gdm -- privilege escalation vulnerability

Sebastian Krahmer reports:

It was discovered that the GNOME Display Manager (gdm) cleared the cache
directory, which is owned by an unprivileged user, with the privileges of the
root user. A race condition exists in gdm where a local user could take
advantage of this by writing to the cache directory between ending the session
and the signal to clean up the session, which could lead to the execution of
arbitrary code as the root user.