This is a note to let you know that I've just added the patch titled
epoll: annotate racy check
to the 6.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
epoll-annotate-racy-check.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit fe93a1a27106b78dc9de1a6130245230cf441b00
Author: Christian Brauner <brauner@xxxxxxxxxx>
Date: Wed Sep 25 11:05:16 2024 +0200
epoll: annotate racy check
[ 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>
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 7221072f39fad..f296ffb57d052 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -703,7 +703,8 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)
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);
@@ -1467,7 +1468,8 @@ 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;
/*
