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Category:   OS (UNIX)  >   FreeBSD Kernel Vendors:   FreeBSD
FreeBSD Kernel nullfs(5) Cross-Mount Links May Let Local Users Bypass Access Controls
SecurityTracker Alert ID:  1029015
SecurityTracker URL:
CVE Reference:   CVE-2013-5710   (Links to External Site)
Date:  Sep 10 2013
Impact:   Modification of system information, Modification of user information
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): 8.3, 8.4, 9.1, 9.2
Description:   A vulnerability was reported in FreeBSD. A local user can bypass filesystem access controls in certain cases.

A local user with read access to a nullfs view and write access to a different nullfs view into the same filesystem can create a hard link from the read-only view to the other view to be able to gain write access on an ostensibly read-only file system.

Konstantin Belousov reported this vulnerability.

Impact:   A local user can bypass filesystem access controls in certain cases.
Solution:   The vendor has issued a fix.

The vendor's advisory is available at:

Vendor URL: (Links to External Site)
Cause:   Access control error

Message History:   This archive entry has one or more follow-up message(s) listed below.
Oct 28 2013 (McAfee Issues Fix for McAfee Firewall Enterprise) FreeBSD Kernel nullfs(5) Cross-Mount Links May Let Local Users Bypass Access Controls
McAfee has issued a fix for McAfee Firewall Enterprise.

 Source Message Contents

Subject:  FreeBSD Security Advisory FreeBSD-SA-13:13.nullfs

Hash: SHA1

FreeBSD-SA-13:13.nullfs                                     Security Advisory
                                                          The FreeBSD Project

Topic:          Cross-mount links between nullfs(5) mounts

Category:       core
Module:         nullfs
Announced:      2013-09-10
Credits:        Konstantin Belousov
Affects:        All supported versions of FreeBSD.
Corrected:      2013-09-10 10:07:21 UTC (stable/9, 9.2-STABLE)
                2013-09-10 10:08:20 UTC (releng/9.2, 9.2-RC1-p2)
                2013-09-10 10:08:20 UTC (releng/9.2, 9.2-RC2-p2)
                2013-09-10 10:08:20 UTC (releng/9.2, 9.2-RC3-p1)
                2013-09-10 10:15:33 UTC (releng/9.1, 9.1-RELEASE-p7)
                2013-09-10 10:12:09 UTC (stable/8, 8.4-STABLE)
                2013-09-10 10:14:19 UTC (releng/8.4, 8.4-RELEASE-p4)
                2013-09-10 10:13:14 UTC (releng/8.3, 8.3-RELEASE-p11)
CVE Name:       CVE-2013-5710

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

I.   Background

The nullfs(5) filesystem allows all or a part of an already mounted
filesystem to be made available in a different part of the global
filesystem namespace.  It is commonly used to make a set of files
available to multiple chroot(2) or jail(2) environments without
replicating the files in each environment.  A common idiom, described
in the FreeBSD Handbook, is to mount one subtree of a filesystem
read-only within a jail's filesystem namespace, and mount a different
subtree of the same filesystem read-write.

II.  Problem Description

The nullfs(5) implementation of the VOP_LINK(9) VFS operation does not
check whether the source and target of the link are both in the same
nullfs instance.  It is therefore possible to create a hardlink from a
location in one nullfs instance to a file in another, as long as the
underlying (source) filesystem is the same.

III. Impact

If multiple nullfs views into the same filesystem are mounted in
different locations, a user with read access to one of these views and
write access to another will be able to create a hard link from the
latter to a file in the former, even though they are, from the user's
perspective, different filesystems.  The user may thereby gain write
access to files which are nominally on a read-only filesystem.

IV.  Workaround

No workaround is available, but systems which do not use the nullfs(5)
filesystem, or do not null-mount different subtrees of the same source
filesystem with different permissions, are not vulnerable.

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

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

# fetch
# fetch
# gpg --verify nullfs.patch.asc

b) Apply the patch.

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

c) Recompile your kernel as described in
<URL:> and reboot the

3) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/8/                                                         r255445
releng/8.3/                                                       r255446
releng/8.4/                                                       r255447
stable/9/                                                         r255443
releng/9.1/                                                       r255448
releng/9.2/                                                       r255444
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://

Or visit the following URL, replacing NNNNNN with the revision number:


VII. References



The latest revision of this advisory is available at
Version: GnuPG v1.4.14 (FreeBSD)


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