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FreeBSD jail
FreeBSD Security Advisory FreeBSD-SA-07:01.jail
FreeBSD Security Advisory FreeBSD-SA-07:01.jail

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============================================================================FreeBSD-SA-07:01.jail                                       Security Advisory
                                                          The FreeBSD Project

Topic:          Jail rc.d script privilege escalation

Category:       core
Module:         etc_rc.d
Announced:      2007-01-11
Credits:        Dirk Engling
Affects:        All FreeBSD releases since 5.3
Corrected:      2007-01-11 18:16:58 UTC (RELENG_6, 6.2-STABLE)
                2007-01-11 18:17:24 UTC (RELENG_6_2, 6.2-RELEASE)
                2007-01-11 18:18:08 UTC (RELENG_6_1, 6.1-RELEASE-p12)
                2007-01-11 18:18:35 UTC (RELENG_6_0, 6.0-RELEASE-p17)
                2007-01-11 18:18:57 UTC (RELENG_5, 5.5-STABLE)
                2007-01-11 18:19:33 UTC (RELENG_5_5, 5.5-RELEASE-p10)
CVE Name:       CVE-2007-0166

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

I.   Background

The jail(2) system call allows a system administrator to lock a process
and all of its descendants inside an environment with a very limited
ability to affect the system outside that environment, even for
processes with superuser privileges.  It is an extension of, but
far more powerful than, the traditional UNIX chroot(2) system call.

The host's jail rc.d(8) script can be used to start and stop jails
automatically on system boot/shutdown.

II.  Problem Description

In multiple situations the host's jail rc.d(8) script does not check if
a path inside the jail file system structure is a symbolic link before
using the path.  In particular this is the case when writing the
output from the jail start-up to /var/log/console.log and when
mounting and unmounting file systems inside the jail directory

III. Impact

Due to the lack of handling of potential symbolic links the host's jail
rc.d(8) script is vulnerable to "symlink attacks".  By replacing
/var/log/console.log inside the jail with a symbolic link it is
possible for the superuser (root) inside the jail to overwrite files
on the host system outside the jail with arbitrary content.  This in
turn can be used to execute arbitrary commands with non-jailed
superuser privileges.

Similarly, by changing directory mount points inside the jail file
system structure into symbolic links, it may be possible for a jailed
attacker to mount file systems which were meant to be mounted inside
the jail at arbitrary points in the host file system structure, or to
unmount arbitrary file systems on the host system.

NOTE WELL: The above vulnerabilities occur only when a jail is being
started or stopped using the host's jail rc.d(8) script; once started
(and until stopped), running jails cannot exploit this.

IV.  Workaround

If the sysctl(8) variable security.jail.chflags_allowed is set to 0
(the default), setting the "sunlnk" system flag on /var, /var/log,
/var/log/console.log, and all file system mount points and their
parent directories inside the jail(s) will ensure that the console
log file and mount points are not replaced by symbolic links.  If
this is done while jails are running, the administrator must check
that an attacker has not replaced any directories with symlinks
after setting the "sunlnk" flag.

V.   Solution

NOTE WELL: The solution described changes the default location of the
"console.log" for jails from /var/log/console.log inside each jail to
/var/log/jail_${jail_name}_console.log on host system.  If this is a
problem, it may be possible to create a hard link from the new position
of the console log file to a location inside the jail.  A new rc.conf(5)
variable, jail_${jail_name}_consolelog, can be used to change the
location of console.log files on a per-jail basis.

In addition, the solution described below does not fully secure jail
configurations where two jails have overlapping directory trees and a
file system is mounted inside the overlap.  Overlapping directory
trees can occur when jails share the same root directory; when a jail
has a root directory which is a subdirectory of another jail's root
directory; or when a part of the file system space of one jail is
mounted inside the file system space of another jail, e.g., using
nullfs or unionfs.

To handle overlapping jails safely the administrator must set the
sysctl(8) variable security.jail.chflags_allowed to 0 (the default)
and manually set the "sunlnk" file/directory flag on all mount points
and all parent directories of mount points.  If this is done while
jails are running, the adminstrator must check that an attacker has
not replaced any directories with symlinks after setting the "sunlnk"

Perform one of the following:

1) Upgrade your vulnerable system to 5-STABLE, or 6-STABLE, or to the
RELENG_6_1, RELENG_6_0, or RELENG_5_5 security branch dated after the
correction date.

2) To patch your present system:

The following patches have been verified to apply to FreeBSD 5.5, 6.0,
and 6.1 systems.

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

[FreeBSD 5.5]
# fetch http://security.FreeBSD.org/patches/SA-07:01/jail5.patch 
# fetch http://security.FreeBSD.org/patches/SA-07:01/jail5.patch.asc 

[FreeBSD 6.0]
# fetch http://security.FreeBSD.org/patches/SA-07:01/jail60.patch 
# fetch http://security.FreeBSD.org/patches/SA-07:01/jail60.patch.asc 

[FreeBSD 6.1]
# fetch http://security.FreeBSD.org/patches/SA-07:01/jail61.patch 
# fetch http://security.FreeBSD.org/patches/SA-07:01/jail61.patch.asc 

b) Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch
# install -o root -g wheel -m 555 etc/rc.d/jail /etc/rc.d

VI.  Correction details

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

Branch                                                           Revision
- -------------------------------------------------------------------------
  src/UPDATING                                            1.342.
  src/UPDATING                                             1.416.
  src/UPDATING                                            1.416.
  src/UPDATING                                             1.416.
- -------------------------------------------------------------------------

VII. References


The latest revision of this advisory is available at
Version: GnuPG v1.4.6 (FreeBSD)


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