USB: serial/keyspan_pda, fix potential tty NULL dereferences

Make sure that we check the return value of tty_port_tty_get.
Sometimes it may return NULL and we later dereference that.

There are several places to check. For easier handling,
tty_port_tty_get is moved directly to the palce where needed in
keyspan_pda_rx_interrupt.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/keyspan_pda.c

@@ -173,6 +173,7 @@ static void keyspan_pda_wakeup_write(struct work_struct *work)
 		container_of(work, struct keyspan_pda_private, wakeup_work);
 	struct usb_serial_port *port = priv->port;
 	struct tty_struct *tty = tty_port_tty_get(&port->port);
+	if (tty)
 		tty_wakeup(tty);
 	tty_kref_put(tty);
 }
@@ -206,7 +207,7 @@ static void keyspan_pda_request_unthrottle(struct work_struct *work)
 static void keyspan_pda_rx_interrupt(struct urb *urb)
 {
 	struct usb_serial_port *port = urb->context;
-	struct tty_struct *tty = tty_port_tty_get(&port->port);
+	struct tty_struct *tty;
 	unsigned char *data = urb->transfer_buffer;
 	int retval;
 	int status = urb->status;
@@ -223,7 +224,7 @@ static void keyspan_pda_rx_interrupt(struct urb *urb)
 		/* this urb is terminated, clean up */
 		dbg("%s - urb shutting down with status: %d",
 		    __func__, status);
-		goto out;
+		return;
 	default:
 		dbg("%s - nonzero urb status received: %d",
 		    __func__, status);
@@ -233,12 +234,14 @@ static void keyspan_pda_rx_interrupt(struct urb *urb)
 	/* 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 (urb->actual_length) {
+		if (tty && urb->actual_length) {
 			tty_insert_flip_string(tty, data + 1,
 						urb->actual_length - 1);
 			tty_flip_buffer_push(tty);
 		}
+		tty_kref_put(tty);
 		break;
 	case 1:
 		/* status interrupt */
@@ -265,8 +268,6 @@ static void keyspan_pda_rx_interrupt(struct urb *urb)
 		dev_err(&port->dev,
 			"%s - usb_submit_urb failed with result %d",
 			__func__, retval);
-out:
-	tty_kref_put(tty);		     
 }