linux-kernel - Re: [PATCH 0/2] exec: Use sane stack rlimit for setuid exec

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Date:   Fri, 7 Jul 2017 15:19:07 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Serge Hallyn <serge@...lyn.com>,
        John Johansen <john.johansen@...onical.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Michal Hocko <mhocko@...nel.org>,
        Ben Hutchings <ben@...adent.org.uk>,
        Hugh Dickins <hughd@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        James Morris <james.l.morris@...cle.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Ingo Molnar <mingo@...nel.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Paul Moore <paul@...l-moore.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 0/2] exec: Use sane stack rlimit for setuid exec

On Fri, Jul 7, 2017 at 2:55 PM, Andy Lutomirski <luto@...nel.org> wrote:
> On Fri, Jul 7, 2017 at 12:56 PM, Kees Cook <keescook@...omium.org> wrote:
>> As discussed with Linus and Andy, we need to reset the stack rlimit
>> before we do memory layouts when execing a privilege-gaining (e.g.
>> setuid) program. This moves security_bprm_secureexec() earlier (with
>> required changes), and then lowers the stack limit when appropriate.
>
> As I see it, there are two cases to harden:
>
> 1. Bad guy has a high rlimit and runs a setuid program with crazy
> large arguments.  This is improved by this patch.  It's not entirely
> clear to me exactly what problem is solved, though, except that the
> rest of the exec code does not sanely check that we haven't used too
> much stack.  How about putting a check later on to make sure that
> we're not running low on stack rather than hoping we got the
> arithmetic right?

The rest of the exec uses a relatively fixed amount of space. (AT_*,
etc.) I didn't see any other dynamic stack usage, but maybe I missed
it?

>
> 2. Bad guy wants to trigger stack exhaustion in a setuid program at a
> controlled location and thus sets a crazy low rlimit.  This isn't
> addressed at all by this patch, but I assume it's what grsecurity was
> trying to do.  FWIW, I seem to recall that a lot of setuid attacks use
> intentionally weird rlimits to trigger unexpected signals.

It looks like they were protecting against 1:

        if (((!uid_eq(bprm->cred->euid, current_euid())) ||
(!gid_eq(bprm->cred->egid, current_egid()))) &&
            (old_rlim[RLIMIT_STACK].rlim_cur > (8 * 1024 * 1024)))
                current->signal->rlim[RLIMIT_STACK].rlim_cur = 8 * 1024 * 1024;

For 2, I think we need another examination of how things will fail
with too low a limit.

-Kees

-- 
Kees Cook
Pixel Security