linux-kernel - Re: [PATCH v9 09/11] seccomp: introduce writer locking

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Date:	Thu, 10 Jul 2014 02:25:12 -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 v9 09/11] seccomp: introduce writer locking

On Wed, Jul 9, 2014 at 11:55 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 07/09, Oleg Nesterov wrote:
>>
>> On 06/27, Kees Cook wrote:
>> >
>> >  static u32 seccomp_run_filters(int syscall)
>> >  {
>> > -   struct seccomp_filter *f;
>> > +   struct seccomp_filter *f = ACCESS_ONCE(current->seccomp.filter);
>>
>> I am not sure...
>>
>> This is fine if this ->filter is the 1st (and only) one, in this case
>> we can rely on rmb() in the caller.
>>
>> But the new filter can be installed at any moment. Say, right after that
>> rmb() although this doesn't matter. Either we need smp_read_barrier_depends()
>> after that, or smp_load_acquire() like the previous version did?
>
> Wait... and it seems that seccomp_sync_threads() needs smp_store_release()
> when it sets thread->filter = current->filter by the same reason?
>
> OTOH. smp_store_release() in seccomp_attach_filter() can die, "current"
> doesn't need a barrier to serialize with itself.

I have lost track of what you're suggesting to change. :)

Since rmb() happens before run_filters, isn't the ACCESS_ONCE
sufficient? We only care that TIF_SECCOMP, mode, and some filter is
valid. In a tsync thread race, it's okay to use not use the deepest
filter node in the list, it just needs A filter.

-Kees

-- 
Kees Cook
Chrome OS Security
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