usbnet: ipheth: check that DPE points past NCM header

By definition, a DPE points at the start of a network frame/datagram.
Thus it makes no sense for it to point at anything that's part of the
NCM header. It is not a security issue, but merely an indication of
a malformed DPE.

Enforce that all DPEs point at the data portion of the URB, past the
NCM header.

Fixes: a2d274c62e44 ("usbnet: ipheth: add CDC NCM support")
Cc: [e-mail]
Signed-off-by: Foster Snowhill <[e-mail]>
Reviewed-by: Jakub Kicinski <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>

drivers/net/usb/ipheth.c

···
 	dpe = ncm0->dpe16;
 	while (le16_to_cpu(dpe->wDatagramIndex) != 0 &&
 	       le16_to_cpu(dpe->wDatagramLength) != 0) {
-		if (le16_to_cpu(dpe->wDatagramIndex) >= urb->actual_length ||
+		if (le16_to_cpu(dpe->wDatagramIndex) < IPHETH_NCM_HEADER_SIZE ||
+		    le16_to_cpu(dpe->wDatagramIndex) >= urb->actual_length ||
 		    le16_to_cpu(dpe->wDatagramLength) > urb->actual_length -
 		    le16_to_cpu(dpe->wDatagramIndex)) {
 			dev->net->stats.rx_length_errors++;