FAILED: patch "[PATCH] aio: prevent eventfd/epoll recursion on poll" failed to apply to 5.10-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x 398e3fa789aa5ffddc8689d8cbc7b31e1824130c
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107061424-failed-5.10-398e3fa789aa@kernel.org' --subject-prefix 'PATCH 5.10.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 398e3fa789aa5ffddc8689d8cbc7b31e1824130c Mon Sep 17 00:00:00 2001
From: Daehyeon Ko <4ncienth@gmail.com>
Date: Thu, 13 Aug 2026 22:38:43 +0900
Subject: [PATCH] aio: prevent eventfd/epoll recursion on poll

aio_poll_wake() can complete a keyed poll request inline while the
provider waitqueue lock is held.  If the polled file is an epoll instance
and the AIO result eventfd is watched by that same instance, signaling the
result can feed back into epoll and try to take the provider waitqueue lock
again.

For example, a timerfd wake can follow this path:

  timerfd -> ep_poll_callback -> aio_poll_wake -> aio_complete
          -> eventfd_signal -> ep_poll_callback -> ep_poll_safewake

The second ep_poll_safewake() recursively acquires ep->poll_wait.lock.
eventfd_signal_allowed() does not prevent this because the wake chain did
not begin in eventfd, so the task's eventfd recursion bit is still clear
when aio_poll_wake() decides to complete inline.

An unprivileged process can construct this graph and make a CPU spin on the
recursive lock.  DEBUG_SPINLOCK reports "BUG: spinlock recursion", and
lockdep reports the same epoll waitqueue lock as both held and requested.

Always defer eventfd-backed poll completions through the existing
aio_poll_put_work() path.  At this point the request has already been
detached from the waitqueue and active request list, so the work item can
publish the completion after the provider callback releases its lock.
Poll completions without a result eventfd remain inline.

Fixes: e8693bcfa0b4 ("aio: allow direct aio poll comletions for keyed wakeups")
Cc: stable@vger.kernel.org
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Link: https://patch.msgid.link/20260813133843.2933127-1-4ncienth@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/aio.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/aio.c b/fs/aio.c
index d78acc69f4870..92241a24d9e28 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1869,7 +1869,12 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
 		list_del_init(&req->wait.entry);
 		list_del(&iocb->ki_list);
 		iocb->ki_res.res = mangle_poll(mask);
-		if (iocb->ki_eventfd && !eventfd_signal_allowed()) {
+		/*
+		 * We hold an arbitrary provider waitqueue lock here.  Signaling a
+		 * result eventfd can feed back through epoll and try to take the same
+		 * lock again.  Defer all eventfd-backed poll completions.
+		 */
+		if (iocb->ki_eventfd) {
 			iocb = NULL;
 			INIT_WORK(&req->work, aio_poll_put_work);
 			schedule_work(&req->work);
-- 
2.53.0