[PATCH] perf: Fix mmap_count accounting on the perf_mmap_close() race path

[Aohan Mei <ljp1205831794@gmail.com>]

From: Aohan Mei <henrymei@tencent.com>

perf_mmap_rb()'s raced path (a concurrent perf_mmap_close() has dropped
rb->mmap_count to 0 but is still blocked on event->mmap_mutex inside
refcount_dec_and_mutex_lock()) installs a fresh ring buffer and then
does refcount_set(&event->mmap_count, 1).

That is wrong: the blocked closer still holds a pending decrement and
the count is 1 at this point. The refcount_set() leaves the count at 1,
so once perf_mmap() drops the mutex the closer observes a 1->0
transition, detaches the *new* buffer via ring_buffer_attach(event,
NULL) and drops its last reference, freeing pages that the racing
mmap() is about to map. A later munmap() of that mapping then finds
event->rb == NULL and crashes the kernel in perf_mmap_close().

Restore the pre-59741451b49c accounting on the raced path: the count is
guaranteed non-zero there, because the closer's decrement of
event->mmap_count only happens while holding mmap_mutex, which the
mapper holds. Use refcount_inc(). Keep refcount_set(..., 1) for the
genuine first mmap, where the 0->1 transition is required and
refcount_inc() would WARN.

Fixes: 59741451b49c ("perf: Identify the 0->1 transition for event::mmap_count")
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Cc: stable@vger.kernel.org
Assisted-by: CodeBuddy:Kimi-K3
Signed-off-by: Aohan Mei <henrymei@tencent.com>
---
 kernel/events/core.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4638544205f2..adcc04ca05f8 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7273,6 +7273,7 @@ static int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,
 	long extra = 0, user_extra = nr_pages;
 	struct perf_buffer *rb;
 	int rb_flags = 0;
+	bool raced = false;
 
 	nr_pages -= 1;
 
@@ -7314,6 +7315,7 @@ static int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,
 		 * event and continue as if !event->rb
 		 */
 		ring_buffer_attach(event, NULL);
+		raced = true;
 	}
 
 	if (!perf_mmap_calc_limits(vma, &user_extra, &extra))
@@ -7338,7 +7340,21 @@ static int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,
 	perf_event_update_userpage(event);
 
 	perf_mmap_account(vma, user_extra, extra);
-	refcount_set(&event->mmap_count, 1);
+
+	/*
+	 * On the raced path above, a concurrent perf_mmap_close() can
+	 * still have a pending decrement of event->mmap_count: it sits
+	 * blocked inside refcount_dec_and_mutex_lock() on event->mmap_mutex
+	 * (which we hold) with the count still at 1. Using
+	 * refcount_set(..., 1) here would make that closer observe a 1->0
+	 * transition once we drop the mutex, causing it to detach and free
+	 * the buffer we just installed, while this mmap() still maps it.
+	 * The count is guaranteed non-zero on the raced path, so increment.
+	 */
+	if (raced)
+		refcount_inc(&event->mmap_count);
+	else
+		refcount_set(&event->mmap_count, 1);
 
 	return 0;
 }
-- 
2.43.7