Bluetooth: btusb: Fix UAF of btusb_data by rx_work

btusb_close() and btusb_flush() cancel data->rx_work with the
asynchronous cancel_delayed_work(), so if btusb_rx_work() is already
running on another CPU it keeps running after the cancel returns.

btusb_disconnect() calls hci_unregister_dev(), which invokes
btusb_close(), and then frees the btusb_data. A still running
btusb_rx_work() then dereferences the freed data:

	while ((skb = skb_dequeue(&data->acl_q)))
		data->recv_acl(data->hdev, skb);

Use cancel_delayed_work_sync() instead. In btusb_close() the cancel also
has to happen after btusb_stop_traffic(), otherwise an URB completion
racing with the cancel can requeue the work right after it has been
waited for.

Fixes: 800fe5ec302e ("Bluetooth: btusb: Add support for queuing during polling interval")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

drivers/bluetooth/btusb.c

diff --git a/drivers/bluetooth/btusb.c b/drivers/bluetooth/btusb.c
index 03039ce..b429634 100644
--- a/drivers/bluetooth/btusb.c
+++ b/drivers/bluetooth/btusb.c
@@ -2096,18 +2096,24 @@ static int btusb_close(struct hci_dev *hdev)
 
 	BT_DBG("%s", hdev->name);
 
-	cancel_delayed_work(&data->rx_work);
 	cancel_work_sync(&data->work);
 	cancel_work_sync(&data->waker);
 
-	skb_queue_purge(&data->acl_q);
-
 	clear_bit(BTUSB_ISOC_RUNNING, &data->flags);
 	clear_bit(BTUSB_BULK_RUNNING, &data->flags);
 	clear_bit(BTUSB_INTR_RUNNING, &data->flags);
 	clear_bit(BTUSB_DIAG_RUNNING, &data->flags);
 
 	btusb_stop_traffic(data);
+
+	/* rx_work must only be canceled once the URBs that can rearm it are
+	 * gone, and it must be canceled synchronously since btusb_disconnect()
+	 * frees the btusb_data it dereferences right after hci_unregister_dev().
+	 */
+	cancel_delayed_work_sync(&data->rx_work);
+
+	skb_queue_purge(&data->acl_q);
+
 	btusb_free_frags(data);
 
 	err = usb_autopm_get_interface(data->intf);
@@ -2133,7 +2139,7 @@ static int btusb_flush(struct hci_dev *hdev)
 
 	BT_DBG("%s", hdev->name);
 
-	cancel_delayed_work(&data->rx_work);
+	cancel_delayed_work_sync(&data->rx_work);
 
 	skb_queue_purge(&data->acl_q);