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

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Date:	Wed, 25 Jun 2014 07:51:25 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	"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 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?

-Kees

-- 
Kees Cook
Chrome OS Security
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