing their own programs which manipulate ACLs. When an ACL passed to the kernel, the kernel verifies that the entries are sorted or else a failure will occur. In this manner, efficiency is achieved while still enforcing a system-defined ordering.
This alternative is simple. reduces the possibility of user error, and allows for more efficient access determination.
The following methods may be used to evaluate access when multiple groups are used:
The first entry which matches one of the subject's groups might be used to determine access. While this is an efficient method, it does not take notice of the possibility of other groups granting access.
The entry which matches one of the subject's groups and grants the least access might be used. This method does not recognize the possibility that all the groups together might grant or deny the desired access.
The entry which matches one of the subject's groups and grants the most access might be used. This method also does not recognize the possibility that all the groups together might grant or deny access.
ANDing the permissions of all the entries which match groups of the subject is another possible method. This approach may be considered too restrictive, since even one entry which grants access may be overruled by other entries which deny access.
ORing the permissions of all the entries which match groups of the subject is also a possibility. This method may be considered too permissive, since the maximum permissions allowed by all the matching entries taken together is the result. However, the same effect can be achieved currently, through the user simply invoking the newgrp command to change to the group with the d1esired access or by opening the same file twice from two different groups which together provide the desired access.
Concern has been expressed that this scheme violates the wording in the TCSEC, for DAC at class B3. The TCSEC states: Furthermore, for each such named object, it shall be possible to specify a list of named individuals and a list of groups of named individuals for which no access to the object is to be given. The ORing of groups, however, does not present a conflict with the class B3 DAC requirement, as it still allows the user to specify groups that shall have no access.
The preferred alternative is for these calls to suffer no additional side effects due to the presence of ACLs. This can be achieved by not storing explicit IDs in the owner and owning group ACL entries. An advantage of this alternative is that the ACL entries for object owner and object owning group can be readily distinguished syntactically from the other user and group entries.
Complete failure is a poor alternative since these mechanisms change the file mode, not just the file permission bits. For example, a program should be able to do a legitimate operation such as changing the setgid bit on any file.
Partial failure means that these mechanisms would make the requested changes but return an error value different from -1. This is a poor alternative for two reasons: it does not make good sense to succeed while returning failure. and programs often do not differentiate between error return values.
Other alternatives attempt to minimize surprises to the caller by changing ACL entries. The first of these alternatives is to mask the access permissions in all the object's additional entries. Access permissions for entries with specific user and specific group are ANDed with the supplied user and group access permissions. Access permissions for entries with only a specific user are ANDed with supplied permissions for the user, and permissions for entries with only a specific group are ANDed with supplied permissions for the group. While this meets POSIX requirements, programs that wish to change only the file mode (non-access) bits will have the masking occur as an undesirable side effect. Another alternative is to disable the additional entries. This implicitly requires a new mechanism to enable entries that have been disabled. POSIX requirements are also satisfied by this alternative, but the same problems exist as in the previous alternative; programs using these mechanisms to change the non-access file mode bits will have entries disabled as an undesirable side effect. Still another alternative is to delete the additional entries. This has similar advantages and disadvantages as ACL entry disabling. It is simpler since there is no need for an ACL entry enabling mechanism. Information given by the user, however, is deleted without warning.
The preferred method is to make no changes to these mechanisms. The mechanisms will affect only file permission bits and ACL entries for the object owner or group. While this does not provide non-ACL cognizant programs with expected results for operations on objects with ACLs, it is not perceived as a serious problem. This alternative is consistent with the preferred alternative for mechanisms which access object attributes as well (see below).
It may also be desirable to add other types of ACL features to mechanisms. For example, one might wish to add the capability during file creation to adopt a specific ACL. For changes of this type, parameters of existing mechanisms should not be changed, and new parameters should not be added. New mechanisms should be created which make use of existing ones. For example. creat may need to be modified to take ACLs into account, but the parameter list should not change. Instead of adding an ACL parameter to creat, a new system call (i.e. with some other name) should be used. which takes the ACL as a parameter and then uses creat.
Another alternative is to find all the entries in the ACL that apply to the user-ID and group-ID of the subject, just like a permissive access check. Then OR all the associated permissions together, and return the results in the appropriate file permission bits (user, group, and other). While this alternative integrates the idea of ACLs into mechanisms that access object attributes9 the context of the mechanisms affects the result returned to the point where the meaning of what the mechanisms return is somewhat clouded.
The preferred alternative is to make no changes to these mechanisms. The mechanisms will continue to return the file permission bits as if ACLs did not exist. Another mechanism must then be used to find out if the file has an ACL, and if so, what its entries are. While this alternative does not provide all information to subjects that don't know about ACLs, it does not change the current behavior of these mechanisms.
It is also important for portability that programs use the available access control mechanisms in an appropriate manner, so that the security policy is interpreted correctly. For instance, at the system call level, the permission information returned by the use of stat may not be sufficient to determine allowed access; other information such as ACL contents may have to be evaluated as well.
Regarding the addition of ACL functionality, existing mechanisms should not be changed, and new parameters should not be added. Instead, new mechanisms should be created which make use of existing ones.
For system calls, there are at least two alternative types of designs. Each depends on how the ACL is viewed. In one approach, the ACL is a series of independent records which can be individually manipulated using calls similar to open, read, write, and close. This approach has a nice parallel to the way files are read and written, but may be viewed as overly complicated given the relative infrequency of ACL modification. In the other approach. the ACL is considered a single unit and is not changed record-by-record, but instead always manipulated as a whole. This approach uses a "get" and "set" concept for ACL operations, where an ACL, as a whole, is retrieved, modified locally, and then replaced [3]. This approach is simple and reflects the growing trend towards get/set type operations.
It may also be reasonable to extend the "get" and "set" concept to apply to default ACLs as well as to the ACL associated with an object. This is a natural extension of the way ACLs would be manipulated, and default ACL operations may be easily added to the recommended system call interface described below.
There are also two possible methods for implementing these calls. One option is to use separate system calls for each of the ACL operations (i.e., getacl, setacl). The other option is to have one ACL system call that can be invoked with a number of command arguments indicating the desired ACL operation [3]. An example of a useful additional command argument is one that would return the number of entries in the ACL. This method conserves the number of system calls, and provides the flexibility to add ACL commands via command arguments. Additionally, using this method, designers are free to implement library functions based on the system call with particular command flags.
For commands, the same issues apply as for system calls. In a system with ACLs, however, there will be a need for commands to not only manipulate ACLs, but also to show and manipulate all discretionary access control information. These commands should include. at a minimum:
A change to a shared ACL results in a change to the discretionary access on all objects using this ACL. This result may be considered to be a side-effect or a desired feature depending on the circumstance. Additionally, it may be difficult to determine which objects are sharing a specific named ACL, and a user may mistakenly grant access to an object that was not intended.
Another problem with named ACLs is that as objects they may themselves be required to contain discretionary access controls. This suggests the idea of recursive ACLs, a situation to be avoided.
While this alternative maintains compatibility with existing programs, it is not a very practical solution. Depending on the relationship of the file permission bits and the ACL, the absence of default ACLs may not make sense. For instance, in a pure ACL implementation, the absence of default ACLs would result in no initial protection on newly created files. Additionally, this alternative would not encourage the use of ACLs by new programs, and would prevent ACL creation by old programs. ACLs could not propagate through the system and hence their usability would be lost.
Requiring default ACLs may cause incompatibilities for an old program that only looks at the file permission bits when it creates an object. Also, for many users, the umask may be a sufficient tool for limiting the permissions on an object when it is created. The main advantage of requiring default ACLs is that the usability of ACLs is greatly improved. Additionally, since an ACL is associated with an object in a single atomic operation, the possibility of a temporarily insecure state is avoided.
This section has really only addressed default ACLs on file system objects. IPC objects are not part of the file system name space, and therefore require further consideration. IPC objects are relatively short lived. and are generally not manipulated by users at the command level as are files. Based on these characteristics default ACLs on IPC objects are probably not needed, and their use is not recommended.
Consider the following options in the context of masking the ACL entries by the file group class permission bits as recommended in the ACL Evaluation section. Also note that these options are discussed with respect; to the ACL entry types as described in the ACL Entry Type and Format section. Additional mechanisms in the ACL which allow direct modification of the file group class permission bits at file creation are not precluded.
The following is a review of some of the important recommendations presented in the paper. An access control list mechanism was chosen to extend the current DAC mechanism. When considering the types of access provided in the UNIX system, additional access modes need not be defined, however they should also not be precluded. The recommended ACL entry type was that of user or group entries. The main advantages of this solution are conformance with the UNIX system method of identification through either the user-ID or the group-ID, and simplicity for the user. The method in which file protection bits and ACLs interact is a very important and complex issue given the conflicting goals of security and compatibility. The recommendation of masking the ACL entries by the group field of the protection bits was chosen as the most accommodating solution considering these goals. A system defined ordering of the ACL entries was preferred and it was recommended that the access allowed for a user in multiple groups should be the sum of all access allowed for each group represented in the ACL. Considering other multiple group issues9 it was recommended to provide the multiple concurrent group capability along with some method of subsetting. It was also recommended that default ACLs be provided and that they originate from the parent directory of the newly created object.
It is important to note that although these and other specific recommendations were given, it is certainly possible to design an acceptable class B3, POSIX-compliant UNIX system following some of the other alternatives. In fact, there are issues where the recommended solution may not be superior to another alternative and the designer should consider his/her own specific requirements when making a choice in those areas. It must also be pointed out that building a system following all the recommendations presented in this paper will not guarantee a full class B3 system. There are many additional class B3 requirements that go beyond the interface specification.
This document will detail the DAC interfaces and their run-time behavior.
The goals of this ACL mechanism were:
However, an ACL, which consists of one or more user entries, one or more group entries, one class entry, and one other entry, is not a constant length (in the following example, * indicates zero or more occurrences of the preceding entry type):
There is a direct mapping between the ACL and the file permission bits. Specifically, the file owner class permission bits will always be equal to the permissions of the ACL entry for the object owner (they may be the same bits depending upon the implementation). Additionally, the file other class permission bits will always be equal to the ACL other entry permissions. And the file group class permission bits will always be equal to the ACL class entry permissions. Typically, the file group class permission bits are set to the maximum permissions allowed to the additional user entries, the owning group entry, and the additional group entries.
Whenever a file is created on a file system that supports ACLs, the ACL will contain a user entry for the object owner, a group entry for the object owning group, a class entry for the file group class permissions, and an other entry for the rest of the world. For compatibility with the current mechanism, if the ACL contains no additional user or additional group entries, the permissions in the group entry for the object owning group and the class entry must be the same.
EXAMPLE:
$ ls -l foo
-rw-rw-rw- 1 craig demo 53 Mar 6 17:37 foo
$ chmod 600 foo
$ ls -l foo
-rw------- 1 craig demo 53 Mar 6 17:37 foo
In the new DAC mechanism, using a pure ACL, there will be two new commands getacl and setacl (there will be a new function, acl, for which these commands provide a user interface). The getacl command will be used to display the ACL and the setacl command will be used to change the ACL.
These commands will be used in much the same way that Is and chmod are used. A user would first execute getacl to look at the ACL and then use setacl to make the desired changes. Because the ACL is not a fixed size, it may be difficult to manipulate. In order to simplify the use of ACLs the following example shows how the ACL may be easily manipulated using a text editor to give greater flexibility (note that changes may also be specified on the setacl command line).
EXAMPLE:
#the output of getacl is redirected to the file tmp
$ getacl bar > tmp
#the file tmp is edited and the line in italics is inserted
$ vi tmp
# file: bar
# owner: craig
# group: demo
user::rw-
group::rw-
group:guest:r--
class:rw-
other:rw-
#setacl is executed and the contents of the file tmp become the new ACL for bar
$ setacl -f tmp bar
#the output from getacl for the file bar is displayed
$ getacl bar
# file: bar
# owner: craig
# group: demo
user::rw-
group::rw-
group:guest:r--
class:rw-
other:rw-
Entries in the user category shall be sorted numerically by user ID from lowest to highest, except for the object owner entry which always precedes all other user entries.
Entries in the group category shall be sorted numerically by group ID from lowest to highest, except for the object owning group entry, which always precedes all other group entries.
Entries in the default:user category shall be sorted numerically by user ID from lowest to highest, except for the default object owner entry, which always precedes all other default user entries. Entries in the default:group category shall be sorted numerically by group ID from lowest to highest, except for the default object owning group entry, which always precedes all other default group entries. The proper ordering of entries required by the acl function can be obtained by the use of the aclsort function. ACL entries given as input to the setacl command need not be sorted; the sorting will be performed by the setacl command.
The permissions that may be specified in an ACL entry are read(r), write(w), and execute/search(x).
When the setacl command is executed, the file owner class permission bits will be set to the permissions specified for the owner and the file other class permission bits will be set to the permissions specified for other. As an option, the file group class permission bits will be manipulated such that they reflect the maximum permission that the ACL permits to members of the file group class (any ACL entry other than the object owner or other). Otherwise, the file group class permission bits will be set to the permissions specified by the class entry. Therefore, if the file group class only allows read permission then additional user entries and any group entries in the ACL will not grant write or execute permission.
This ACL scheme supports finer discretionary access controls than the current mechanism, while maintaining compatibility with the current permissions mechanism. The DAC information may be changed in one atomic operation, avoiding the possibility of an intermediate insecure state. Finer controls can be specified via the ACL, including explicit specification of users disallowed any access to the object. Additionally, the file permission bits provide a summary of all access rights.
Rationale: The ACL scheme described here will allow entries to be either permissive or restrictive. In general, an entry that results in less permission than the file other class permissions would grant would be considered restrictive. An entry that results in more permission than the file other class permissions would grant would be considered permissive. In the event that a file with an ACL is exported to a non-ACL system, the loss of permissive entries would not present a security problem; however, the absence of support for restrictive entries may allow a process to have permission that it would not have been granted on a system with ACLs. This behavior must be described in the documentation.
Note, this is a logical description of the access check. The physical code sequence may be different for better performance.
Discretionary Access Check Algorithm:
Note, the user ACL entry for the object owner matches the file owner class permission bits.
Note, the permissions of the additional user or group ACL entries further restrict the access specified by the file group class permission bits. Also, the class ACL entry matches the file group class permission bits.
Note, the other ACL entry matches the file other class permission bits.
EXAMPLE 1:
# create file foo
$ > foo
#execute ls -l and getacl on the file foo
$ ls -l foo
-rw-r--r-- 1 craig demo 0 Mar 620:27 foo
$ getacl foo
# file: foo
# owner: Craig
# group: demo
user::rw-
group::r--
class:r--
other:r---
EXAMPLE 2:
#execute getacl and Is -I on the file, run.sh, with added ACL entries
$ ls -I run.sh
-rwxr-xr-x+ 1 Craig demo 73 Mar 620:27 run.sh
$ getacl run.sh
# file: run.sh
# owner: Craig
# group: demo
user::rwx
user:fred:r-x
user:larry:--x
group::r-x
group:guest:---
class:r-x
other:r-x
EXAMPLE 3:
#use the chmod command on a file with an ACL
#use getacl to report both the ACL entries and the effective permissions
$ chmod 644 run.sh
$ ls -l run.sh
-rw-r--r--+ 1 craig demo 73 Mar 620:27 run.sh
$ getacl run.sh
# file: run.sh
# owner: craig
#group: demo
user::rw-
user:fred:r-x #effective:r--
user:larry:--x #effective:---
group::r-x #effective:r--
group guest:---
class:r--
other:r--
The user ID of the file is set to the effective user ID of the invoking process. The group ID of the file depends upon the mode of the containing directory. If the S_ISGID bit is not set on the directory, the group ID of the file is set to the effective group ID of the invoking process. If the S_ISGID bit is set on the directory, the group ID of the file is set to the group ID of the containing directory.
Each function that creates a new file supplies an initial value for the file permission bits. This initial value is then merged with the file mode creation mask (umask) of the invoking process and with any default ACL entries of the containing directory to form the file permission bits and ACL of the new file.
Although in many cases the process umask is sufficient to assign default permissions, users making explicit use of ACLs may desire default ACLs. The default ACL scheme must sensibly interact with the existing DAC mechanism, including umask.
The default ACL entries specify permissions for users and/or groups and/or others, that will be assigned to a new file. These default ACL entries are associated with a directory. Note, an ACL on a directory may contain entries that control access to the directory and entries (defaults) used for new file creation in that directory.
The process of creating the file permission bits and the ACL for the new file is called "ACL Merge". First, any mode parameter is transformed into the equivalent ACL form. For example, the mode 0664 is equivalent to user::rw-, group::rw-, class:rw-, other:r--. Also, the complement of the umask is used to obtain the equivalent ACL. Thus, the umask 022 is equivalent to user::rwx, group::r-x, class:r-x, other:r-x.
Two ACLs are merged by first logically sorting both ACLs into one ACL. Then any pair of matching entries are replaced with an entry that has permissions formed by ANDing the matched entries. Thus a permission is in the merged entry only if it was previously in both entries.
The first ACL merge is with the initial mode from the file creation function and with the process file mode creation mask. The second ACL merge is with any default entries from the containing directory. The result is the ACL for the new file. The file permission bits are then set from the user, class, and other ACL entries. Note, this may be different from the setacl command with the -r option since this merge does not set the file group class permission bits to the maximum permission of the file group class entries.
Finally, if the new object is a directory, then any default entries from the containing directory are copied to the new ACL. That is, the default ACL entries of the new directory are the same as the default ACL entries of the containing directory.
An example of the ACL merge operation is shown in the following figure:
FIGURE MISSING!
Note that although Is will still report these permissions. they will not be the only permissions evaluated during an access check. The output of Is will continue to be the maximum permission that may be granted, but there may be additional discretionary access control information (ACL entries) that was added to the object. In order to indicate that additional entries exist, Is-I will display the character "+" to the right of the current permissions display if an ACL is present. Therefore, when additional discretionary access control information has been added, in the form of ACL entries (as shown in the examples on previous pages), a user will need to use the newly provided command, getacl, to get a full view of the current discretionary access controls in effect. Although chmod will still modify the file permission bits, it will not change any additional discretionary access control information (i .e., ACL entries for additional users and additional groups) added to the object. To change these additional entries if they exist, the user will need to use the setacl command.
When the owner of an object is changed, the result will be identical to the current behavior. If the owner is changed to a user ID for which an additional user entry already exists in the ACL, the additional user entry is not changed but the user entry for the object owner will take precedence during an access check. When the group of an object is changed. the result will be identical to the current behavior. If the group is changed to a group ID for which an additional group entry already exists in the ACL, the additional group entry is not changed but the group entry for the object owning group will take precedence during an access check (except in the case of multiple concurrent groups, where all group entries are given equal treatment).
When the ACL contains no additional user or additional group entries, the permissions in the group entry for the object owning group and in the class entry must be the same. This behavior is the same as the current mechanism since the file permission bits can only specify at most three different permissions.
This command gives the user an interface to a pure ACL mechanism, allowing a finer granularity for file access. Note that this command only supports the file system objects: e.g., regular files, special files, directories, and named pipes. For simplicity, these objects are referred to as "files".
SYNOPSIS:
setacl [-r] -f acl_file file
where:
When the additional user entries or additional group entries of the ACL are modified, the file group class permission bits may also need to be modified to reflect the maximum permission allowed by these entries.
The -r, recalculate, option will result in the permissions specified in the class entry being ignored and replaced by the maximum permission needed for the file group class. For example, if there are no additional user entries or additional group entries, the permission of the group entry for the file owning group is used for the class entry.
A directory may contain default ACL entries. These entries may be of the type default:user, default:group, default:class, or default:other. For default:user entries, if no user ID is specified, this entry will apply to the file owner permissions. Additional default:user entries must have a unique user ID specified. For default:group entries, if no group ID is specified, this entry will apply to the file owning group permissions. Additional default:group entries must have a unique group ID specified. If there are no additional default:user entries or additional default:group entries, then the permissions of the default:group and the default:class must be the same.
If a file is created in a directory which contains default ACL entries the entries will be added to the newly created file. Note that the default permissions specified for the file owner class, file group class, and file other class will be constrained by the umask and the mode( specified in the file creation call. If default ACL entries are specified for a file which is not a directory the command will fail {11}, see ERRORS AND RETURNS.
With no options and arguments {1}, see ERRORS AND RETURNS. If the MAC or DAC check fails when a request is made to modify the ACL (2), see ERRORS AND RETURNS. If the file named file does not exist {6}, see ERRORS AND RETURNS.
If options are specified, the validity of the option-arguments will be checked. If an invalid option is specified {3a}, see ERRORS AND RETURNS. The arguments must be processed in the order specified (e.g., if the modify option is specified with a user, followed by the delete option with ihe same user, the entry will be deleted).
For the -m, -s, and -d options, if uid is not a valid login name or a valid user ID {3b}, or if gid is not a valid group name or a valid group ID {3c}, or if a specified perm is not r, w, x, -, or a specified operm is not an octal digit {3d}, see ERRORS AND RETURNS.
The -m option is used to add a new ACL entry or change an existing ACL entry.
If an entry already exists for the specified uid or gid. the specified permissions (perm operm) will replace the current permissions. If an entry does not exist for the specified uid or gid, an entry will be created. Note that an entry with no permissions will result in the specified uid or gid being denied access (any permissions) to the file. To specify no access in an entry being modified or added, either 0 should be specified for operm or - should be specified for perm.
The -s option is used to replace the ACL information on a file. The effect of using this option is that all entries are removed, and replaced by the newly specified ACL. If -s is specified with -d, -f, or -m {5}, see ERRORS AND RETURNS. There must be exactly one user entry specified for the file owner, exactly one group entry specified for the file owning group, exactly one class entry specified for the file group class, and exactly one other entry specified. If there is no user entry specified for the file owner. or no group entry specified for the file owning group or no class entry specified for the file group class, or no other entry specified {8}, see ERRORS AND RETURNS. There may be additional user ACL entries and additional group ACL entries specified. If duplicate entries are specified {9}, see ERRORS AND RETURNS.
The -d option is used to delete an existing entry from the ACL. If a matching entry is not found {4a}, see ERRORS AND RETURNS. Otherwise, the matching entry will be deleted. The user entry for the file owner, the group entry for the file owning group, the class entry, and the other entry may not be deleted from the ACL. If an attempt is made to delete one of these entries {4b}, see ERRORS AND RETURNS.
(Note: deleting an entry may have different effects than removing all the specified permissions for an entry. If an entry is deleted and a search is later done for the user or group identity that appeared in the entry, this identity might match another entry and then be given the permissions specified in this other entry. If the original entry remained with no permissions and a search was done for this identity. the search might match this entry and the subject would be denied access.)
The -f option is used to assign the ACL information contained in the file named acl_file to the specified file(s). If -f is specified with -d. -s. or -m {5}, see ERRORS AND RETURNS. If the file named acl_file does not exist {6}, see ERRORS AND RETURNS. The file named acl_file must be readable by the invoking subject. If it is not readable {2}, see ERRORS AND RETURNS.
If the entire file named acl_file contains correct external representation(s) for ACL entries, the ACL for the specified file(s) will be (removed and) replaced with the ACL whose external representation is contained in the file named acl_file. Each external representation of an ACL entry, contained in the file named acl_file, must be on a separate line and must be in the following format:
There must be exactly one user entry specified for the file owner, exactly one group entry specified for the file owning group, exactly one class entry specified for the file group class, and exactly one other entry specified. If not, see ERRORS AND RETURNS. There may be additional user ACL entries and additional group ACL entries specified. If duplicate entries are specified {9}, see ERRORS AND RETURNS.
Validity checks are performed on all entries. If an invalid entry is encountered {7}; see ERRORS AND RETURNS. If the exact problem can be determined an additional message may be displayed {3b}{3c}{3d}, see ERRORS AND RETURNS.
The character "#" will be used to indicate a comment. All characters starting with the #. to the end of the line will be ignored. Note that this includes any effective permissions (#effective:rwx) displayed by getacl.
This command may be executed on a file system that does not support ACLs. If ACL entries are specified which do not map into the base permissions {10}, see ERRORS AND RETURNS, otherwise the base permissions will be set.
ERRORS AND RETURNS: Following is a list of error conditions and the corresponding error message that should be output when this condition occurs.
setacl [-r] -f acljile file
{1} No options or arguments:
information associated with a file. It will allow the file owner or a
process with appropriate permission or appropriate privilege to
perform the following functions:
1. display the owner, group, and ACL for the specified file(s),
2. display the default ACL for a directory.
Note that this command only supports the file system objects: e.g.,
regular files, special files, directories. and named pipes. For
simplicity, these objects are referred to as "files".
SYNOPSIS:
getacl [-ad] file
PROCESSING: With no arguments (1), see ERRORS AND RETURNS. If MAC or
DAC check fails when a request is made to display the ACL information
(2) see ERRORS AND RETURNS. With invalid options (3), see ERRORS AND
RETURNS. If the file named file does not exist (4), see ERRORS AND
RETURNS.
With the -a option specified, the filename, owner, group, and the ACL of
the file will be displayed. With the -d option specified, the filename,
owner, group, and the default ACL of the file will be displayed, if it
exists. If the specified file does not support default ACLs (e.g., it is not a
directory) only the filename, owner, and group will be displayed. With
no option specified, both the ACL and the default ACL (if it exists) of the
file will be displayed.
This command may be executed on a file system that does not support
ACLs. It will report the ACL based on the base permission bits.
ERRORS AND RETURNS: Following is a list of error conditions and the
corresponding error message that should be output when this condition
occurs.
usage: getacl [-ad] file
(1) No arguments:
UX:getacl: ERROR: incorrect usage
usage:...
(2) If MAC or DAC check fails when a request is made to display the ACL
information:
UX:getacl: ERROR: permission denied for "file"
(3) incorrect/unknown option specified:
UX:getacl: ERROR: illegal option -- "-optio,t"
usage:...
(4) file does not exist:
UX:setacl: ERROR: file "file" not found
OUTPUT: When an ACL is displayed, the external representation of the ACL will
be as follows:
# file: filename
# owner: uid
# group: gid
user::perm
inser:uid:perm
group::perm
group:gid:perm
class:perm
other:perm
default :user: :perm
default :user:uid:perm
defai,lt :group: :perm
default :group :gid :perm
default :class :perm
default :other :perm
The ACL entries will be displayed in the order listed above (the user
entry for the file owner, followed by zero or more additional user
entries, followed by the group entry [or the file owning group, followed
by zero or more additional group entries, followed by the class entry
for the file group class, followed by the entry for other). When the
specified file is a directory the entries described above may be
followed by default entries (the default user entry for the file
owner, followed by zero or more additional default:user entries,
followed by the default:group entry for the file owning group,
followed by zero or more additional default:group entries, followed
by the default:class entry for the file group class, followed by the
entry for default:other). Note that these default ACL entries are
never used in an access check.
If more than one file is specified, a blank line will be displayed
before the ACL of the next file is displayed.
The first line displays the name of the file, next the file owner, and then the file owning group. The user entry without a user ID indicates the permissions that will be granted to the owner of the file. The additional user entries indicate the permissions that will be granted to the specified user. The group entry without a group indicates the permissions that will be granted to the group of the file. The additional group entries indicate the permissions that will be granted to the specified group. The class entry indicates the permissions that will be granted to the file group class. The other entry indicates the permissions that will be granted to others.
The default entries (default:user, default:group, default:class, and default:other) may only exist for directories, and indicate the default user, group, class, and other entries respectively that will be merged with the ACL for a new file created within the directory.
The uid is a login name, or a user ID (only if there is no login name associated with the user ID); gid is a group name. or a group ID (only if there is no group name associated with the group ID); and perm is a three character string composed of the letters representing the separate discretionary access controls, r (read). w (write), x (execute/search), or the character -. The perm will be displayed in the following order: rwx. If a permission is not granted by this ACL entry. the placeholder. "-", will appear. For example. if the user does not have write permission. but does have read and execute permission, r-x will be output.
The file group class permission bits constrain the ACL (represent the most access that any entry in the ACL may have). If a user executes the chmod command and changes the file group class permission bits this may change the permissions that would be granted based on the ACL alone. This behavior is necessary for the save-restore model (all permissions are temporarily removed via chmod 000 file and then restored) to work correctly. In order to indicate that the file permission bits are more restrictive than an ACL entry, getacl will display the ACL entry as described above with an additional tab followed by a sharp sign and the effective permissions.
Note that output from getacl will be in the correct format for input to setacl. Therefore, if the output is redirected into a file (e.g., getacl junk > entries), this file can be used as input to setacl (e.g., setacl -f entries junk.new). In this way, a user can easily assign one file's ACL information to another file.
EXAMPLES:
1) File with several ACL entries:
#file:fred
#owner:craig
# group: demo
user::rwx
user:spy:---
user:larry:rw-
group::r--
class:rw-
other:---
2) Same file. after a "chmod 700 fred":
#file:fred
$ owner: craig
$ group: demo
user::rwx
user:spy:---
user:larry:rw- #effective:---
group::r-- $effective:---
class:---
other:---
3) Directory with ACL entries including default ACL entries:
$ file: foodir
$ owner: craig
$ group: demo
user::rwx
user:spy:---
user:larry:rwx
group::r-x
class:rwx
other:r--
default:user::rwx
default user :larry :rwx
default:rnser:worm:- --
default :group:demo:r -
default:other:---
discretionary permission information associated with a file. It will
allow the file owner or a process with appropriate permission or
appropriate privilege to perform the following functions:
1. get or set a file's ACL information in an atomic operation.
2. return the number of entries contained in an file's ACL.
Note that this call only supports the file system objects: e.g., regular
files, special files, directories, and named pipes. For simplicity,
these objects are referred to as "files".
SYNOPSIS:
#include <tbd.h>
int acl(const char *path, int cmd. int nentries, struct acl "aclbufp)
Three values for cmd will be supported: ACLSET, ACLGET, and ACL_CNT.
The value of nentries is the number of ACL entries that can fit in the
user-supplied ACL buffer for an ACL_GET or the number actually present
for an ACL_SET; and aclbufp is a pointer to the user-supplied buffer of
ACL entry structures. The buffer will consist of an array of four
(USER_OBJ, GROUP_OBJ, CLASS_OBJ, and OTHER_OBJ entries are required) or
more occurrences of the following structure:
struct acl [
int a_type;
uid_t a_id;
ushort a_perm;
];
Twelve values of a_type will be supported to specify the type of entry:
(six for access checking and six for defaults), USER_OBJ, USER,
GROUP_OBJ, GROUP, CLASS_OBJ, OTHER_OBJ, DEF_USER_OBJ, DEF_USER,
DEF_GROUP_OBJ, DEF_GROUP, DEF_CLASS_OBJ, and DEF_OTHER_OBJ.
When a_type is USER or DEF_USER, aid will be a user id, and when a_type
is GROUP or DEF_GROUP, aid will be a group id. When a_type is USER_OBJ,
GROUP_OBJ, CLASS_OBJ, OTHER_OBJ, DEF_USER_OBJ,
DEF_GROUP_OBJ, DEF_CLASS_OBJ, or DEF_OTHER_OBJ, a_id will not be
used. The permissions for the entry will be contained in a_perm.
PROCESSING: When the specified cmd is ACL_CNT, the return value from the call
will be the number of ACL entries for the filename pointed to by path.
The values of nentries and aclbitfp will be ignored. If the user does
not pass the DAC and MAC checks to see the ACL. the acl call will fail
(see ERRORS AND RETURNS).
When the specified cmd is ACL_GET. the ACL information for the filename
pointed to by path will be retrieved and the ACL entries will be
placed in the buffer pointed to by aclbufp. The value of nentries is
the number of entries that can be held in the allocated buffer. If
the number of ACL entries in the ACL is greater than the value of
nentries (that is. the buffer space allocated to hold the files ACL
entries is less than nentries times the size of an entry), the acl
call will fail (see ERRORS AND RETURNS). On success. the return value
from this call will be the number of ACL entries retrieved. On any
error, the contents of the acl structures pointed to by aclbufp are
indeterminate. If the user does not pass the DAC and MAC checks to see
the ACL, the acl call will fail (see ERRORS AND RETURNS).
When the specified cmd is ACL_SET, ACL entries currently in the buffer
pointed to by aclbufp, for the filename pointed to by path, will be
set if all required checks are passed. The contents of nentries shall be
the number of ACL entries in the buffer, pointed to by aclbufp, to be
copied. On success, the return value from this call will be 0. If the
invoking user does not pass the DAC and MAC checks to set an ACL, the
act call will fail (see ERRORS AND RETURNS). If an error occurs,
either due to DAC and MAC checks or the validation check listed below,
there will be no change to the current ACL information. Before the ACL
entries are actually set, validation checks will be performed to
determine that the ACL entries are in the following order:
a) a user entry for the file owner (USER_OBJ),
b) additional user entries (USER),
c) a group entry for the file owning group (GROUP_OBJ),
d) additional group entries (GROUP),
e) a class entry for the file group class (CLASS_OBJ),
f) an entry for other (OTHER_OBJ),
g) default user entry for the file owner (DEF_USER_OBJ),
h) default additional user entries (DEF_USER),
i) default group entry for the file owning group (DEF_GROUP_OBJ),
j) default additional group entries (DEF_GROUP),
k) default class entry for file group class (DEF_CLASS_OBJ).
l) default entry for other (DEF_OTHER_OBJ),
The entry in classes a), c), e), and f) must always exist. The entry for classes a), c), e), f), g), i), k), and l) do not use the a_id field. Classes b) and h) may contain zero or more entries and the entries must be sorted by uid (lowest to highest). Classes d) and j) may contain zero or more entries and the entries must be sorted by gid (lowest to highest). (this ordering should be done with the aclsort function).
Class g), h), i), j), k), and l) entries are only applicable for directories. If an attempt is made to set default ACL entries on a file that is not a directory, the call will fail (see ERRORS AND RETURNS).
Validation of the ACL will be performed. If entries containing duplicate uids or gids are found, or there is not exactly one user entry specified for the file owner, one group entry specified for the file owning group, one class entry specified for the file group class, and one other entry specified, or there are no additional user and group entries and the permissions of the class entry are not equal to the permissions of the group entry, or there are no additional default:user and default:group entries and the permissions of the default:class entry is not equal to the permissions of the default:group entry, the call will fail (see ERRORS AND RETURNS).
The file owner class permission bits will be changed, such that they are equal to the permissions specified for the user entry of the file owner in the ACL. The file group class permission bits will be changed, such that they are equal to the permissions specified for the class ACL entry. The file other class permission bits will be changed, such that they are equal to the permissions specified for the other ACL entry.
This function may be executed on a file system that does not support ACLs. With ACL_GET as the cmd it will report the ACL based on the file permission bits. With ACL_SET as the cmd, if ACL entries are specified which do not map into the file permission bits, see ERRORS AND RETURNS, otherwise the file permission bits will be set.
A design may constrain the maximum number of ACL entries that are written, with a system-wide tunable parameter, aclmax. If the number ofACL entries exceeds the value of aclmax the function will fail (see ERRORS AND RETURNS).
ERRORS AND RETURNS: If the acl call is unsuccessful, a value of -l will be
returned and errno will be set to indicate the error. Only
implementation-independent errnos are presented.
Under the following conditions, the function act will fail and will
set errno to the specified value (note: unless otherwise stated, the
errno applies to ACL_CNT, ACL_GET, and ACL_SET):
ENOTDIR if a component of the path prefix is not a directory
ENOTDIR if an attempt is made to set a default ACL on a file
type other than a directory
ENOENT if a component of the pathname should exist but does not
EACCES if the DAC and/or MAC check fails
EINVAL if cmd is not ACL_CNT, ACL_GET, or ACL_SET
EINVAL if cmd is ACL_SET and the ACL entries do not pass the
validation check
ENOSPC if cmd is ACLGET and the space required for the
file's ACL entries exceeds nentries
ENOSPC if cmd is ACL,SET and there is insufficient space
in the file system to store the ACL
EINVAL if the number of acl entries exceeds the value of aclmax
ENOSYS if the file system type does not support ACLs, and
additional entries are specified
entries (including default ACL entries) and sort them into the correct
order to be accepted by the act or the aclipc function. It will
optionally calculate the maximum permissions needed for the object group
class and set the class ACL entry.
SYNOPSIS:
$include <tbd.h>
int aclsort(int nentries, int calclass, struct acl *aclbufp)
Where the value of nentries is the number of ACL entries, the value of
calclass if non-zero indicates to recalculate the class entry, and
aclbufp is a pointer to ACL entry structures.
PROCESSING: A call to aclsort will result in the contents of the buffer
being sorted in the following order:
a) a user entry for the object owner,
b) additional user entries.
c) a group entry for the object owning group,
d) additional group entries,
e) a class entry for the file group class,
f) an entry for other,
g) default user entry for the object owner.
h) default additional user entries,
i) default group entry for the object owning group.
j) default additional group entries,
k) default class entry for the file group class,
l) default entry for other.
Classes a), c), e), and f) must each have exactly one entry, if not,
see ERRORS AND RETURNS. Classes g), i), k), and l) must have zero or
one entry, if not, see ERRORS AND RETURNS. Entries will be sorted in
increasing order, by user ID in classes b) and h), and by group ID in
classes d) and j). Following sorting, a check will be performed to verify that no
duplicate entries (more than one entry containing the same user ID or the same
group ID) exist. If duplicate entries are found, see ERRORS AND RETURNS.
If there are no entries in classes b) and d). the function will set
the permission field, a_perm. in the class entry e) to that of the group
entry c). If there are entries in classes b) or d) and the calclass
argument is non-zero, the function will set the permission field,
a_perm, of the class entry to the maximum permission of the entries in
the file group class. Otherwise, the class entry permissions will remain
unchanged.
If there are no entries in classes h) and j), the function will set
the permissions in the default class entry k) to that of the default
entry i).
Upon success. aclsort will return the value 0.
ERRORS AND RETURNS: If the aclsort function is unsuccessful due to duplicate
entries, the return value will be the position (entry number) of the
first duplicate entry. If there is less than one user entry for the
object owner, group entry for the object owning group, class entry for
the file group class, or other entry specified, a value of -l will be
returned. If there is more than one user entry for the object owner,
group entry for the object owning group, class entry for the file
group class, or entry specified, they will be treated as duplicate
entries, and the return value will be the position of the duplicate
entry.
If the aclsort function is unsuccessful for any other reason, a value of
-l will be returned.
1. it allows a subject to change the file mode. including the
permissions for the file owner class. the file group class, and the
file other class of a file.
Note that the chmod command will not require any modifications.
SYNOPSIS: No change.
PROCESSING: Any permissions changes made with the chmod command or function
will update the file permission bits. This includes changing the file
owner ACL entry, the class ACL entry, and the other ACL entry if
the corresponding group(s) of bits are changed by this call. Any
additional ACL entries will not be affected. Note. the permissions
granted by such additional entries are constrained by the file group
class permission bits. If no additional user and no additional group
entries exist, the file group class permission bits will also
represent the permissions for the owning group of the file.
ERRORS AND RETURNS: No change.
OUTPUT: No change.
1. it allows a subject to change the owner and/or group of a file.
Note that the chown system call/command and the chgrp command will not
require any modifications.
SYNOPSIS: No change.
PROCESSING: When the owner of a file is changed, the result will be identical
to the current behavior. If the owner is changed to a user ID. for
which an additional user entry already exists in the ACL, the additional
user entry is not changed but the user entry for the file owner will
take precedence during an access check. When the group of a file is
changed. the result will be identical to the current behavior. If the
group is changed to a group ID, for which an additional group entry
already exists in the ACL, the additional group entry is not changed but
the group entry for the file owning group will take precedence during an
access check (except in the case of multiple concurrent groups, where
all group entries are given equal treatment).
ERRORS AND RETURNS: No change.
OUTPUT: No change.
EXAMPLES: The following examples illustrate the operation of the chown
function. For each example, there is a "before" state showing the
output of getacl, the chown function that is executed, and the "after"
state output.
EXAMPLE 1:
BEFORE:
$ file: filel
$ owner: larry
$ group: guest
user::rwx
group::r--
class:r- -
other:---
CALL: chown(filel, lisa, demo)
AFTER:
$file:filel
$ owner: lisa
$ group: demo
user::rwx
group::r--
class:r--
other:---
EXAMPLE 2:
BEFORE:
$ file: file2
$ owner: tarry
$ group: guest
user::rwx
user:fred:r--
group::r--
group:dev:r--
class:r--
other:---
CALL: chown(file2. lisa demo)
AFTER:
$ file: file2
$ owner: lisa
$ group: demo
user::rwx
user:fred:r---
group::r--
group:dev:r--
class:r--
other:---
EXAMPLE 3:
BEFORE:
$ file: file3
$ owner: larry
$ group: guest
user::rwx
user:lisa:r--
user:fred:r--
group::r--
group:dev:r--
group:demo:r--
class:r--
other:---
CALL: chown(file3. lisa, demo)
AFTER:
$ file: file3
$ owner: lisa
$ group: demo
user::rwx
user:lisa:r--
user:fred:r--
group::r--
group:dev:r--
group:demo:r--
class:r--
other:---
Note in EXAMPLE 3, a user entry contains a user ID that is the same as the file owner. In this case the file owner entry takes precedence. Also in EXAMPLE 3, a group entry contains a group ID that is the same as the owning group of the file. If multiple concurrent groups are not being used, the object owning group entry takes precedence.
discretionary permission information associated with an IPC object. It
will allow the object owner or a process with appropriate permission
or appropriate privilege to perform the following functions:
1. get or set an IPC object's ACL information in an atomic operation.
2. return the number of entries contained in an IPC object's ACL.
Note that this call only supports the IPC objects: e.g., shared memory
segments. semaphores. and message queues. For simplicity, these
objects are referred to as "IPC objects" in the remainder of this
description.
SYNOPSIS:
#include <tbd.h>
int aclipc(int type, int id, int cmd, int nentries, struct acl
*aclbufp)
Three values for type will be supported: IPC_SHM, IPC_SEM, and IPC_MSG.
If type is IPC_SHM, id must be a valid shmid returned by shmget. If type
is IPC_SEM, id must be a valid semid returned by semget. If type is
IPC_MSG, id must be a valid msgid returned by msgget. Three values for
cmd will be supported: ACL_SET. ACL_GET, and ACL_CNT. The value of
nentries is the number of ACL entries that can fit in the user
-supplied ACL buffer for an ACL_GET or the number actually present for
an ACL_SET; and aclbufp is a pointer to the user-supplied buffer of ACL
entry structures. The buffer will consist of an array of four
(USER_OBJ, GROUP_OBJ, CLASS_OBJ, and OTHER_OBJ entries are required)
or more occurrences of the following structure:
struct acl {
int a_type;
uid_t a_id;
ushort a_perm;
};
Six values of a_type will be supported to specify the type of entry:
USER_OBJ, USER, GROUP_OBJ, GROUP, CLASS_OBJ, and OTHER_OBJ.
When a_type is USER, a_id will be a user id, and when a_type is GROUP,
a_id will be a group id. When a_type is USER_OBJ, GROUP_OBJ, CLASS_OBJ,
or OTHER_OBJ, a_id will not be used. The permissions for the entry will
be contained in a_perm.
PROCESSING: When the specified cmd is ACL_CNT, the return value from the call
will be the number of ACL entries for the IPC object specified by type
and id. The values of nentries and aclbufp will be ignored. If the
invoking user does not pass the DAC or MAC checks to see the ACL. the
aclipc call will fail (see ERRORS AND RETURNS).
When the specified cmd is ACL_GET the ACL information for the IPC object
specified by type and id will be retrieved and the ACL entries will be
placed in the buffer pointed to by aclbufp. The value of nentries is
the number of entries that can be held in the buffer. If the number of
ACL entries in the ACL is greater than the value of nentries (the buffer
space allocated to hold the file's ACL entries is less than nentries
times the size of an entry), the aclipc call will fail (see ERRORS AND
RETURNS). On success. the return value from this call will be the number
of ACL entries retrieved. On any error, the contents of the acl
structures pointed to by aclbufp are indeterminate. If the user does
not pass the DAC and MAC checks to see the ACL, the aclipc call will
fail (see ERRORS AND RETURNS).
When the specified cmd is ACL_SET, ACL entries currently in the buffer,
pointed to by aclbufp, for the IPC object specified by type and id, will
be set if all required checks are passed. The contents of nentries shall
be the number of ACL entries in the buffer pointed to by aclbufp to be
copied. On success, the return value from this call will be 0. If the
invoking subject does not pass the DAC and MAC checks to set an ACL the
aclipc call will fail (see ERRORS AND RETURNS). If an error occurs,
either due to DAC or MAC checks or the validation check listed below,
there will be no change to the current ACL information. Before the ACL
entries are actually set, validation checks will be performed to
determine that the ACL entries are in the following order:
a) a user entry for the IPC object owner (USER_OBJ),
b) additional user entries (USER),
c) a group entry for the IPC object owning group (GROUP_OBJ),
d) additional group entries (GROUP),
e) a class entry for the IPC group class (CLASS_OBJ),
f) an entry for other (OTHER_OBJ).
The entries in class a), c), e), and f) must always exist. The entry
for class a), c), e), and f) do not use the aid field. Class b) may
contain zero or more entries and the entries must be sorted by uid (lowest
to highest). Class d) may contain zero or more entries and the entries must be
sorted by gid (lowest to highest). (this ordering should be done with the aclsort
function).
Validation of the ACL will be performed. If entries containing duplicate
uids or gids are found. or there is not exactly; one user entry for
the object owner, one group entry for the object owning group. one class
entry for the IPC group class, or one other entry specified. or there
are no additional user and group entries and the permissions of the
class entry are not equal to the permissions of the group entry, the
call will fail (see ERRORS AND RETURNS).
The IPC owner permission bits will be changed, such that they are
equal to the permissions specified for the user entry of the object
owner in the ACL. The IPC group class permission bits will be
changed, such that they are equal to the permissions specified for the
class ACL entry. The IPC other class permission bits will be changed,
such that they are equal to the permissions specified for the other
ACL entry.
A design may constrain the maximum number of ACL entries that are
written, with a system-wide tunable parameter, aclmax. If the number of
ACL entries exceeds the value of aclmax the function will fail (see
ERRORS AND RETURNS).
ERRORS AND RETURNS: If the aclipc call is unsuccessful, a value of -l will be
returned and errno will be set to indicate the error. Only
implementation-independent errnos are presented.
Under the following conditions, the function aclipc will fail and will
set errno to the specified value (note: if cmd is unspecified, the errno
applies to ACL_CNT, ACL_GET, and ACL_SET):
EINVAL if type is not IPC_SHM, IPC_SEM, or IPC_MSG
EINVAL if the value of id is (1) not a valid message_queue_identifier
and the type was IPC_MSG, (2) not a valid semaphore_identifier
and the type was IPC_SEM, or (3) not a valid shared memory,
identifier and the type was IPC_SHM
EINVAL if cmd is not ACL_CNT, ACL_GET, or ACL_SET
EINVAL if cmd is ACL_SET and the ACL entries do not pass
the validation check
EACCES if the DAC and/or MAC check fails
ENOSPC if cmd is ACL_GET and the space required for the
IPC's object ACL entries exceeds nentries
ENOMEM if cmd is ACL_SET and there is insufficient
space to store the ACL
EINVAL if the number of acl entries exceeds the value of aclmax
functionality:
1. they allow a subject to change the user ID. group ID, and
permissions on IPC objects.
SYNOPSIS: No change.
PROCESSING: No change.
ERRORS AND RETURNS: No change.
5200.28-STD, December 1985.
[2] National Computer Security Center, A Guide to Understanding Discretionary Access Control in Trusted Systems, NCSC-TG-003 Version-I, September 1987.
[3] UNIX System Access Control List Proposal, C. Rubin, AT&T. May 15. 1988.
[4] Adding Access Control Lists To UNIX, A. Silverstein, B. McMahon. G. Nuss,
Hewlett-Packard Co.. March 12. 1988.
[5] Discretionary Access Control System Functions, D. H. Steves, IBM, March 14, 1988.
[6] P1003 .6 Security Extension Proposal: Discretionary Access Control Semantics, W. Olin Sibert. Oxford Systems Inc., May 18. 1988.
[7] PlOO3.6 Supplementary Document: Discretionary Access Control. Problems in P1003.1 Draft 12 , W. Olin Sibert, Oxford Systems Inc.. May 18, 1988.
[8] P1003.6 Supplementary Document: Comments on Hewlett-Packard ACL Proposal, W. Olin Sibert. Oxford Systems Inc., May 18,1988.
[9] Extending The UNIX Protection Model with Access Control Lists, G. Fernandez, L. Allen, Apollo Computer Inc., June 1988.
[10] On Incorporating Access Control Lists into the UNIX Operating System S. M. Kramer, SecureWare Inc., June 1988.
[11] Trusted UNIX Discretionary Access and Privilege Control Mechanisms, B.D.
Wilner, Infosystems Technology Inc.. June 2,1988.
(12] Access Control List Design, Hewlett Packard, October 21, 1988.
[13] Proposal for Adding Access Control Lists to POSIX, P. B. Flinn, SecureWare Inc., July 25, 1988.
[14] Discretionary Access Control Proposal, H. L. Hall, Digital Equipment Corporation, Oct. 1988.
[15] Portable Operating System Interface for Computer Environments IEEE Std. 1003.1-1988