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FreeBSD-SA-09:14.devfs                                      Security Advisory
                                                          The FreeBSD Project

Topic:          Devfs / VFS NULL pointer race condition

Category:       core
Module:         kern
Announced:      2009-10-02
Credits:        Przemyslaw Frasunek
Affects:        FreeBSD 6.x and 7.x
Corrected:      2009-05-18 10:41:59 UTC (RELENG_7, 7.2-STABLE)
                2009-10-02 18:09:56 UTC (RELENG_7_2, 7.2-RELEASE-p4)
                2009-10-02 18:09:56 UTC (RELENG_7_1, 7.1-RELEASE-p8)
                2009-10-02 18:09:56 UTC (RELENG_6, 6.4-STABLE)
                2009-10-02 18:09:56 UTC (RELENG_6_4, 6.4-RELEASE-p7)
                2009-10-02 18:09:56 UTC (RELENG_6_3, 6.3-RELEASE-p13)

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:http://security.FreeBSD.org/>.

I.   Background

The device file system (devfs) provides access to system devices, such as
storage devices and serial ports, via the file system namespace.

VFS is the Virtual File System, which abstracts file system operations in
the kernel from the actual underlying file system.

II.  Problem Description

Due to the interaction between devfs and VFS, a race condition exists
where the kernel might dereference a NULL pointer.

III. Impact

Successful exploitation of the race condition can lead to local kernel
privilege escalation, kernel data corruption and/or crash.

To exploit this vulnerability, an attacker must be able to run code with user
privileges on the target system.

IV.  Workaround

An errata note, FreeBSD-EN-09:05.null has been released simultaneously to
this advisory, and contains a kernel patch implementing a workaround for a
more broad class of vulnerabilities.  However, prior to those changes, no
workaround is available.

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to 6-STABLE, or 7-STABLE, or to the
RELENG_7_2, RELENG_7_1, RELENG_6_4, or RELENG_6_3 security branch
dated after the correction date.

2) To patch your present system:

The following patches have been verified to apply to FreeBSD 6.3, 6.4,
7.1, and 7.2 systems.

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

[FreeBSD 6.x]
# fetch http://security.FreeBSD.org/patches/SA-09:14/devfs6.patch
# fetch http://security.FreeBSD.org/patches/SA-09:14/devfs6.patch.asc

[FreeBSD 7.x]
# fetch http://security.FreeBSD.org/patches/SA-09:14/devfs7.patch
# fetch http://security.FreeBSD.org/patches/SA-09:14/devfs7.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
system.

VI.  Correction details

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

CVS:

Branch                                                           Revision
  Path
- -------------------------------------------------------------------------
RELENG_6
  src/sys/fs/devfs/devfs_vnops.c                               1.114.2.17
RELENG_6_4
  src/UPDATING                                            1.416.2.40.2.11
  src/sys/conf/newvers.sh                                  1.69.2.18.2.13
  src/sys/fs/devfs/devfs_vnops.c                           1.114.2.16.2.2
RELENG_6_3
  src/UPDATING                                            1.416.2.37.2.18
  src/sys/conf/newvers.sh                                  1.69.2.15.2.17
  src/sys/fs/devfs/devfs_vnops.c                           1.114.2.15.2.1
RELENG_7
  src/sys/fs/devfs/devfs_vnops.c                                1.149.2.9
RELENG_7_2
  src/UPDATING                                             1.507.2.23.2.7
  src/sys/conf/newvers.sh                                   1.72.2.11.2.8
  src/sys/fs/devfs/devfs_vnops.c                            1.149.2.8.2.2
RELENG_7_1
  src/UPDATING                                            1.507.2.13.2.11
  src/sys/conf/newvers.sh                                   1.72.2.9.2.12
  src/sys/fs/devfs/devfs_vnops.c                            1.149.2.4.2.2
- -------------------------------------------------------------------------

Subversion:

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/6/                                                         r197715
releng/6.4/                                                       r197715
releng/6.3/                                                       r197715
stable/7/                                                         r192301
releng/7.2/                                                       r197715
releng/7.1/                                                       r197715
- -------------------------------------------------------------------------

VII. References

The latest revision of this advisory is available at
http://security.FreeBSD.org/advisories/FreeBSD-SA-09:14.devfs.asc
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