USB: serial: iuu_phoenix: fix memory corruption

The driver would happily overwrite its write buffer with user data in
256 byte increments due to a removed buffer-space sanity check.

Fixes: 5fcf62b ("tty: iuu_phoenix: fix locking.")
Cc: stable <stable@vger.kernel.org>     # 2.6.31
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/iuu_phoenix.c

@@ -697,14 +697,16 @@ static int iuu_uart_write(struct tty_struct *tty, struct usb_serial_port *port,
 	struct iuu_private *priv = usb_get_serial_port_data(port);
 	unsigned long flags;
 
-	if (count > 256)
-		return -ENOMEM;
-
 	spin_lock_irqsave(&priv->lock, flags);
 
+	count = min(count, 256 - priv->writelen);
+	if (count == 0)
+		goto out;
+
 	/* fill the buffer */
 	memcpy(priv->writebuf + priv->writelen, buf, count);
 	priv->writelen += count;
+out:
 	spin_unlock_irqrestore(&priv->lock, flags);
 
 	return count;