Messages in this thread | | | Date | Sat, 2 Mar 2013 21:56:08 -0600 | From | "Serge E. Hallyn" <> | Subject | Re: user ns: arbitrary module loading |
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Quoting Kees Cook (keescook@google.com): > On Sat, Mar 2, 2013 at 4:57 PM, Serge E. Hallyn <serge@hallyn.com> wrote: > > 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. > > Well, not just randomfs. Any module that modprobe in the init ns can find.
right
> > 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. > > I'd be curious to hear the objections. It seems pretty nice to me to
Wait, sorry, I mis-spoke. The objection would have been to requiring CAP_SYS_MODULE, which is different. Sorry!
> add a new argument to every request_module() that specifies the > "subsystem" it expects a module to load from. Maybe pass > "request_module=filesystem" or "...=netdev" to the modprobe call. And
That would be useful for adding to the separation of privileges, i.e. helping contain the leaking of posix caps. It sounds good to me.
> then in init_module(), check the userargs for which subsystem was > requested and look up in a table for the entry point module symbol for > that subsystem to require. e.g. for "request_module=filesystem", > require that the module contains the "register_filesystem" symbol, > etc. > > > But how about if we > > add a check for 'current_user_ns() == &init_user_ns' at that place > > instead? > > Well, we'd need to mostly revert > 57eccb830f1cc93d4b506ba306d8dfa685e0c88f ("mount: consolidate > permission checks") since get_fs_type() is being called before > may_mount() right now. (And then, as you suggest, we should strengthen > the test.) I think this will require either more plumbing into > get_fs_type (something like "bool load_module_if_missing") or the > subsystem verification stuff in request_module. I think the latter is > MUCH nicer as it covers this problem in all places, not just this > "mount" case.
My first instinct was to say I'd like to have the kernel 100% belonging to the init_user_ns, with child user namespaces having zero ability to induce loading of any kernel modules, period. So a check for current being in init_user_ns at request_module itself.
However (thinking more) that seems maybe wrong. You don't need privs to induce the loading of a new binfmt module right? The host's /lib/modules and module blacklists should be set up right by the admin (or distro)... If we require that the host admin manually modprobe every module which a task in a child user namespace might need, that goes counter to the goal of kernel modules.
> > Eric Biederman, do you have any objections to that?
-serge
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