io_uring: fix lockup with timeouts

mainline inclusion
from mainline-5.6-rc5
commit f0e20b89
category: feature
bugzilla: https://bugzilla.openeuler.org/show_bug.cgi?id=27
CVE: NA
---------------------------

There is a recipe to deadlock the kernel: submit a timeout sqe with a
linked_timeout (e.g.  test_single_link_timeout_ception() from liburing),
and SIGKILL the process.

Then, io_kill_timeouts() takes @ctx->completion_lock, but the timeout
isn't flagged with REQ_F_COMP_LOCKED, and will try to double grab it
during io_put_free() to cancel the linked timeout. Probably, the same
can happen with another io_kill_timeout() call site, that is
io_commit_cqring().
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: Nyangerkun <yangerkun@huawei.com>
Reviewed-by: Nzhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: NCheng Jian <cj.chengjian@huawei.com>

fs/io_uring.c

@@ -994,6 +994,7 @@ static void io_kill_timeout(struct io_kiocb *req)
 	if (ret != -1) {
 		atomic_inc(&req->ctx->cq_timeouts);
 		list_del_init(&req->list);
+		req->flags |= REQ_F_COMP_LOCKED;
 		io_cqring_fill_event(req, 0);
 		io_put_req(req);
 	}