From: Christian Brauner brauner@kernel.org
[ 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@google.com Link: https://lore.kernel.org/r/20240925-fungieren-anbauen-79b334b00542@brauner Reviewed-by: Jan Kara jack@suse.cz Reported-by: syzbot+3b6b32dc50537a49bb4a@syzkaller.appspotmail.com Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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;
/*