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

To: Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v8 5/9] seccomp: split mode set routines
From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 25 Jun 2014 09:10:32 -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>
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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.

--Andy

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