Re: user ns: arbitrary module loading

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Kees Cook (keescook@google.com):
> 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

So the concern is root in a child user namespace doing

	mount -t randomfs <...>

in which case do_new_mount() checks ns_capable(), not capable(),
before trying to load a module for randomfs.

As well as (secondly) the fact that there is no enforcement on
the format of the module names (i.e. fs-*).

Kees, from what I've seen the GRKERNSEC_MODHARDEN won't be acceptable.
At least Eric Paris is strongly against it.  But how about if we
add a check for 'current_user_ns() == &init_user_ns' at that place
instead?

Eric Biederman, do you have any objections to that?

thanks,
-serge