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

To: Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v8 5/9] seccomp: split mode set routines
From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 25 Jun 2014 10:38:17 -0700
Cc: Kees Cook <keescook@chromium.org>, LKML <linux-kernel@vger.kernel.org>, "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>, Alexei Starovoitov <ast@plumgrid.com>, Andrew Morton <akpm@linux-foundation.org>, Daniel Borkmann <dborkman@redhat.com>, Will Drewry <wad@chromium.org>, Julien Tinnes <jln@chromium.org>, David Drysdale <drysdale@google.com>, Linux API <linux-api@vger.kernel.org>, "x86@kernel.org" <x86@kernel.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, linux-mips@linux-mips.org, linux-arch <linux-arch@vger.kernel.org>, linux-security-module <linux-security-module@vger.kernel.org>
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On Wed, Jun 25, 2014 at 10:32 AM, Oleg Nesterov <oleg@redhat.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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