[PATCH 1/3 v4] kernel/watch_queue: Remove dangling pipe reference 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).

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.

Signed-off-by: Siddh Raman Pant <code@xxxxxxxx>
---
Changes in v4:
- Brought the lines towards the start rather than the end.
- Removed incorrect NULLing of wqueue->pipe->watch_queue.
The latter was pointed out by Eric Biggers <ebiggers@xxxxxxxxxx>
in reply to v3.

Changes in v3:
- Restored the original unlock order, and clear before unlock.
- Used READ_ONCE() in post path.
This was explained by David Howells <dhowells@xxxxxxxxxx> in
reply to v1.

Changes in v2:
- Removed the superfluous ifdef guard.

 kernel/watch_queue.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index a6f9bdd956c3..8999c4e3076d 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;
@@ -606,6 +606,9 @@ void watch_queue_clear(struct watch_queue *wqueue)
 	/* Prevent new notifications from being stored. */
 	wqueue->defunct = true;
 
+	/* This pipe will get freed by caller, and we are anyways clearing. */
+	wqueue->pipe = NULL;
+
 	while (!hlist_empty(&wqueue->watches)) {
 		watch = hlist_entry(wqueue->watches.first, struct watch, queue_node);
 		hlist_del_init_rcu(&watch->queue_node);
-- 
2.35.1