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FreeBSD 5.x File System Access Control Lists

 In conjunction with file system enhancements like snapshots, FreeBSD 5.0 and later offers the security of File System Access Control Lists (ACLs)... 
FreeBSD 5.x File System Access Control Lists- -
                                       
File System Access Control Lists
Contributed by Tom Rhodes and Pat GUO. 
In conjunction with file system enhancements like snapshots, FreeBSD 5.0 and later offers the security of File System Access Control Lists (ACLs).
Access Control Lists extend the standard UNIX® permission model in a highly compatible (POSIX®.1e) way. This feature permits an administrator to make use of and take advantage of a more sophisticated security model.
To enable ACL support for UFS file systems, the following:
options UFS_ACL
must be compiled into the kernel. If this option has not been compiled in, a warning message will be displayed when attempting to mount a file system supporting ACLs. This option is included in the GENERIC kernel. ACLs rely on extended attributes being enabled on the file system. Extended attributes are natively supported in the next generation UNIX file system, UFS2.
Note: A higher level of administrative overhead is required to configure extended attributes on UFS1 than on UFS2. The performance of extended attributes on UFS2 is also substantially higher. As a result, UFS2 is generally recommended in preference to UFS1 for use with access control lists.
ACLs are enabled by the mount-time administrative flag, acls, which may be added to /etc/fstab. The mount-time flag can also be automatically set in a persistent manner using 
tunefs(8)
to modify a superblock ACLs flag in the file system header. In general, it is preferred to use the superblock flag for several reasons:


  • The mount-time ACLs flag cannot be changed by a remount (
    mount(8)
    -u), only by means of a complete 
    umount(8)
    and fresh 
    mount(8)
    . This means that ACLs cannot be enabled on the root file system after boot. It also means that you cannot change the disposition of a file system once it is in use.

  • Setting the superblock flag will cause the file system to always be mounted with ACLs enabled even if there is not an fstab entry or if the devices re-order. This prevents accidental mounting of the file system without ACLs enabled, which can result in ACLs being improperly enforced, and hence security problems.


Note: We may change the ACLs behavior to allow the flag to be enabled without a complete fresh 
mount(8)
, but we consider it desirable to discourage accidental mounting without ACLs enabled, because you can shoot your feet quite nastily if you enable ACLs, then disable them, then re-enable them without flushing the extended attributes. In general, once you have enabled ACLs on a file system, they should not be disabled, as the resulting file protections may not be compatible with those intended by the users of the system, and re-enabling ACLs may re-attach the previous ACLs to files that have since had their permissions changed, resulting in other unpredictable behavior.


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