io_uring: sanitize double poll handling

There's a bit of confusion on the matching pairs of poll vs double poll,
depending on if the request is a pure poll (IORING_OP_POLL_ADD) or
poll driven retry.

Add io_poll_get_double() that returns the double poll waitqueue, if any,
and io_poll_get_single() that returns the original poll waitqueue. With
that, remove the argument to io_poll_remove_double().

Finally ensure that wait->private is cleared once the double poll handler
has run, so that remove knows it's already been seen.

Cc: stable@vger.kernel.org # v5.8
Reported-by: syzbot+7f617d4a9369028b8a2c@syzkaller.appspotmail.com
Fixes: 18bceab ("io_uring: allow POLL_ADD with double poll_wait() users")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -4649,9 +4649,24 @@ static bool io_poll_rewait(struct io_kiocb *req, struct io_poll_iocb *poll)
 	return false;
 }
 
-static void io_poll_remove_double(struct io_kiocb *req, void *data)
+static struct io_poll_iocb *io_poll_get_double(struct io_kiocb *req)
 {
-	struct io_poll_iocb *poll = data;
+	/* pure poll stashes this in ->io, poll driven retry elsewhere */
+	if (req->opcode == IORING_OP_POLL_ADD)
+		return (struct io_poll_iocb *) req->io;
+	return req->apoll->double_poll;
+}
+
+static struct io_poll_iocb *io_poll_get_single(struct io_kiocb *req)
+{
+	if (req->opcode == IORING_OP_POLL_ADD)
+		return &req->poll;
+	return &req->apoll->poll;
+}
+
+static void io_poll_remove_double(struct io_kiocb *req)
+{
+	struct io_poll_iocb *poll = io_poll_get_double(req);
 
 	lockdep_assert_held(&req->ctx->completion_lock);
 
@@ -4671,7 +4686,7 @@ static void io_poll_complete(struct io_kiocb *req, __poll_t mask, int error)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 
-	io_poll_remove_double(req, req->io);
+	io_poll_remove_double(req);
 	req->poll.done = true;
 	io_cqring_fill_event(req, error ? error : mangle_poll(mask));
 	io_commit_cqring(ctx);
@@ -4711,7 +4726,7 @@ static int io_poll_double_wake(struct wait_queue_entry *wait, unsigned mode,
 			       int sync, void *key)
 {
 	struct io_kiocb *req = wait->private;
-	struct io_poll_iocb *poll = req->apoll->double_poll;
+	struct io_poll_iocb *poll = io_poll_get_single(req);
 	__poll_t mask = key_to_poll(key);
 
 	/* for instances that support it check for an event match first: */
@@ -4725,6 +4740,8 @@ static int io_poll_double_wake(struct wait_queue_entry *wait, unsigned mode,
 		done = list_empty(&poll->wait.entry);
 		if (!done)
 			list_del_init(&poll->wait.entry);
+		/* make sure double remove sees this as being gone */
+		wait->private = NULL;
 		spin_unlock(&poll->head->lock);
 		if (!done)
 			__io_async_wake(req, poll, mask, io_poll_task_func);
@@ -4808,7 +4825,7 @@ static void io_async_task_func(struct callback_head *cb)
 	if (hash_hashed(&req->hash_node))
 		hash_del(&req->hash_node);
 
-	io_poll_remove_double(req, apoll->double_poll);
+	io_poll_remove_double(req);
 	spin_unlock_irq(&ctx->completion_lock);
 
 	if (!READ_ONCE(apoll->poll.canceled))
@@ -4919,7 +4936,7 @@ static bool io_arm_poll_handler(struct io_kiocb *req)
 	ret = __io_arm_poll_handler(req, &apoll->poll, &ipt, mask,
 					io_async_wake);
 	if (ret || ipt.error) {
-		io_poll_remove_double(req, apoll->double_poll);
+		io_poll_remove_double(req);
 		spin_unlock_irq(&ctx->completion_lock);
 		kfree(apoll->double_poll);
 		kfree(apoll);
@@ -4951,14 +4968,13 @@ static bool io_poll_remove_one(struct io_kiocb *req)
 {
 	bool do_complete;
 
+	io_poll_remove_double(req);
+
 	if (req->opcode == IORING_OP_POLL_ADD) {
-		io_poll_remove_double(req, req->io);
 		do_complete = __io_poll_remove_one(req, &req->poll);
 	} else {
 		struct async_poll *apoll = req->apoll;
 
-		io_poll_remove_double(req, apoll->double_poll);
-
 		/* non-poll requests have submit ref still */
 		do_complete = __io_poll_remove_one(req, &apoll->poll);
 		if (do_complete) {