USB: serial: quatech2: Fix null-ptr-deref in qt2_process_read_urb()

This patch addresses a null-ptr-deref in qt2_process_read_urb() due to an incorrect bounds check in the following:

       if (newport > serial->num_ports) {
               dev_err(&port->dev,
                       "%s - port change to invalid port: %i\n",
                       __func__, newport);
               break;
       }

The condition doesn't account for the valid range of the serial->port buffer, which is from 0 to serial->num_ports - 1. When newport is equal to serial->num_ports, the assignment of "port" in the following code is out-of-bounds and NULL:

       serial_priv->current_port = newport;
       port = serial->port[serial_priv->current_port];

The fix checks if newport is greater than or equal to serial->num_ports indicating it is out-of-bounds.

---
 drivers/usb/serial/quatech2.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index a317bdbd00ad..72fe83a6c978 100644
@@ -503,7 +503,7 @@ static void qt2_process_read_urb(struct urb *urb)
 
 				newport = *(ch + 3);
 
-				if (newport > serial->num_ports) {
+				if (newport >= serial->num_ports) {
 					dev_err(&port->dev,
 						"%s - port change to invalid port: %i\n",
 						__func__, newport);

2.39.5

Thanks for the update. I've applied the patch now after adding Greg's Reviewed-by tag (for v2).

For your future contributions, try to remember to include any Reviewed-by or Tested-by tags when updating the patch unless the changes are non-trivial.

There should typically also be a short change log here under the --- line to indicate what changes from previous versions.

It is also encouraged to write the commit message in imperative mood (add, change, fix) and to avoid the phrase "this patch". There are some more details in

Documentation/process/submitting-patches.rst

Something to keep in mind for the future, but this patch already looks really good.

Johan