Unverified Commit 33e92e9e authored by Christian Brauner's avatar Christian Brauner
Browse files

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: default avatarChristian Brauner (Amutable) <brauner@kernel.org>
parent d30deeb8

include/linux/eventpoll.h

+10 −6
Original line number Diff line number Diff line
@@ -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;