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SubjectRe: user ns: arbitrary module loading
From
On Mon, Mar 4, 2013 at 10:21 AM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
> Kees Cook <keescook@google.com> writes:
>
>> On Mon, Mar 4, 2013 at 12:29 AM, Mathias Krause <minipli@googlemail.com> wrote:
>>> On Sun, Mar 03, 2013 at 09:48:50AM -0800, Kees Cook wrote:
>>>> 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);
>>>>
>>>> [...]
>>>>
>>>> Several of these come from hardcoded values, though (e.g. crypto, chipreg).
>>>
>>> Well, crypto does not. Try the code snippet below on a system with
>>> CONFIG_CRYPTO_USER_API=y. It'll abuse the above request_module() call
>>> to load any module the user requests -- iregardless of being contained
>>> in a user ns or not.
>>
>> Oh ew. Yeah, I must have missed the path through the user api. Arg.
>
> I will let someone else write the patch that adds the module aliases to
> crypto.
>
> It seems worth doing even outside of any security concerns as it just
> makes the reqest to modprobe make more sense, and allows the existing
> modprobe policy controls to work.
>
> Whereas an ill-formed string just doesn't tell modprobe enough to really
> act intelligently.
>
>>> ---8<---
>>> /* Loading arbitrary modules using crypto api since v2.6.38
>>> *
>>> * - minipli
>>> */
>>> #include <linux/if_alg.h>
>>> #include <sys/socket.h>
>>> #include <unistd.h>
>>> #include <stdlib.h>
>>> #include <string.h>
>>> #include <stdio.h>
>>>
>>> #ifndef AF_ALG
>>> #define AF_ALG 38
>>> #endif
>>>
>>>
>>> int main(int argc, char **argv) {
>>> struct sockaddr_alg sa_alg = {
>>> .salg_family = AF_ALG,
>>> .salg_type = "hash",
>>> };
>>> int sock;
>>>
>>> if (argc != 2) {
>>> printf("usage: %s MODULE_NAME\n", argv[0]);
>>> exit(1);
>>> }
>>>
>>> sock = socket(AF_ALG, SOCK_SEQPACKET, 0);
>>> if (sock < 0) {
>>> perror("socket(AF_ALG)");
>>> exit(1);
>>> }
>>>
>>> strncpy((char *) sa_alg.salg_name, argv[1], sizeof(sa_alg.salg_name));
>>> bind(sock, (struct sockaddr *) &sa_alg, sizeof(sa_alg));
>>> close(sock);
>>>
>>> return 0;
>>> }
>>> --->8---
>>>
>>> If people care about unprivileged users not being able to load arbitrary
>>> modules, could someone please fix this in crypto API, then? Herbert?
>>
>> So, should this get a prefix too? Maybe we need to change the
>> request_module primitive to request_module(prefix, fmt, args) to stop
>> these request_module("%s", name) things from continuing to exist...
>
> Something like the patch below?
>
> diff --git a/kernel/kmod.c b/kernel/kmod.c
> index 56dd349..859aa3a 100644
> --- a/kernel/kmod.c
> +++ b/kernel/kmod.c
> @@ -131,6 +131,10 @@ int __request_module(bool wait, const char *fmt, ...)
> #define MAX_KMOD_CONCURRENT 50 /* Completely arbitrary value - KAO */
> static int kmod_loop_msg;
>
> + /* Require that calls to request module have a little structure */
> + if (fmt[0] == '%')
> + return -EINVAL;
> +
> /*
> * We don't allow synchronous module loading from async. Module
> * init may invoke async_synchronize_full() which will end up

Something like that, but that'll break some things that do stuff like %s-suffix.

-Kees

--
Kees Cook
Chrome OS Security


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