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=============================================================================
FreeBSD-SA-96:10                                            Security Advisory
Revised: Wed May 22 00:20:23 PDT 1996                           FreeBSD, Inc.

Topic:          system stability compromise via mount_union program

Category:       core
Module:         unionfs
Announced:      1996-05-17
Affects:        FreeBSD 2.0, 2.0.5, 2.1, 2.1-stable, and 2.2-current
Corrected:      (workaround) 2.1-stable and 2.2-current as of 1996-05-17
Source:         4.4BSD (lite)
FreeBSD only:   no

Patches:        ftp://freebsd.org/pub/CERT/patches/SA-96:10/

=============================================================================

I.   Background    

     A bug was found in the union file system code which can allow
     an unprivileged local user to compromise system stability.
     This problem is present in all source code and binary
     distributions of FreeBSD version 2.x released before 1996-05-18.

     All FreeBSD users are encouraged to use the workaround provided
     until the FreeBSD Project distributes a full solution.


II.  Problem Description

     The union filesystem code had problems with certain mount ordering
     problems.  By executing a certain sequence of mount_union commands,
     an unprivileged local user may cause a system reload.

     NOTE: This is a different problem than the one discussed in
     FreeBSD SA-96:09.  The workaround for this vulnerability is
     similar to the one discussed in 96:09, but the proper solution
     for the unauthorized access problem in 96:09 does not address
     this vulnerability.


III. Impact

     The problem could allow local users to compromise system stability.

     This vulnerability can only be exploited by users with a valid
     account on the local system.


IV. Solution(s)

     The FreeBSD project is currently developing a solution to this
     problem,  however the proper solution will not be available until
     a future FreeBSD release.  We do not anticipate releasing patches
     for previous versions of FreeBSD due to the extensive nature of this
     fix.  This security advisory will be updated as new information is
     made available.

V. Workaround

     This vulnerability can quickly and easily be limited by removing
     the setuid permission bit from the mount_union program.  This
     workaround will work for all versions of FreeBSD affected by
     this problem.
     
     As root, execute the command:

          % chmod u-s /sbin/mount_union

     then verify that the setuid permissions of the files have been
     removed.  The permissions array should read "-r-xr-xr-x" as
     shown here:

           % ls -l /sbin/mount_union
           -r-xr-xr-x  1 root  bin   53248 Apr 26 04:40 /sbin/mount_union

     In addition to changing the permissions on the executable files,
     if you have the source code installed, we suggest patching the
     sources so that mount_union will not be installed with the
     setuid bit set:

    *** /usr/src/sbin/mount_union/Makefile      Sun Nov 20 14:47:52 1994
    --- /usr/src/sbin/mount_union/Makefile      Fri May 17 10:36:09 1996
    ***************
    *** 8,14 ****
      CFLAGS+= -I${.CURDIR}/../../sys -I${MOUNT}
      .PATH:    ${MOUNT}
      
    - BINOWN= root
    - BINMODE=4555
    - 
      .include 
    --- 8,11 ----

=============================================================================
FreeBSD, Inc.

Web Site:                       http://www.freebsd.org/
Confidential contacts:          security-officer@freebsd.org
PGP Key:                        ftp://freebsd.org/pub/CERT/public_key.asc
Security notifications:         security-notifications@freebsd.org
Security public discussion:     security@freebsd.org

Notice: Any patches in this document may not apply cleanly due to
        modifications caused by digital signature or mailer software.
        Please reference the URL listed at the top of this document
        for original copies of all patches if necessary.
=============================================================================

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