io_uring/net: fix sendzc double notif flush

refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 5823 at lib/refcount.c:28 refcount_warn_saturate+0x15a/0x1d0 lib/refcount.c:28
RIP: 0010:refcount_warn_saturate+0x15a/0x1d0 lib/refcount.c:28
Call Trace:
 <TASK>
 io_notif_flush io_uring/notif.h:40 [inline]
 io_send_zc_cleanup+0x121/0x170 io_uring/net.c:1222
 io_clean_op+0x58c/0x9a0 io_uring/io_uring.c:406
 io_free_batch_list io_uring/io_uring.c:1429 [inline]
 __io_submit_flush_completions+0xc16/0xd20 io_uring/io_uring.c:1470
 io_submit_flush_completions io_uring/io_uring.h:159 [inline]

Before the blamed commit, sendzc relied on io_req_msg_cleanup() to clear
REQ_F_NEED_CLEANUP, so after the following snippet the request will
never hit the core io_uring cleanup path.

io_notif_flush();
io_req_msg_cleanup();

The easiest fix is to null the notification. io_send_zc_cleanup() can
still be called after, but it's tolerated.

Reported-by: syzbot+cf285a028ffba71b2ef5@syzkaller.appspotmail.com
Tested-by: syzbot+cf285a028ffba71b2ef5@syzkaller.appspotmail.com
Fixes: cc34d83 ("io_uring/net: don't clear REQ_F_NEED_CLEANUP unconditionally")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/e1306007458b8891c88c4f20c966a17595f766b0.1742643795.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/net.c

@@ -1440,6 +1440,7 @@ int io_send_zc(struct io_kiocb *req, unsigned int issue_flags)
 	 */
 	if (!(issue_flags & IO_URING_F_UNLOCKED)) {
 		io_notif_flush(zc->notif);
+		zc->notif = NULL;
 		io_req_msg_cleanup(req, 0);
 	}
 	io_req_set_res(req, ret, IORING_CQE_F_MORE);
@@ -1500,6 +1501,7 @@ int io_sendmsg_zc(struct io_kiocb *req, unsigned int issue_flags)
 	 */
 	if (!(issue_flags & IO_URING_F_UNLOCKED)) {
 		io_notif_flush(sr->notif);
+		sr->notif = NULL;
 		io_req_msg_cleanup(req, 0);
 	}
 	io_req_set_res(req, ret, IORING_CQE_F_MORE);