From 33e92e9ecf48c08cb4807e9a36f9eb01619c1a1e Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Thu, 23 Apr 2026 11:56:11 +0200 Subject: eventpoll: refresh eventpoll_release() fast-path comment The old comment justified the lockless READ_ONCE(file->f_ep) check with "False positives simply cannot happen because the file is on the way to be removed and nobody ( but eventpoll ) has still a reference to this file." That reasoning was the root of the UAF fixed in "eventpoll: fix ep_remove struct eventpoll / struct file UAF": __ep_remove() could clear f_ep while another close raced past the fast path and freed the watched eventpoll / recycled the struct file slot. With ep_remove() now pinning @file via epi_fget() across the f_ep clear and hlist_del_rcu(), the invariant is re-established for the right reason: anyone who might clear f_ep holds @file alive for the duration, so a NULL observation really does mean no concurrent eventpoll path has work left on this file. Refresh the comment accordingly so the next reader doesn't inherit the broken model. Link: https://patch.msgid.link/20260423-work-epoll-uaf-v1-8-2470f9eec0f5@kernel.org Signed-off-by: Christian Brauner (Amutable) --- include/linux/eventpoll.h | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) (limited to 'include/linux/eventpoll.h') diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h index ea9ca0e4172a..728fb5dee5ed 100644 --- a/include/linux/eventpoll.h +++ b/include/linux/eventpoll.h @@ -39,12 +39,16 @@ static inline void eventpoll_release(struct file *file) { /* - * Fast check to avoid the get/release of the semaphore. Since - * we're doing this outside the semaphore lock, it might return - * false negatives, but we don't care. It'll help in 99.99% of cases - * to avoid the semaphore lock. False positives simply cannot happen - * because the file in on the way to be removed and nobody ( but - * eventpoll ) has still a reference to this file. + * Fast check to skip the slow path in the common case where the + * file was never attached to an epoll. Safe without file->f_lock + * because every f_ep writer excludes a concurrent __fput() on + * @file: + * - ep_insert() requires the file alive (refcount > 0); + * - ep_remove() holds @file pinned via epi_fget() across the + * write; + * - eventpoll_release_file() runs from __fput() itself. + * We are in __fput() here, so none of those can race us: a NULL + * observation truly means no epoll path has work left on @file. */ if (likely(!READ_ONCE(file->f_ep))) return; -- cgit v1.2.3