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

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Date:	Wed, 25 Jun 2014 09:54:01 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Will Drewry <wad@...omium.org>,
	Julien Tinnes <jln@...omium.org>,
	David Drysdale <drysdale@...gle.com>,
	Linux API <linux-api@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, linux-mips@...ux-mips.org,
	linux-arch <linux-arch@...r.kernel.org>,
	linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v8 5/9] seccomp: split mode set routines

On Wed, Jun 25, 2014 at 9:10 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Wed, Jun 25, 2014 at 7:51 AM, Kees Cook <keescook@...omium.org> wrote:
>> On Wed, Jun 25, 2014 at 6:51 AM, Oleg Nesterov <oleg@...hat.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
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