From: oleg@redhat•com (Oleg Nesterov)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH v9 09/11] seccomp: introduce writer locking
Date: Thu, 10 Jul 2014 19:35:52 +0200 [thread overview]
Message-ID: <20140710173552.GA27410@redhat.com> (raw)
In-Reply-To: <CAGXu5jKNUn0OcXPyTmqbHwQ_GPMNTeajyrxpd2xAtzjTRFyhpg@mail.gmail.com>
On 07/10, Kees Cook wrote:
>
> On Thu, Jul 10, 2014 at 8:24 AM, Oleg Nesterov <oleg@redhat•com> wrote:
> >
> > Just to simplify. Suppose TIF_SECCOMP was set a long ago. This thread
> > has a single filter F1 and it enters seccomp_run_filters().
> >
> > Right before it does ACCESS_ONCE() to read the pointer, another thread
> > does seccomp_sync_threads() and sets .filter = F2.
> >
> > If ACCESS_ONCE() returns F1 - everything is fine. But it can see the new
> > pointer F2, and in this case we need a barrier to ensure that, say,
> > LOAD(F2->prog) will see all the preceding changes in this memory.
>
> And the rmb() isn't sufficient for that?
But it has no effect if the pointer was changed _after_ rmb() was already
called.
And, you need a barrier _after_ ACCESS_ONCE().
(Unless, again, we know that this is the first filter, but this is only
by accident).
> Is another barrier needed
> before assigning the filter pointer to make sure the contents it
> points to are flushed?
I think smp_store_release() should be moved from seccomp_attach_filter()
to seccomp_sync_threads(). Although probably it _should_ work either way,
but at least this looks confusing because a) "current" doesn't need a
barrier to serialize wuth itself, and b) it is not clear why it is safe
to change the pointer dereferenced by another thread without a barrier.
> What's the least time-consuming operation I can use in run_filters?
As I said smp_read_barrier_depends() (nop unless alpha) or
smp_load_acquire() which you used in the previous version.
And to remind, afaics smp_load_acquire() in put_filter() should die ;)
Oleg.
next prev parent reply other threads:[~2014-07-10 17:35 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-06-27 23:22 [PATCH v9 0/11] seccomp: add thread sync ability Kees Cook
2014-06-27 23:22 ` [PATCH v9 01/11] seccomp: create internal mode-setting function Kees Cook
2014-06-27 23:22 ` [PATCH v9 02/11] seccomp: extract check/assign mode helpers Kees Cook
2014-06-27 23:22 ` [PATCH v9 03/11] seccomp: split mode setting routines Kees Cook
2014-06-27 23:22 ` [PATCH v9 04/11] seccomp: add "seccomp" syscall Kees Cook
2014-06-27 23:22 ` [PATCH v9 05/11] ARM: add seccomp syscall Kees Cook
2014-06-27 23:22 ` [PATCH v9 06/11] MIPS: " Kees Cook
2014-06-27 23:22 ` [PATCH v9 07/11] sched: move no_new_privs into new atomic flags Kees Cook
2014-06-27 23:22 ` [PATCH v9 08/11] seccomp: split filter prep from check and apply Kees Cook
2014-06-27 23:22 ` [PATCH v9 09/11] seccomp: introduce writer locking Kees Cook
2014-07-09 18:42 ` Oleg Nesterov
2014-07-09 18:55 ` Oleg Nesterov
2014-07-10 9:25 ` Kees Cook
2014-07-10 15:24 ` Oleg Nesterov
2014-07-10 16:54 ` Kees Cook
2014-07-10 17:35 ` Oleg Nesterov [this message]
2014-07-09 18:59 ` Oleg Nesterov
2014-06-27 23:22 ` [PATCH v9 10/11] seccomp: allow mode setting across threads Kees Cook
2014-06-27 23:23 ` [PATCH v9 11/11] seccomp: implement SECCOMP_FILTER_FLAG_TSYNC Kees Cook
2014-07-09 18:05 ` Oleg Nesterov
2014-07-10 9:17 ` Kees Cook
2014-07-10 15:08 ` Oleg Nesterov
2014-07-10 16:03 ` Kees Cook
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