[tip: perf/urgent] perf/core: Fix a refcount leak in attach_perf_ctx_data()

[tip-bot2 for Namhyung Kim]

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     cca4980630b3c7a85f53cb43c6018184ce5d4e37
Gitweb:        https://git.kernel.org/tip/cca4980630b3c7a85f53cb43c6018184ce5d4e37
Author:        Namhyung Kim <namhyung@xxxxxxxxxx>
AuthorDate:    Sun, 20 Sep 2026 16:16:39 -07:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 23 Sep 2026 11:48:35 +02:00

perf/core: Fix a refcount leak in attach_perf_ctx_data()

The attach_perf_ctx_data() can race on global and !global cases.  The
global case is protected by global_ctx_data_rwsem and shares a single
reference count using perf_ctx_data.global field.

But when it races with !global case, it may miss to set the global field
and result in a reference count leak.

 CPU1                                  CPU2
 ----------------------------------------------------------------
 attach_task_ctx_data(.global=1)       attach_task_ctx_data(.global=0)
   cd1 = alloc_perf_ctx_data();          cd2 = alloc_perf_ctx_data();
                                         //    { .global = 0, .refcount = 1 };

                                         try_cmpxchg(); // success,
                                         // task->perf_ctx_data = cd2
   try_cmpxhg(); // fail; old = cd2
   refcount_inc_not_zero(&old->refcount); // success
     // old.refcount = 2
   free_perf_ctx_data(cd1);

Then later detach_global_ctx_data() will see the data but it's not
marked as global, so it won't call detach_task_ctx_data().

Fixes: 506e64e710ff ("perf: attach/detach PMU specific data")
Assisted-by: Sashiko.dev:Gemini-3.1-pro
Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/20260920231639.11910-1-namhyung@xxxxxxxxxx
---
 kernel/events/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index db7b76d..e180134 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5454,6 +5454,8 @@ attach_task_ctx_data(struct task_struct *task, struct kmem_cache *ctx_cache,
 		}
 
 		if (refcount_inc_not_zero(&old->refcount)) {
+			if (global)
+				old->global = true;
 			free_perf_ctx_data(cd); /* unused */
 			return 0;
 		}