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.

Reported-by: syzbot <[email protected]>
Closes: https://syzkaller.appspot.com/bug?extid=506479ebf12fe435d01a
Fixes: f7a33e6 ("USB: serial: add quatech2 usb to serial driver")
Cc: <[email protected]>      # 3.5
Signed-off-by: Qasim Ijaz <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Johan Hovold <[email protected]>

Author: qasim-ijaz

drivers/usb/serial/quatech2.c

@@ -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);