USB: mct_u232: add sanity checking in probe

commit 4e9a0b05 upstream.

An attack using the lack of sanity checking in probe is known. This
patch checks for the existence of a second port.

CVE-2016-3136
Signed-off-by: Oliver Neukum <ONeukum@suse.com>
[johan: add error message ]
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/usb/serial/mct_u232.c

@@ -376,14 +376,21 @@ static void mct_u232_msr_to_state(struct usb_serial_port *port,
 
 static int mct_u232_port_probe(struct usb_serial_port *port)
 {
+	struct usb_serial *serial = port->serial;
 	struct mct_u232_private *priv;
 
+	/* check first to simplify error handling */
+	if (!serial->port[1] || !serial->port[1]->interrupt_in_urb) {
+		dev_err(&port->dev, "expected endpoint missing\n");
+		return -ENODEV;
+	}
+
 	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
 	if (!priv)
 		return -ENOMEM;
 
 	/* Use second interrupt-in endpoint for reading. */
-	priv->read_urb = port->serial->port[1]->interrupt_in_urb;
+	priv->read_urb = serial->port[1]->interrupt_in_urb;
 	priv->read_urb->context = port;
 
 	spin_lock_init(&priv->lock);