netdev - Re: [PATCH v11 07/12] seccomp: add SECCOMP_RET

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Date:	Mon, 27 Feb 2012 11:14:04 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Oleg Nesterov <oleg@...hat.com>, Will Drewry <wad@...omium.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-doc@...r.kernel.org, kernel-hardening@...ts.openwall.com,
	netdev@...r.kernel.org, x86@...nel.org, arnd@...db.de,
	davem@...emloft.net, hpa@...or.com, mingo@...hat.com,
	peterz@...radead.org, rdunlap@...otime.net, mcgrathr@...omium.org,
	tglx@...utronix.de, eparis@...hat.com, serge.hallyn@...onical.com,
	djm@...drot.org, scarybeasts@...il.com, indan@....nu,
	pmoore@...hat.com, akpm@...ux-foundation.org, corbet@....net,
	eric.dumazet@...il.com, markus@...omium.org,
	coreyb@...ux.vnet.ibm.com
Subject: Re: [PATCH v11 07/12] seccomp: add SECCOMP_RET_ERRNO

On Mon, Feb 27, 2012 at 10:35 AM, Andrew Lutomirski <luto@....edu> wrote:
> On Mon, Feb 27, 2012 at 10:14 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> On 02/27, Kees Cook wrote:
>>>
>>> On Mon, Feb 27, 2012 at 9:11 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>>> > On 02/24, Will Drewry wrote:
>>> >>
>>> >>  static u32 seccomp_run_filters(int syscall)
>>> >>  {
>>> >>       struct seccomp_filter *f;
>>> >> -     u32 ret = SECCOMP_RET_KILL;
>>> >>       static const struct bpf_load_fn fns = {
>>> >>               bpf_load,
>>> >>               sizeof(struct seccomp_data),
>>> >>       };
>>> >> +     u32 ret = SECCOMP_RET_ALLOW;
>>> >>       const void *sc_ptr = (const void *)(uintptr_t)syscall;
>>> >>
>>> >> +     /* Ensure unexpected behavior doesn't result in failing open. */
>>> >> +     if (unlikely(current->seccomp.filter == NULL))
>>> >> +             ret = SECCOMP_RET_KILL;
>>> >
>>> > Is "seccomp.filter == NULL" really possible?
>>>
>>> It should not be, but I'm much more comfortable with this failing
>>> closed. I think it's important to be as defensive as possible with
>>> this code given its intended use.
>>
>> Can't resists... Sorry, I know I am troll but personally I think
>> in this case the most defensive code is BUG_ON(->filter == NULL)
>> or at least WARN_ON().
>
> Linus will probably object because he objected (correctly) to a very
> similar problem in my old vsyscall emulation series.  A userspace
> security feature shouldn't have a failure mode in which it confuses
> the kernel and results in an oops, unless the situation is really
> unrecoverable.  So WARN_ON plus do_exit would be okay but BUG_ON would
> not.

Yeah, actually, add WARN_ON would be preferred here because it should
be an impossible situation. It should still fail closed, though:

     /* Ensure unexpected behavior doesn't result in failing open. */
     if (WARN_ON(current->seccomp.filter == NULL))
             return SECCOMP_RET_KILL;

-Kees

-- 
Kees Cook
ChromeOS Security
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