Re: [PATCH 2/4] perf: Fix irq work dereferencing garbage

[Frederic Weisbecker <frederic@kernel.org>]

Le Fri, May 02, 2025 at 12:29:18PM +0200, Peter Zijlstra a écrit :
> > @@ -13951,18 +13943,25 @@ perf_event_exit_event(struct perf_event *event,
> >  	/*
> >  	 * Child events can be freed.
> >  	 */
> > -	if (is_child) {
> > -		if (parent_event) {
> > -			mutex_unlock(&parent_event->child_mutex);
> > -			/*
> > -			 * Kick perf_poll() for is_event_hup();
> > -			 */
> > -			perf_event_wakeup(parent_event);
> > +	if (parent_event) {
> > +		mutex_unlock(&parent_event->child_mutex);
> > +		/*
> > +		 * Kick perf_poll() for is_event_hup();
> > +		 */
> > +		perf_event_wakeup(parent_event);
> 
> Should not this perf_event_wakeup() be inside the next if() as well?
> doing anything on parent_event when !ATTACH_CHILD seems dodgy.

Good point!

> 
> > +
> > +		/*
> > +		 * Match the refcount initialization. Make sure it doesn't happen
> > +		 * twice if pmu_detach_event() calls it on an already exited task.
> > +		 */
> > +		if (attach_state & PERF_ATTACH_CHILD) {
> >  			/*
> >  			 * pmu_detach_event() will have an extra refcount.
> > +			 * perf_pending_task() might have one too.
> >  			 */
> >  			put_event(event);
> >  		}
> > +
> >  		return;
> >  	}
> 
> This is a *much* saner patch, thank you!
> 
> So the thing I worried about... which is why I chose for the TOMBSTONE
> thing, is that this second invocation will now dereference parent_event,
> even though we've already released our reference count on it. 
> 
> This is essentially a use-after-free.
> 
> The thing that makes it work is RCU. And I think we're good, since the
> fail case is two perf_event_exit_event() invocations on the same event,
> separated by an RCU grace period, and I don't think this can happen.
> 
> But its a shame we can't reliably detect that.. Oh well.

It's not RCU but the reference count of the child that protects it.
In a second invocation, pmu_unregister() still holds a reference to
the child and that protects the parent as well because the reference
to the parent is only dropped once the child has dropped its own.

Hopefully that is one less opportunity for a headache :-)

-- 
Frederic Weisbecker
SUSE Labs