usbip: fix stub_send_ret_submit() vulnerability to null transfer_buffer

stub_send_ret_submit() handles urb with a potential null transfer_buffer,
when it replays a packet with potential malicious data that could contain
a null buffer. Add a check for the condition when actual_length > 0 and
transfer_buffer is null.
Reported-by: NSecunia Research <vuln@secunia.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/usbip/stub_tx.c

@@ -167,6 +167,13 @@ static int stub_send_ret_submit(struct stub_device *sdev)
 		memset(&pdu_header, 0, sizeof(pdu_header));
 		memset(&msg, 0, sizeof(msg));
 
+		if (urb->actual_length > 0 && !urb->transfer_buffer) {
+			dev_err(&sdev->udev->dev,
+				"urb: actual_length %d transfer_buffer null\n",
+				urb->actual_length);
+			return -1;
+		}
+
 		if (usb_pipetype(urb->pipe) == PIPE_ISOCHRONOUS)
 			iovnum = 2 + urb->number_of_packets;
 		else