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

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Date:	Wed, 25 Jun 2014 10:38:17 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Kees Cook <keescook@...omium.org>,
	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 10:32 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 06/25, Andy Lutomirski wrote:
>>
>> Write the filter, then smp_mb (or maybe a weaker barrier is okay),
>> then set the bit.
>
> Yes, exactly, this is what I meant. Plas rmb() in __secure_computing().
>
> But I still can't understand the rest of your discussion about the
> ordering we need ;)

Let me try again from scratch.

Currently there are three relevant variables: TIF_SECCOMP,
seccomp.mode, and seccomp.filter.  __secure_computing needs
seccomp.mode and seccomp.filter to be in sync, and it wants (but
doesn't really need) TIF_SECCOMP to be in sync as well.

My suggestion is to rearrange it a bit.  Move mode into seccomp.filter
(so that filter == NULL implies no seccomp) and don't check
TIF_SECCOMP in secure_computing.  Then turning on seccomp is entirely
atomic except for the fact that the seccomp hooks won't be called if
filter != NULL but !TIF_SECCOMP.  This removes all ordering
requirements.

Alternatively, __secure_computing could still BUG_ON(!seccomp.filter).
In that case, filter needs to be set before TIF_SECCOMP is set, but
that's straightforward.

--Andy
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