user ns: arbitrary module loading

The rearranging done for user ns has resulted in allowing arbitrary
kernel module loading[1] (i.e. re-introducing a form of CVE-2011-1019)
by what is assumed to be an unprivileged process.

At present, it does look to require at least CAP_SETUID along the way
to set up the uidmap (but things like the setuid helper newuidmap
might soon start providing such a thing by default).

It might be worth examining GRKERNSEC_MODHARDEN in grsecurity, which
examines module symbols to verify that request_module() for a
filesystem only loads a module that defines "register_filesystem"
(among other things).

-Kees

[1] https://twitter.com/grsecurity/status/307473816672665600

--
Kees Cook
Chrome OS Security