Bluetooth: use hdev->workqueue when queuing hdev->{cmd,ncmd}_timer works

syzbot is reporting attempt to schedule hdev->cmd_work work from system_wq
WQ into hdev->workqueue WQ which is under draining operation [1], for
commit c8efcc2589464ac7 ("workqueue: allow chained queueing during
destruction") does not allow such operation.

The check introduced by commit 877afadad2dce8aa ("Bluetooth: When HCI work
queue is drained, only queue chained work") was incomplete.

Use hdev->workqueue WQ when queuing hdev->{cmd,ncmd}_timer works because
hci_{cmd,ncmd}_timeout() calls queue_work(hdev->workqueue). Also, protect
the queuing operation with RCU read lock in order to avoid calling
queue_delayed_work() after cancel_delayed_work() completed.

Link: https://syzkaller.appspot.com/bug?extid=243b7d89777f90f7613b [1]
Reported-by: syzbot <[e-mail]>
Signed-off-by: Tetsuo Handa <[e-mail]>
Fixes: 877afadad2dce8aa ("Bluetooth: When HCI work queue is drained, only queue chained work")
Signed-off-by: Luiz Augusto von Dentz <[e-mail]>

net/bluetooth/hci_core.c

···
 
 	/* Cancel these to avoid queueing non-chained pending work */
 	hci_dev_set_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE);
+	/* Wait for
+	 *
+	 *    if (!hci_dev_test_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE))
+	 *        queue_delayed_work(&hdev->{cmd,ncmd}_timer)
+	 *
+	 * inside RCU section to see the flag or complete scheduling.
+	 */
+	synchronize_rcu();
+	/* Explicitly cancel works in case scheduled after setting the flag. */
 	cancel_delayed_work(&hdev->cmd_timer);
 	cancel_delayed_work(&hdev->ncmd_timer);
 
···
 			if (res < 0)
 				__hci_cmd_sync_cancel(hdev, -res);
 
+			rcu_read_lock();
 			if (test_bit(HCI_RESET, &hdev->flags) ||
 			    hci_dev_test_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE))
 				cancel_delayed_work(&hdev->cmd_timer);
 			else
-				schedule_delayed_work(&hdev->cmd_timer,
-						      HCI_CMD_TIMEOUT);
+				queue_delayed_work(hdev->workqueue, &hdev->cmd_timer,
+						   HCI_CMD_TIMEOUT);
+			rcu_read_unlock();
 		} else {
 			skb_queue_head(&hdev->cmd_q, skb);
 			queue_work(hdev->workqueue, &hdev->cmd_work);

net/bluetooth/hci_event.c

···
 {
 	cancel_delayed_work(&hdev->cmd_timer);
 
+	rcu_read_lock();
 	if (!test_bit(HCI_RESET, &hdev->flags)) {
 		if (ncmd) {
 			cancel_delayed_work(&hdev->ncmd_timer);
 			atomic_set(&hdev->cmd_cnt, 1);
 		} else {
 			if (!hci_dev_test_flag(hdev, HCI_CMD_DRAIN_WORKQUEUE))
-				schedule_delayed_work(&hdev->ncmd_timer,
-						      HCI_NCMD_TIMEOUT);
+				queue_delayed_work(hdev->workqueue, &hdev->ncmd_timer,
+						   HCI_NCMD_TIMEOUT);
 		}
 	}
+	rcu_read_unlock();
 }
 
 static u8 hci_cc_le_read_buffer_size_v2(struct hci_dev *hdev, void *data,