Re: [tip:perf/core] [perf] da916e96e2: BUG:KASAN:null-ptr-deref_in_put_event

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Apr 15, 2025 at 12:08:40PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 15, 2025 at 10:14:05AM +0100, James Clark wrote:
> > On 15/04/2025 5:46 am, Oliver Sang wrote:
> 
> > > yes, below patch fixes the issues we observed for da916e96e2. thanks
> > > 
> > > Tested-by: kernel test robot <oliver.sang@intel.com>
> > > 
> > 
> > Also fixes the same issues we were seeing:
> > 
> > Tested-by: James Clark <james.clark@linaro.org>
> 
> Excellent, thank you both! Now I gotta go write me a Changelog :-)

Hmm, so while writing Changelog, I noticed something else was off. The
case where event->parent was set to EVENT_TOMBSTONE now didn't have a
put_event(parent) anymore. So that needs to be put back in as well.

Frederic, afaict this should still be okay, since if we're detached,
then nothing will try and access event->parent in the free path.

Also, nothing in perf_pending_task() will try and access either
event->parent or event->pmu.

---
Subject: perf: Fix event->parent life-time issue
From: Peter Zijlstra <peterz@infradead.org>
Date: Tue Apr 15 12:12:52 CEST 2025

Due to an oversight in merging da916e96e2de ("perf: Make
perf_pmu_unregister() useable") on top of 56799bc03565 ("perf: Fix
hang while freeing sigtrap event"), it is now possible to hit
put_event(EVENT_TOMBSTONE), which makes the computer sad.

This also means that for the event->parent == EVENT_TOMBSTONE, the
put_event() matching inherit_event() has gone missing.

Previously this was done in perf_event_release_kernel() after calling
perf_remove_from_context(), but with it delegated to put_event(), this
case is now entirely missed, leading to leaks.

Fixes: da916e96e2de ("perf: Make perf_pmu_unregister() useable")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/events/core.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2343,6 +2343,7 @@ static void perf_child_detach(struct per
 	 * not being a child event. See for example unaccount_event().
 	 */
 	event->parent = EVENT_TOMBSTONE;
+	put_event(parent_event);
 }
 
 static bool is_orphaned_event(struct perf_event *event)
@@ -5688,7 +5689,7 @@ static void put_event(struct perf_event
 	_free_event(event);
 
 	/* Matches the refcount bump in inherit_event() */
-	if (parent)
+	if (parent && parent != EVENT_TOMBSTONE)
 		put_event(parent);
 }