FAILED: patch "[PATCH] USB: xhci: dbc: fix tty registration race" failed to apply to 5.4-stable tree

[<gregkh@xxxxxxxxxxxxxxxxxxx>]

The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@xxxxxxxxxxxxxxx>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From 880de403777376e50bdf60def359fa50a722006f Mon Sep 17 00:00:00 2001
From: Johan Hovold <johan@xxxxxxxxxx>
Date: Fri, 8 Oct 2021 12:25:45 +0300
Subject: [PATCH] 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: stable@xxxxxxxxxxxxxxx      # 4.16
Cc: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
Signed-off-by: Johan Hovold <johan@xxxxxxxxxx>
Signed-off-by: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20211008092547.3996295-4-mathias.nyman@xxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

diff --git a/drivers/usb/host/xhci-dbgtty.c b/drivers/usb/host/xhci-dbgtty.c
index 6e784f2fc26d..eb46e642e87a 100644
--- a/drivers/usb/host/xhci-dbgtty.c
+++ b/drivers/usb/host/xhci-dbgtty.c
@@ -408,40 +408,38 @@ static int xhci_dbc_tty_register_device(struct xhci_dbc *dbc)
 		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);