CVE-2025-21689

In the Linux kernel, the following vulnerability has been resolved:

USB: serial: quatech2: fix null-ptr-deref in qt2processread_urb()

This patch addresses a null-ptr-deref in qt2processread_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->numports - 1. When newport is equal to serial->numports, 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.