io_uring/poll: don't reissue in case of poll race on multishot request

commit 8caa03f10bf92cb8657408a6ece6a8a73f96ce13 upstream.

A previous commit fixed a poll race that can occur, but it's only
applicable for multishot requests. For a multishot request, we can safely
ignore a spurious wakeup, as we never leave the waitqueue to begin with.

A blunt reissue of a multishot armed request can cause us to leak a
buffer, if they are ring provided. While this seems like a bug in itself,
it's not really defined behavior to reissue a multishot request directly.
It's less efficient to do so as well, and not required to rearm anything
like it is for singleshot poll requests.

Cc: stable@vger.kernel.org
Fixes: 6e5aedb9324a ("io_uring/poll: attempt request issue after racy poll wakeup")
Reported-and-tested-by: Olivier Langlois <olivier@trillion01.com>
Link: https://github.com/axboe/liburing/issues/778
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/io_uring/poll.c b/io_uring/poll.c
index f2f9f17..ab5ae47 100644 (file)
--- a/io_uring/poll.c
+++ b/io_uring/poll.c
@@ -281,8 +281,12 @@ static int io_poll_check_events(struct io_kiocb *req, bool *locked)
                         * to the waitqueue, so if we get nothing back, we
                         * should be safe and attempt a reissue.
                         */
-                       if (unlikely(!req->cqe.res))
+                       if (unlikely(!req->cqe.res)) {
+                               /* Multishot armed need not reissue */
+                               if (!(req->apoll_events & EPOLLONESHOT))
+                                       continue;
                                return IOU_POLL_REISSUE;
+                       }
                }
                if (req->apoll_events & EPOLLONESHOT)
                        return IOU_POLL_DONE;