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 Reported-by: syzbot+3b6b32dc50537a49bb4a@xxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx> --- fs/eventpoll.c | 3 ++- include/linux/eventpoll.h | 2 +- 2 files changed, 3 insertions(+), 2 deletions(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index f53ca4f7fced..fa766695f886 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -823,7 +823,8 @@ 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); diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h index 3337745d81bd..0c0d00fcd131 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; /* -- 2.45.2
