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FreeBSD-SA-02:44.filedesc Security Advisory
                                                          The FreeBSD Project

Topic: file descriptor leak in fpathconf

Category: core
Module: kernel
Announced: 2003-01-07
Credits: Joost Pol <joostpine.nl>
Affects: FreeBSD 4.3-RELEASE and later versions
Corrected: 2002-11-11 01:43:31 UTC (RELENG_4)
                2003-01-06 12:37:52 UTC (RELENG_4_7)
                2003-01-06 12:38:21 UTC (RELENG_4_6)
                2003-01-07 15:17:16 UTC (RELENG_4_5)
                2003-01-07 15:17:40 UTC (RELENG_4_4)
                2003-01-06 21:20:54 UTC (RELENG_5_0)
FreeBSD only: YES

0. Revision History

2003-01-06 v1.0 Initial release.
2003-01-07 v1.1 Added information regarding bug in FreeBSD 5.x.
                  Added correction details for RELENG_4_5, RELENG_4_4.

I. Background

The fpathconf system call provides a method for applications to
determine the current value of a configurable system limit or option
variable associated with a pathname or file descriptor.

II. Problem Description

A programming error in the fpathconf system call can result in the
given file descriptor's reference count being erroneously incremented.

A similar problem exists in the developer preview versions of FreeBSD
5.0, affecting the lseek(2), dup(2), and other system calls.

III. Impact

A local attacker may cause the operating system to crash by repeatedly
calling fpathconf on a file descriptor until the reference count wraps
to a negative value, and then calling close on that file descriptor.

Similarly, it may be possible to cause a file descriptor to reference
unallocated kernel memory, but remain valid. If a new file is later
opened and the kernel allocates the new file structure at the same
memory location, then an attacker may be able to gain read or write
access to that file. This may in turn lead to privilege escalation.

IV. Workaround

There is no workaround.

V. Solution

The following patch has been verified to apply to FreeBSD 4.4, 4.5,
4.6, and 4.7 systems.

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-02:44/filedesc.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-02:44/filedesc.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.

Path Revision
  Branch
- -------------------------------------------------------------------------
src/sys/kern/kern_descrip.c
  RELENG_4 1.81.2.15
  RELENG_4_7 1.81.2.14.4.1
  RELENG_4_6 1.81.2.14.2.1
  RELENG_4_5 1.81.2.9.2.3
  RELENG_4_4 1.81.2.8.2.3
  RELENG_5_0 1.169.2.2
src/sys/kern/vfs_syscalls.c
  RELENG_5_0 1.297.2.2
- -------------------------------------------------------------------------

VII. References

<URL:http://www.pine.nl/press/pine-cert-20030101.txt>
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