usbnet: ipheth: fix possible overflow in DPE length check

Originally, it was possible for the DPE length check to overflow if
wDatagramIndex + wDatagramLength > U16_MAX. This could lead to an OoB
read.

Move the wDatagramIndex term to the other side of the inequality.

An existing condition ensures that wDatagramIndex < urb->actual_length.

Fixes: a2d274c ("usbnet: ipheth: add CDC NCM support")
Cc: stable@vger.kernel.org
Signed-off-by: Foster Snowhill <forst@pen.gy>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Forst

drivers/net/usb/ipheth.c

@@ -243,8 +243,8 @@ static int ipheth_rcvbulk_callback_ncm(struct urb *urb)
 	while (le16_to_cpu(dpe->wDatagramIndex) != 0 &&
 	       le16_to_cpu(dpe->wDatagramLength) != 0) {
 		if (le16_to_cpu(dpe->wDatagramIndex) >= urb->actual_length ||
-		    le16_to_cpu(dpe->wDatagramIndex) +
-		    le16_to_cpu(dpe->wDatagramLength) > urb->actual_length) {
+		    le16_to_cpu(dpe->wDatagramLength) > urb->actual_length -
+		    le16_to_cpu(dpe->wDatagramIndex)) {
 			dev->net->stats.rx_length_errors++;
 			return retval;
 		}