Re: [PATCH 2/3] seccomp: release task filters when the task exits

Oleg Nesterov
2024-05-22T10:37:27Z

On 05/22, Andrei Vagin wrote:
>
> > On Thu, May 16, 2024 at 6:10 AM Oleg Nesterov <oleg@redhat.com> wrote:
> > >
> > > On 05/16, Oleg Nesterov wrote:
> > > >
> > > > On 05/15, Andrei Vagin wrote:
> > > > >
> > > > > seccomp_sync_threads and seccomp_can_sync_threads should be considered too.
> > > >
> > > > Yes. But we only need to consider them in the multi-thread case, right?
> > > > In this case exit_signals() sets PF_EXITING under ->siglock, so they can't
> > > > miss this flag, seccomp_filter_release() doesn't need to take siglock.
> > > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> PF_EXITING is set without holding ->siglock if tsk->signal has the
> SIGNAL_GROUP_EXIT flag. I think it can be a case when one thread is in
> seccomp_sync_threads and others are exiting.

Yes, I forgot this.

> > > Hmm. I thought we have something smp_mb__after_unlock(), but it seems we
> > > don't. So we can't add a fast-path
>
> We have smp_mb__after_unlock_lock in include/linux/rcupdate.h.

This is another thing.

But sorry for confusion, this doesn't really matter, we could you a plain mb().
I mean, I was thinking about something like

seccomp_filter_release:

smp_mb();
if (!READ_ONCE(tsk->seccomp.filter))
return;

spin_lock_irq(siglock);
orig = tsk->seccomp.filter;
...

but then seccomp_sync_threads() should do something like

orig = READ_ONCE(thread->seccomp.filter);

smp_store_release(&thread->seccomp.filter,
caller->seccomp.filter);

smp_mb(); // pairs with mb() in seccomp_filter_release()

if (READ_ONCE(thread->flags) & PF_EXITING) {
WRITE_ONCE(thread->seccomp.filter, orig);
continue;
}
__seccomp_filter_release(orig);

...

too subtle even _if_ correct, and I am not sure at all this would be correct.

> > > Cough... Now that I look at seccomp_can_sync_threads() I think it too
> > > doesn't need the PF_EXITING check.
> > >
> > > If it is called before seccomp_filter_release(), this doesn't really
> > > differ from the case when it is called before do_exit/exit_signals.
> > >
> > > If it is called after seccomp_filter_release(), then is_ancestor()
> > > must be true.
> > >
> > > But perhaps I missed something, I won't insist, up to you.
> > >
>
> You are right, this check isn't required in seccomp_can_sync_threads, but
> I decided that it is better to be consistent with seccomp_sync_threads.

OK, agreed.

Oleg.