[PATCH v2] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Siddh Raman Pant]

If not done, a reference to a freed pipe remains in the watch_queue,
as this function is called before freeing a pipe in free_pipe_info()
(see line 834 of fs/pipe.c).

This causes a UAF when post_one_notification tries to access the pipe on
a key update, which is reported by syzbot.

Bug report: https://syzkaller.appspot.com/bug?id=1870dd7791ba05f2ea7f47f7cbdde701173973fc
Reported-and-tested-by: syzbot+c70d87ac1d001f29a058 at syzkaller.appspotmail.com

Signed-off-by: Siddh Raman Pant <code at siddh.me>
---
Changes since v1:
- Removed the superfluous ifdef guard.

 kernel/watch_queue.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index bb9962b33f95..0357e5c6cf18 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -637,8 +637,15 @@ void watch_queue_clear(struct watch_queue *wqueue)
 		spin_lock_bh(&wqueue->lock);
 	}
 
-	spin_unlock_bh(&wqueue->lock);
 	rcu_read_unlock();
+
+	/* Clearing the watch queue, so we should clean the associated pipe. */
+	if (wqueue->pipe) {
+		wqueue->pipe->watch_queue = NULL;
+		wqueue->pipe = NULL;
+	}
+
+	spin_unlock_bh(&wqueue->lock);
 }
 
 /**
-- 
2.35.1