linux-kernel - Re: user ns: arbitrary module loading

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Date:	Mon, 4 Mar 2013 10:41:04 -0800
From:	Kees Cook <keescook@...gle.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Mathias Krause <minipli@...glemail.com>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Brad Spengler <spender@...ecurity.net>,
	Al Viro <viro@...iv.linux.org.uk>,
	Eric Paris <eparis@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	PaX Team <pageexec@...email.hu>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: user ns: arbitrary module loading

On Mon, Mar 4, 2013 at 10:21 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Kees Cook <keescook@...gle.com> writes:
>
>> On Mon, Mar 4, 2013 at 12:29 AM, Mathias Krause <minipli@...glemail.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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