It has been pointed out that some files in /proc and /sys can be used
to break out of containers. However, if those filesystems are mounted
read-only, most of the known exploits are mitigated, since they rely
on writing some file in those filesystems.

This does not replace security modules (like SELinux or AppArmor), it
is just another layer of security. Likewise, it doesn't mean that the
other mitigations (shadowing parts of /proc or /sys with bind mounts)
are useless. Those measures are still useful. As such, the shadowing
of /proc/kcore is still enabled with both LXC and native drivers.

Special care has to be taken with /proc/1/attr, which still needs to
be mounted read-write in order to enable the AppArmor profile. It is
bind-mounted from a private read-write mount of procfs.

All that enforcement is done in dockerinit. The code doing the real
work is in libcontainer. The init function for the LXC driver calls
the function from libcontainer to avoid code duplication.

Docker-DCO-1.1-Signed-off-by: Jérôme Petazzoni <[email protected]> (github: jpetazzo)

There is not need for the remount hack, we use aa_change_onexec so the
apparmor profile is not applied until we exec the users app.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <[email protected]> (github: crosbymichael)