[PATCH v3] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Siddh Raman Pant <code@siddh.me>]

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.

We also need to use READ_ONCE() in post_one_notification() to prevent the
compiler from optimising and loading a non-NULL value from wqueue->pipe.

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

Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
Changes in v3:
- Restore the original unlock order, and clear before unlock.
- Use READ_ONCE() in post path.

This was explained by David Howells <dhowells@redhat.com> in
reply to v1. Not added Suggested-by since he didn't reply yet.

Changes in v2:
- Removed the superfluous ifdef guard.

 kernel/watch_queue.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index bb9962b33f95..617425e34252 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -99,7 +99,7 @@ static bool post_one_notification(struct watch_queue *wqueue,
 				  struct watch_notification *n)
 {
 	void *p;
-	struct pipe_inode_info *pipe = wqueue->pipe;
+	struct pipe_inode_info *pipe = READ_ONCE(wqueue->pipe);
 	struct pipe_buffer *buf;
 	struct page *page;
 	unsigned int head, tail, mask, note, offset, len;
@@ -637,6 +637,12 @@ void watch_queue_clear(struct watch_queue *wqueue)
 		spin_lock_bh(&wqueue->lock);
 	}
 
+	/* 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);
 	rcu_read_unlock();
 }
-- 
2.35.1