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UNIRAS Brief - 55/04 - FreeBSD Security Bulletin - shmat reference counting bug


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   UNIRAS (UK Govt CERT) Briefing Notice - 55/04 dated 06.02.04  Time: 11:45
 UNIRAS is part of NISCC(National Infrastructure Security Co-ordination Centre)
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  UNIRAS material is also available from its website at www.uniras.gov.uk and
         Information about NISCC is available from www.niscc.gov.uk
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FreeBSD Security Bulletin:

shmat reference counting bug


A programming error in the shmat(2) system call can result in a shared memory 
segment's reference count being erroneously incremented.

FreeBSD-SA-04:02.shmat                                      Security Advisory
                                                          The FreeBSD Project

Topic:          shmat reference counting bug

Category:       core
Module:         kernel
Announced:      2004-02-05
Credits:        Joost Pol <joost@xxxxxxx>
Affects:        All FreeBSD releases
Corrected:      2004-02-04 18:00:40 UTC (RELENG_4)
                2004-02-04 18:00:47 UTC (RELENG_5_2, 5.2-RELEASE-p2)
                2004-02-04 18:00:55 UTC (RELENG_5_1, 5.1-RELEASE-p14)
                2004-02-04 18:01:03 UTC (RELENG_5_0, 5.0-RELEASE-p20)
                2004-02-04 18:01:10 UTC (RELENG_4_9, 4.9-RELEASE-p2)
                2004-02-04 18:01:18 UTC (RELENG_4_8, 4.8-RELEASE-p15)
                2004-02-04 18:01:25 UTC (RELENG_4_7, 4.7-RELEASE-p25)
CVE Name:       CAN-2004-0114
FreeBSD only:   NO

I.   Background

The System V Shared Memory interface provides primitives for sharing
memory segments between separate processes.  FreeBSD supports this
interface when the kernel is built with SYSVSHM option, or the sysvshm
module is loaded.  By default, the FreeBSD kernel is built with the
SYSVSHM option.

The shmat(2) system call, which is part of the System V Shared Memory
interface, is used to attach a shared memory segment to the calling
process's address space.

II.  Problem Description

A programming error in the shmat(2) system call can result in a shared
memory segment's reference count being erroneously incremented.

III. Impact

It may be possible to cause a shared memory segment to reference
unallocated kernel memory, but remain valid.  This could allow a local
attacker to gain read or write access to a portion of kernel memory,
resulting in sensitive information disclosure, bypass of access
control mechanisms, or privilege escalation.

IV.  Workaround

NOTE: These workarounds could cause applications that use shared
memory, such as the X Window System, to exhibit erratic behavior or to
fail completely.

Do one of the following:

1) Disable the System V Shared Memory interface entirely by following
these steps:

   - Remove or comment out any lines mentioning `SYSVSHM' from your
     kernel configuration file, and recompile your kernel as described
     in <URL:http://www.freebsd.org/handbook/kernelconfig.html>.

   - Remove or comment out any lines mentioning `sysvshm' from
     /boot/loader.conf and /etc/rc.conf.

   - On FreeBSD 5.x systems only , System V Shared Memory support may
     be provided as a kld(4).  To be absolutely safe, remove any files
     named `sysvshm.ko' in /modules, /boot, and any subdirectories.

   - Finally, reboot your system.


2) Configure the System V Shared Memory parameters so that no new
shared memory segments may be created, terminate all processes using
shared memory, and delete all existing shared memory segments.  Run
the following commands as root:

   # sysctl -w kern.ipc.shmmax=0
   # echo 'kern.ipc.shmmax=0' >> /etc/sysctl.conf
   # ipcs | awk '/^m/ { print $2 }' | xargs -n 1 ipcrm -m

V.   Solution

Do one of the following:

1) Upgrade your vulnerable system to 4-STABLE, or to the RELENG_5_2,
RELENG_5_1, RELENG_4_9, or RELENG_4_8 security branch dated after the
correction date.

NOTE WELL: Due to release engineering in progress at the time of this
           writing, the RELENG_5_2 security branch (5.2-RELEASE-p2)
           also includes numerous other critical bug fixes, most of
           which are not security related.  Please read src/UPDATING
           for details on these changes.


2) Patch your present system:

The following patch has been verified to apply to FreeBSD 4.x and 5.x

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-04:02/shmat.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-04:02/shmat.patch.asc

b) Apply the patch.

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:http://www.freebsd.org/handbook/kernelconfig.html> and reboot the

VI.  Correction details

The following list contains the revision numbers of each file that was
corrected in FreeBSD.

Path                                                             Revision
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VII. References


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UNIRAS wishes to acknowledge the contributions of FreeBSD for the information
contained in this Briefing. 
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