usb: gadgetfs: restrict upper bound on device configuration size

Andrey Konovalov reported that we were not properly checking the upper
limit before of a device configuration size before calling
memdup_user(), which could cause some problems.

So set the upper limit to PAGE_SIZE * 4, which should be good enough for
all devices.
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/legacy/inode.c

@@ -1762,7 +1762,8 @@ dev_config (struct file *fd, const char __user *buf, size_t len, loff_t *ptr)
 	}
 	spin_unlock_irq(&dev->lock);
 
-	if (len < (USB_DT_CONFIG_SIZE + USB_DT_DEVICE_SIZE + 4))
+	if ((len < (USB_DT_CONFIG_SIZE + USB_DT_DEVICE_SIZE + 4)) ||
+	    (len > PAGE_SIZE * 4))
 		return -EINVAL;
 
 	/* we might need to change message format someday */