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FreeBSD Security AdvisoriesSep 3, 2008 1:13 pm 
Robert WatsonSep 3, 2008 4:14 pm 
Subject:FreeBSD Security Advisory FreeBSD-SA-08:08.nmount
From:FreeBSD Security Advisories (secu@freebsd.org)
Date:Sep 3, 2008 1:13:13 pm
List:org.freebsd.freebsd-security

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============================================================================= FreeBSD-SA-08:08.nmount Security Advisory The FreeBSD Project

Topic: nmount(2) local arbitrary code execution

Category: core Module: sys_kern Announced: 2008-09-03 Credits: James Gritton Affects: FreeBSD 7.0-RELEASE, FreeBSD 7.0-STABLE Corrected: 2008-09-03 19:09:47 UTC (RELENG_7, 7.1-PRERELEASE) 2008-09-03 19:09:47 UTC (RELENG_7_0, 7.0-RELEASE-p4) CVE Name: CVE-2008-3531

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 mount(2) and nmount(2) system calls are used by various utilities in the base system to graft a file system object on to the file system tree to a given mount point. It is possible to allow unprivileged users to utililize these system calls by setting the vfs.usermount sysctl(8) variable.

II. Problem Description

Various user defined input such as mount points, devices, and mount options are prepared and passed as arguments to nmount(2) into the kernel. Under certain error conditions, user defined data will be copied into a stack allocated buffer stored in the kernel without sufficient bounds checking.

III. Impact

If the system is configured to allow unprivileged users to mount file systems, it is possible for a local adversary to exploit this vulnerability and execute code in the context of the kernel.

IV. Workaround

It is possible to work around this issue by allowing only privileged users to mount file systems by running the following sysctl(8) command:

# sysctl vfs.usermount=0

V. Solution

NOTE WELL: Even with this fix allowing users to mount arbitrary media should not be considered safe. Most of the file systems in FreeBSD was not built to protect safeguard against malicious devices. While such bugs in file systems are fixed when found, a complete audit has not been perfomed on the file system code.

Perform one of the following:

1) Upgrade your vulnerable system to 7-STABLE, or to the RELENG_7_0 security branch dated after the correction date.

2) To patch your present system:

The following patches have been verified to apply to FreeBSD 7.0 systems.

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

# fetch http://security.FreeBSD.org/patches/SA-08:08/nmount.patch # fetch http://security.FreeBSD.org/patches/SA-08:08/nmount.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.

Branch Revision Path - ------------------------------------------------------------------------- RELENG_7 src/sys/kern/vfs_mount.c 1.265.2.10 RELENG_7_0 src/UPDATING 1.507.2.3.2.8 src/sys/conf/newvers.sh 1.72.2.5.2.8 src/sys/kern/vfs_mount.c 1.265.2.1.2.2 - -------------------------------------------------------------------------

VII. References

http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2008-3531

The latest revision of this advisory is available at http://security.FreeBSD.org/advisories/FreeBSD-SA-08:08.nmount.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (FreeBSD)

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