USB: xhci: dbc: fix tty registration race

Make sure to allocate resources before registering the tty device to
avoid having a racing open() and write() fail to enable rx or
dereference a NULL pointer when accessing the uninitialised fifo.

Fixes: dfba2174dc42 ("usb: xhci: Add DbC support in xHCI driver")
Cc: [e-mail] # 4.16
Cc: Lu Baolu <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>
Signed-off-by: Mathias Nyman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/host/xhci-dbgtty.c

···
 		return -EBUSY;
 
 	xhci_dbc_tty_init_port(dbc, port);
-	tty_dev = tty_port_register_device(&port->port,
-					   dbc_tty_driver, 0, NULL);
-	if (IS_ERR(tty_dev)) {
-		ret = PTR_ERR(tty_dev);
-		goto register_fail;
-	}
 
 	ret = kfifo_alloc(&port->write_fifo, DBC_WRITE_BUF_SIZE, GFP_KERNEL);
 	if (ret)
-		goto buf_alloc_fail;
+		goto err_exit_port;
 
 	ret = xhci_dbc_alloc_requests(dbc, BULK_IN, &port->read_pool,
 				      dbc_read_complete);
 	if (ret)
-		goto request_fail;
+		goto err_free_fifo;
 
 	ret = xhci_dbc_alloc_requests(dbc, BULK_OUT, &port->write_pool,
 				      dbc_write_complete);
 	if (ret)
-		goto request_fail;
+		goto err_free_requests;
+
+	tty_dev = tty_port_register_device(&port->port,
+					   dbc_tty_driver, 0, NULL);
+	if (IS_ERR(tty_dev)) {
+		ret = PTR_ERR(tty_dev);
+		goto err_free_requests;
+	}
 
 	port->registered = true;
 
 	return 0;
 
-request_fail:
+err_free_requests:
 	xhci_dbc_free_requests(&port->read_pool);
 	xhci_dbc_free_requests(&port->write_pool);
+err_free_fifo:
 	kfifo_free(&port->write_fifo);
-
-buf_alloc_fail:
-	tty_unregister_device(dbc_tty_driver, 0);
-
-register_fail:
+err_exit_port:
 	xhci_dbc_tty_exit_port(port);
 
 	dev_err(dbc->dev, "can't register tty port, err %d\n", ret);