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Gentoo Linux Security Advisory                           GLSA 200404-01
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~                                            http://security.gentoo.org
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~  Severity: Normal
~     Title: Insecure sandbox temporary lockfile vulnerabilities in
~            Portage
~      Date: April 04, 2004
~      Bugs: #21923
~        ID: 200404-01

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Synopsis
========

A flaw has been found in the temporary file handling algorithms for the
sandboxing code used within Portage. Lockfiles created during normal
Portage operation of portage could be manipulated by local users
resulting in the truncation of hard linked files; causing a Denial of
Service attack on the system.

Background
==========

Portage is Gentoo's package management system which is responsible for
installing, compiling and updating any ebuilds on the system through the
Gentoo rsync tree. Under default configurations, most ebuilds run under
a sandbox which prevent the build process writing to the "real" system
outside the build directory - packages are installed into a temporary
location and then copied over safely by Portage instead. During the
process the sandbox wrapper creates lockfiles in the /tmp directory
which are vulnerable to a hard-link attack.

Affected packages
=================

~    -------------------------------------------------------------------
~     Package           /    Vulnerable    /                 Unaffected
~    -------------------------------------------------------------------
~     sys-apps/portage       < 2.0.50-r3                   >= 2.0.50-r3

Description
===========

A flaw in Portage's sandbox wrapper has been found where the temporary
lockfiles are subject to a hard-link attack which allows linkable files
to be overwritten to an empty file. This can be used to damage critical
files on a system causing a Denial of Service, or alternatively this
attack may be used to cause other security risks; for example firewall
configuration data could be overwritten without notice.

The vulnerable sandbox functions have been patched to test for these new
conditions: namely; for the existance of a hard-link which would be
removed before the sandbox process would continue, for the existance of
a world-writable lockfile in which case the sandbox would also remove
it, and also for any mismatches in the UID ( anything but root ) and the
GID ( anything but the group of the sandbox process ).

If the vulnerable files cannot be removed by the sandbox, then the
sandbox would exit with a fatal error warning the adminstrator of the
issue. The patched functions also fix any other sandbox I/O operations
which do not explicitly include the mentioned lockfile.

Impact
======

Any user with write access to the /tmp directory can hard-link a file to
/tmp/sandboxpids.tmp - this file would eventually be replaced with an
empty one; effectively wiping out the file it was linked to as well with
no prior warning. This could be used to potentially disable a vital
component of the system and cause a path for other possible exploits.

This vulnerability only affects systems that have /tmp on the root
partition: since symbolic link attacks are filtered, /tmp has to be on
the same partition for an attack to take place.

Workaround
==========

A workaround is not currently known for this issue. All users are
advised to upgrade to the latest version of Portage.

Resolution
==========

Users should upgrade to Portage 2.0.50-r3 or later:

~    # emerge sync

~    # emerge -pv ">=sys-apps/portage-2.0.50-r3"
~    # emerge ">=sys-apps/portage-2.0.50-r3"

Concerns?
=========

Security is a primary focus of Gentoo Linux and ensuring the
confidentiality and security of our users machines is of utmost
importance to us. Any security concerns should be addressed to
security@gentoo.org or alternatively, you may file a bug at
http://bugs.gentoo.org.
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