io_uring/poll: fix multishot recv missing EOF on wakeup race

[ Upstream commit a68ed2d ]

When a socket send and shutdown() happen back-to-back, both fire
wake-ups before the receiver's task_work has a chance to run. The first
wake gets poll ownership (poll_refs=1), and the second bumps it to 2.
When io_poll_check_events() runs, it calls io_poll_issue() which does a
recv that reads the data and returns IOU_RETRY. The loop then drains all
accumulated refs (atomic_sub_return(2) -> 0) and exits, even though only
the first event was consumed. Since the shutdown is a persistent state
change, no further wakeups will happen, and the multishot recv can hang
forever.

Check specifically for HUP in the poll loop, and ensure that another
loop is done to check for status if more than a single poll activation
is pending. This ensures we don't lose the shutdown event.

Backport notes for linux-6.6.y:
  - In 6.6.y the do-while masks v in the while-condition itself
    (`atomic_sub_return(v & IO_POLL_REF_MASK, ...) & IO_POLL_REF_MASK`),
    so v can carry IO_POLL_RETRY_FLAG / IO_POLL_CANCEL_FLAG bits when
    we reach the multishot branch.  The HUP check therefore compares
    `(v & IO_POLL_REF_MASK) != 1` rather than the upstream
    `v != 1`, to avoid reacting to flag bits.
  - io_poll_issue takes `ts` (struct io_tw_state *) here.

CVE: CVE-2026-23473
Cc: stable@vger.kernel.org # 6.6.y
Fixes: dbc2564 ("io_uring: let fast poll support multishot")
Reported-by: Francis Brosseau <francis@malagauche.com>
Link: axboe/liburing#1549
Signed-off-by: Jens Axboe <axboe@kernel.dk>
[backport for linux-6.6.y, verified 2026-05-01]
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: axboe

io_uring/poll.c

@@ -321,7 +321,13 @@ static int io_poll_check_events(struct io_kiocb *req, struct io_tw_state *ts)
 				return IOU_POLL_REMOVE_POLL_USE_RES;
 			}
 		} else {
-			int ret = io_poll_issue(req, ts);
+			int ret;
+
+			/* multiple refs and HUP, ensure we loop once more */
+			if ((req->cqe.res & (POLLHUP | POLLRDHUP)) &&
+			    (v & IO_POLL_REF_MASK) != 1)
+				v--;
+			ret = io_poll_issue(req, ts);
 			if (ret == IOU_STOP_MULTISHOT)
 				return IOU_POLL_REMOVE_POLL_USE_RES;
 			else if (ret == IOU_REQUEUE)