usbnet: ipheth: fix DPE OoB read

Fix an out-of-bounds DPE read, limit the number of processed DPEs to
the amount that fits into the fixed-size NDP16 header.

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>

drivers/net/usb/ipheth.c

@@ -246,7 +246,7 @@ static int ipheth_rcvbulk_callback_ncm(struct urb *urb)
 		goto rx_error;
 
 	dpe = ncm0->dpe16;
-	while (true) {
+	for (int dpe_i = 0; dpe_i < IPHETH_NDP16_MAX_DPE; ++dpe_i, ++dpe) {
 		dg_idx = le16_to_cpu(dpe->wDatagramIndex);
 		dg_len = le16_to_cpu(dpe->wDatagramLength);
 
@@ -268,8 +268,6 @@ static int ipheth_rcvbulk_callback_ncm(struct urb *urb)
 		retval = ipheth_consume_skb(buf, dg_len, dev);
 		if (retval != 0)
 			return retval;
-
-		dpe++;
 	}
 
 rx_error: