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FreeBSD-SA-04:03.jail                                       Security Advisory
                                                          The FreeBSD Project

Topic:          Jailed processes can attach to other jails

Category:       core
Module:         kernel
Announced:      2004-02-25
Credits:        JAS Group (http://www.cs.mu.oz.au/jas/)
Affects:        FreeBSD 5.1-RELEASE
                FreeBSD 5.2-RELEASE
Corrected:      2004-02-19 23:26:39 UTC (RELENG_5_2, 5.2.1-RC2)
                2004-02-25 20:03:35 UTC (RELENG_5_1, 5.1-RELEASE-p14)
CVE Name:       CAN-2004-0126
FreeBSD only:   YES

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

I.   Background

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

The jail_attach(2) system call, which was introduced in FreeBSD 5
before 5.1-RELEASE, allows a non-jailed process to permanently move
into an existing jail.

II.  Problem Description

A programming error has been found in the jail_attach(2) system call
which affects the way that system call verifies the privilege
level of the calling process.  Instead of failing immediately if the
calling process was already jailed, the jail_attach(2) system call
would fail only after changing the calling process's root directory.

III. Impact

A process with superuser privileges inside a jail could change its
root directory to that of a different jail, and thus gain full read
and write access to files and directories within the target jail.

IV.  Workaround

No workaround is available.

V.   Solution

Do one of the following:

1) Upgrade your vulnerable system to 5.2.1-RELEASE, or to the
RELENG_5_2 or RELENG_5_1 security branch dated after the correction
date.

OR

2) Patch your present system:

The following patch has been verified to apply to FreeBSD 5.1 and 5.2
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-04:03/jail.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-04:03/jail.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_5_2
  src/sys/kern/kern_jail.c                                       1.34.2.1
RELENG_5_1
  src/UPDATING                                                 1.251.2.16
  src/sys/conf/newvers.sh                                       1.50.2.16
  src/sys/kern/kern_jail.c                                       1.33.2.1
- -------------------------------------------------------------------------

VII. References

<URL:http://www.cs.mu.oz.au/jas/>
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