Insecure sandbox temporary lockfile vulnerabilities in Portage
Gentoo Linux Security Advisory
||GLSA 200404-01 / Portage
||April 04, 2004
||April 04, 2004: 01
All supported architectures
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
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.
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.
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.
A workaround is not currently known for this issue. All users are advised
to upgrade to the latest version of the affected package.
Users should upgrade to Portage 2.0.50-r3 or later:
Code Listing 3.1: Resolution
# emerge sync
# emerge -pv ">=sys-apps/portage-2.0.50-r3"
# emerge ">=sys-apps/portage-2.0.50-r3"