[PATCH 1/1] ipc/shm: serialize orphan cleanup with shm_nattch updates

[Ren Wei]

From: Yilin Zhu <zylzyl2333@xxxxxxxxx>

shm_destroy_orphaned() walks the shm idr under shm_ids(ns).rwsem, but
that does not serialize all fields tested by shm_may_destroy().  In
particular, shm_nattch is updated while holding shm_perm.lock, and attach
paths can do that without holding the rwsem.

Do not decide that an orphaned segment is unused before taking the object
lock.  Move the shm_may_destroy() check under shm_perm.lock, matching the
other destroy paths, and unlock the segment when it no longer qualifies
for removal.

Fixes: 4c677e2eefdb ("shm: optimize locking and ipc_namespace getting")
Cc: stable@xxxxxxxxxx
Reported-by: Yuan Tan <yuantan098@xxxxxxxxx>
Reported-by: Yifan Wu <yifanwucs@xxxxxxxxx>
Reported-by: Juefei Pu <tomapufckgml@xxxxxxxxx>
Reported-by: Xin Liu <bird@xxxxxxxxxx>
Signed-off-by: Yilin Zhu <zylzyl2333@xxxxxxxxx>
Signed-off-by: Ren Wei <n05ec@xxxxxxxxxx>
---
 ipc/shm.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index a95dae447707..b3e8a58e177d 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -418,15 +418,17 @@ static int shm_try_destroy_orphaned(int id, void *p, void *data)
 	 * We want to destroy segments without users and with already
 	 * exit'ed originating process.
 	 *
-	 * As shp->* are changed under rwsem, it's safe to skip shp locking.
+	 * shm_nattch can be changed under shm_perm.lock without holding the
+	 * rwsem, so take the object lock before checking shm_may_destroy().
 	 */
 	if (!list_empty(&shp->shm_clist))
 		return 0;
 
-	if (shm_may_destroy(shp)) {
-		shm_lock_by_ptr(shp);
+	shm_lock_by_ptr(shp);
+	if (shm_may_destroy(shp))
 		shm_destroy(ns, shp);
-	}
+	else
+		shm_unlock(shp);
 	return 0;
 }
 
-- 
2.47.3