USB: serial: mos7840: fix another NULL-deref at open

commit 5182c2cf upstream.

Fix another NULL-pointer dereference at open should a malicious device
lack an interrupt-in endpoint.

Note that the driver has a broken check for an interrupt-in endpoint
which means that an interrupt URB has never even been submitted.

Fixes: 3f542974 ("USB: Moschip 7840 USB-Serial Driver")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/serial/mos7840.c

@@ -1058,6 +1058,7 @@ static int mos7840_open(struct tty_struct *tty, struct usb_serial_port *port)
 	 * (can't set it up in mos7840_startup as the structures *
 	 * were not set up at that time.)                        */
 	if (port0->open_ports == 1) {
+		/* FIXME: Buffer never NULL, so URB is not submitted. */
 		if (serial->port[0]->interrupt_in_buffer == NULL) {
 			/* set up interrupt urb */
 			usb_fill_int_urb(serial->port[0]->interrupt_in_urb,
@@ -2385,7 +2386,8 @@ static int mos7840_startup(struct usb_serial *serial)
 	}
 
 	if (serial->num_bulk_in < serial->num_ports ||
-			serial->num_bulk_out < serial->num_ports) {
+			serial->num_bulk_out < serial->num_ports ||
+			serial->num_interrupt_in < 1) {
 		dev_err(&serial->interface->dev, "missing endpoints\n");
 		return -ENODEV;
 	}