[PATCH AUTOSEL 6.6 1/9] epoll: annotate racy check

[Sasha Levin]

From: Christian Brauner <brauner@xxxxxxxxxx>

[ Upstream commit 6474353a5e3d0b2cf610153cea0c61f576a36d0a ]

Epoll relies on a racy fastpath check during __fput() in
eventpoll_release() to avoid the hit of pointlessly acquiring a
semaphore. Annotate that race by using WRITE_ONCE() and READ_ONCE().

Link: https://lore.kernel.org/r/66edfb3c.050a0220.3195df.001a.GAE@xxxxxxxxxx
Link: https://lore.kernel.org/r/20240925-fungieren-anbauen-79b334b00542@brauner
Reviewed-by: Jan Kara <jack@xxxxxxx>
Reported-by: syzbot+3b6b32dc50537a49bb4a@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/eventpoll.c            | 6 ++++--
 include/linux/eventpoll.h | 2 +-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 0ed73bc7d4652..bcaad495930c3 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -741,7 +741,8 @@ static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
 	to_free = NULL;
 	head = file->f_ep;
 	if (head->first == &epi->fllink && !epi->fllink.next) {
-		file->f_ep = NULL;
+		/* See eventpoll_release() for details. */
+		WRITE_ONCE(file->f_ep, NULL);
 		if (!is_file_epoll(file)) {
 			struct epitems_head *v;
 			v = container_of(head, struct epitems_head, epitems);
@@ -1498,7 +1499,8 @@ static int attach_epitem(struct file *file, struct epitem *epi)
 			spin_unlock(&file->f_lock);
 			goto allocate;
 		}
-		file->f_ep = head;
+		/* See eventpoll_release() for details. */
+		WRITE_ONCE(file->f_ep, head);
 		to_free = NULL;
 	}
 	hlist_add_head_rcu(&epi->fllink, file->f_ep);
diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h
index 3337745d81bd6..0c0d00fcd131f 100644
--- a/include/linux/eventpoll.h
+++ b/include/linux/eventpoll.h
@@ -42,7 +42,7 @@ static inline void eventpoll_release(struct file *file)
 	 * because the file in on the way to be removed and nobody ( but
 	 * eventpoll ) has still a reference to this file.
 	 */
-	if (likely(!file->f_ep))
+	if (likely(!READ_ONCE(file->f_ep)))
 		return;
 
 	/*
-- 
2.43.0