USB: serial: keyspan_pda: fix receive sanity checks

commit c528fcb1 upstream.

Make sure to check for short transfers before parsing the receive buffer
to avoid acting on stale data.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2:
 - Adjust context
 - Keep the check for !tty in the data case]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/serial/keyspan_pda.c

@@ -212,6 +212,7 @@ static void keyspan_pda_rx_interrupt(struct urb *urb)
 	struct usb_serial_port *port = urb->context;
 	struct tty_struct *tty;
 	unsigned char *data = urb->transfer_buffer;
+	unsigned int len = urb->actual_length;
 	int retval;
 	int status = urb->status;
 	struct keyspan_pda_private *priv;
@@ -234,20 +235,28 @@ static void keyspan_pda_rx_interrupt(struct urb *urb)
 		goto exit;
 	}
 
+	if (len < 1) {
+		dev_warn(&port->dev, "short message received\n");
+		goto exit;
+	}
+
 	/* see if the message is data or a status interrupt */
 	switch (data[0]) {
 	case 0:
 		tty = tty_port_tty_get(&port->port);
 		 /* rest of message is rx data */
-		if (tty && urb->actual_length) {
-			tty_insert_flip_string(tty, data + 1,
-						urb->actual_length - 1);
+		if (!tty || len < 2)
+			break;
+		tty_insert_flip_string(tty, data + 1, len - 1);
 		tty_flip_buffer_push(tty);
-		}
 		tty_kref_put(tty);
 		break;
 	case 1:
 		/* status interrupt */
+		if (len < 3) {
+			dev_warn(&port->dev, "short interrupt message received\n");
+			break;
+		}
 		dbg(" rx int, d1=%d, d2=%d", data[1], data[2]);
 		switch (data[1]) {
 		case 1: /* modemline change */