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

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Date:	Wed, 25 Jun 2014 09:10:32 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Kees Cook <keescook@...omium.org>
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 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.

--Andy
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