Linux namespaces: It is possible to escape from bind mounts

[Jann Horn <jann () thejh net>]

See here for the corresponding patches:
<http://permalink.gmane.org/gmane.linux.kernel.containers/29173>
<http://permalink.gmane.org/gmane.linux.kernel.containers/29177>

Given that it went over a public mailinglist now, I guess there's
not much sense in keeping it secret anymore.

Containers on Linux normally use bind mounts to restrict how much
of the filesystem is visible for processes inside the container.
However, if an attacker can gain capabilities within such a
container or can create another user and mount namespace within
the existing container, he can do something similar to a
double-chroot attack to break out of the bind mount and gain
access to the full filesystem to which the bind mount refers:

Create folders /A, /A/B, /C, /D inside the namespace.
Bind-mount the /A inside the namespace to /D.
Let a process chdir to /D/B.
Move /D/B over into /C.
The process which chdir'ed to /D/B is now in /C/B, but at the
same time it is in a bind mount with /D as root. It can then
traverse upwards, past what looks like / inside the namespace.

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