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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:http://security.FreeBSD.org/>.

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 http://security.FreeBSD.org/patches/SA-13:13/nullfs.patch
# fetch http://security.FreeBSD.org/patches/SA-13:13/nullfs.patch.asc
# gpg --verify nullfs.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.

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://svn.freebsd.org/base

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

<URL:http://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:http://www.freebsd.org/doc/en/books/handbook/jails-application.html>

<URL:http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2013-5710>

The latest revision of this advisory is available at
<URL:http://security.FreeBSD.org/advisories/FreeBSD-SA-13:13.nullfs.asc>
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