linux-mips
[Top] [All Lists]

Re: [PATCH v8 5/9] seccomp: split mode set routines

To: Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH v8 5/9] seccomp: split mode set routines
From: Kees Cook <keescook@chromium.org>
Date: Wed, 25 Jun 2014 09:54:01 -0700
Cc: Oleg Nesterov <oleg@redhat.com>, LKML <linux-kernel@vger.kernel.org>, "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>, Alexei Starovoitov <ast@plumgrid.com>, Andrew Morton <akpm@linux-foundation.org>, Daniel Borkmann <dborkman@redhat.com>, Will Drewry <wad@chromium.org>, Julien Tinnes <jln@chromium.org>, David Drysdale <drysdale@google.com>, Linux API <linux-api@vger.kernel.org>, "x86@kernel.org" <x86@kernel.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, linux-mips@linux-mips.org, linux-arch <linux-arch@vger.kernel.org>, linux-security-module <linux-security-module@vger.kernel.org>
Dkim-signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20120113; h=mime-version:sender:in-reply-to:references:date:message-id:subject :from:to:cc:content-type; bh=4sCV+YHmOBm1mKPxFZyEVaE3vdJtTI/7qrUHp5SwZQQ=; b=QHBMgGUSPOW7GhHCogJUDB1v3pw1ajFX5NeT6LEXxZ0Oa/R9wC+6buYKlk4U7W+hgC l30qwnDPw/6UHwZllNcFhK89vTi9b4agy3ypotB18jCGB9+loUUjSEpPysdp7XseQYq0 Ah8r7UuNFdJgDSxPmaKXn8Oo98BrA3s8qqkwlARFQxFcibULgSwvK9IV4tUCqCzgEsAq VmCaLVy0HzX/XhqJQRuFWVI+YBldd9Ay3OOofY1H1BvFRdq0QSrG06fhT+rMCQPpMvS2 msQZH7MIUC/rpwvS2owESZTKcuZj0q7h6URNkgrKWctR97qXlBMxeZFob/Kw2lpT5tkp QTEQ==
Dkim-signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=mime-version:sender:in-reply-to:references:date:message-id:subject :from:to:cc:content-type; bh=4sCV+YHmOBm1mKPxFZyEVaE3vdJtTI/7qrUHp5SwZQQ=; b=OSdgx4yTVSviBVQOart1SprFt8omtio1cNeiUXgrlidnj9TBJjXMaN/9iQEKfN7ZXG ZDYPAm0pRa0xgflEqsir4NSUmJz5JSlv3qL9QUhxMepFxPmBA/iw2S4G9+jYVKPaVQRB VQYQ7/rD++tkQ/gcNr7EiCDXiBWa7UrZhTDN8=
In-reply-to: <CALCETrUBNmLnpa+LM91om2RSpR6SjupP-EdefzhU1Me4nv3Dfw@mail.gmail.com>
List-archive: <http://www.linux-mips.org/archives/linux-mips/>
List-help: <mailto:ecartis@linux-mips.org?Subject=help>
List-id: linux-mips <linux-mips.eddie.linux-mips.org>
List-owner: <mailto:ralf@linux-mips.org>
List-post: <mailto:linux-mips@linux-mips.org>
List-software: Ecartis version 1.0.0
List-subscribe: <mailto:ecartis@linux-mips.org?subject=subscribe%20linux-mips>
List-unsubscribe: <mailto:ecartis@linux-mips.org?subject=unsubscribe%20linux-mips>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <1403642893-23107-1-git-send-email-keescook@chromium.org> <1403642893-23107-6-git-send-email-keescook@chromium.org> <20140625135121.GB7892@redhat.com> <CAGXu5jJkFxh4K=40xuh6tu3kUf4oJM8Dry+4upBdRieW3FNLgw@mail.gmail.com> <CALCETrUBNmLnpa+LM91om2RSpR6SjupP-EdefzhU1Me4nv3Dfw@mail.gmail.com>
Sender: linux-mips-bounce@linux-mips.org
On Wed, Jun 25, 2014 at 9:10 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Wed, Jun 25, 2014 at 7:51 AM, Kees Cook <keescook@chromium.org> wrote:
>> On Wed, Jun 25, 2014 at 6:51 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>>> On 06/24, Kees Cook wrote:
>>>>
>>>> +static inline void seccomp_assign_mode(struct task_struct *task,
>>>> +                                    unsigned long seccomp_mode)
>>>> +{
>>>> +     BUG_ON(!spin_is_locked(&task->sighand->siglock));
>>>> +
>>>> +     task->seccomp.mode = seccomp_mode;
>>>> +     set_tsk_thread_flag(task, TIF_SECCOMP);
>>>> +}
>>>
>>> OK, but unless task == current this can race with secure_computing().
>>> I think this needs smp_mb__before_atomic() and secure_computing() needs
>>> rmb() after test_bit(TIF_SECCOMP).
>>>
>>> Otherwise, can't __secure_computing() hit BUG() if it sees the old
>>> mode == SECCOMP_MODE_DISABLED ?
>>>
>>> Or seccomp_run_filters() can see ->filters == NULL and WARN(),
>>> smp_load_acquire() only serializes that LOAD with the subsequent memory
>>> operations.
>>
>> Hm, actually, now I'm worried about smp_load_acquire() being too slow
>> in run_filters().
>>
>> The ordering must be:
>> - task->seccomp.filter must be valid before
>> - task->seccomp.mode is set, which must be valid before
>> - TIF_SECCOMP is set
>>
>> But I don't want to impact secure_computing(). What's the best way to
>> make sure this ordering is respected?
>
> Remove the ordering requirement, perhaps?
>
> What if you moved mode into seccomp.filter?  Then there would be
> little reason to check TIF_SECCOMP from secure_computing; instead, you
> could smp_load_acquire (or read_barrier_depends, maybe) seccomp.filter
> from secure_computing and pass the result as a parameter to
> __secure_computing.  Or you could even remove the distinction between
> secure_computing and __secure_computing -- it's essentially useless
> anyway to split entry hook approaches like my x86 fastpath prototype.

The TIF_SECCOMP is needed for the syscall entry path. The check in
secure_computing() is just because the "I am being traced" trigger
includes a call to secure_computing, which filters out tracing
reasons.

Your fast path work would clean a lot of that up, as you say. But it
still doesn't change the ordering check here. TIF_SECCOMP indicates
seccomp.mode must be checked, so that ordering will remain, and if
mode == FILTER, seccomp.filter must be valid.

Isn't there a way we can force the assignment ordering in seccomp_assign_mode()?

-Kees

-- 
Kees Cook
Chrome OS Security

<Prev in Thread] Current Thread [Next in Thread>