[PATCH] perf: Fix mmap replacement ring lifetime race

[David Lee <david.lee@trailofbits.com>]

perf_mmap_close() drops the ring-local mmap_count before serializing
with perf_mmap() through event->mmap_mutex.  When the last mapping is
being closed, a concurrent mmap can therefore observe a nonzero
event->mmap_count and a zero rb->mmap_count.

In that case perf_mmap_rb() detaches the old ring, installs a replacement,
and resets event->mmap_count to one.  The old close then consumes that
replacement count and detaches the new ring.  Its pages can consequently
be freed while the replacement VMA still maps their PFNs.

Take event->mmap_mutex before updating either count.  This makes the
ring-local and event-global count transitions atomic with respect to
perf_mmap(), so a replacement cannot be installed until the old close has
detached its ring.

Fixes: 59741451b49c ("perf: Identify the 0->1 transition for event::mmap_count")
Bug found and triaged by OpenAI Security Research and 
validated by Trail of Bits.

Assisted-by: Codex:gpt-5.6-sol gpt-5.5-cyber
Signed-off-by: Kyle Zeng <kylebot@openai.com>
---
Trail of Bits has a reproducer for this bug that triggers a
kernel panic and can share if needed.

 kernel/events/core.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index ba5bd6a78..f93327c76 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7053,11 +7053,19 @@ static void perf_mmap_close(struct vm_area_struct *vma)
 		mutex_unlock(&rb->aux_mutex);
 	}
 
+	/*
+	 * Serialize both count updates with perf_mmap() so they cannot
+	 * refer to different ring buffer generations.
+	 */
+	mutex_lock(&event->mmap_mutex);
+
 	if (refcount_dec_and_test(&rb->mmap_count))
 		detach_rest = true;
 
-	if (!refcount_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex))
+	if (!refcount_dec_and_test(&event->mmap_count)) {
+		mutex_unlock(&event->mmap_mutex);
 		goto out_put;
+	}
 
 	ring_buffer_attach(event, NULL);
 	mutex_unlock(&event->mmap_mutex);
-- 
2.53.0