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

From: Oleg Nesterov
Date: Thu Jul 10 2014 - 11:26:57 EST

On 07/10, Kees Cook wrote:
> On Wed, Jul 9, 2014 at 11:55 AM, Oleg Nesterov <oleg@xxxxxxxxxx> 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. :)

Perhaps I am just trying to confuse you and myself ;)


> Since rmb() happens before run_filters, isn't the ACCESS_ONCE
> sufficient?

Yes. But see above. ACCESS_ONCE is sufficient if we read the first filter
installed by another thread, in this case rmb() pairs with mb_before_atomic()
before set_bit(TIF_SECCOMP).

IOW, if this threads sees TIF_SECCOMP, it should also see all modifications
which were done before set_bit, including the data in ->filter points to.

> 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,

Yes, it is fine if we miss yet another filter which was just installed by
another thread.

But, unless I missed something, the problem is that we can get this new

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.


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