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SubjectRe: user ns: arbitrary module loading
On Sat, Mar 2, 2013 at 7:56 PM, Serge E. Hallyn <> wrote:
> Quoting Kees Cook (
>> On Sat, Mar 2, 2013 at 4:57 PM, Serge E. Hallyn <> wrote:
>> > Quoting Kees Cook (
>> >> 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]
>> >
>> > 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.

Several subsystems already have an implicit subsystem restriction
because they load with aliases. (e.g. binfmt-XXXX, net-pf=NNN,
snd-card-NNN, FOO-iosched, etc). This isn't the case for filesystems
and a few others, unfortunately:

$ git grep 'request_module("%.*s"' | grep -vi prefix
crypto/api.c: request_module("%s", name);
drivers/mtd/chips/chipreg.c: if (!drv && !request_module("%s", name))
drivers/mtd/mtdpart.c: if (!parser && !request_module("%s", *types))
drivers/net/wireless/iwlwifi/iwl-drv.c: request_module("%s", op->name);
drivers/staging/rtl8192e/rtllib_wx.c: request_module("%s", tempbuf);
fs/filesystems.c: if (!fs && (request_module("%.*s", len, name) == 0))
net/core/dev_ioctl.c: if (!request_module("%s", name))

Several of these come from hardcoded values, though (e.g. crypto, chipreg).

>> > Eric Biederman, do you have any objections to that?
> -serge


Kees Cook
Chrome OS Security

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