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

[Oleg Nesterov <oleg@redhat.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.