usbnet: ipheth: refactor NCM datagram loop

Introduce an rx_error label to reduce repetitions in the header
signature checks.

Store wDatagramIndex and wDatagramLength after endianness conversion to
avoid repeated le16_to_cpu() calls.

Rewrite the loop to return on a null trailing DPE, which is required
by the CDC NCM spec. In case it is missing, fall through to rx_error.

This change does not fix any particular issue. Its purpose is to
simplify a subsequent commit that fixes a potential OoB read by limiting
the maximum amount of processed DPEs.

Cc: [e-mail] # 6.5.x
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

···
 	struct usb_cdc_ncm_ndp16 *ncm0;
 	struct usb_cdc_ncm_dpe16 *dpe;
 	struct ipheth_device *dev;
+	u16 dg_idx, dg_len;
 	int retval = -EINVAL;
 	char *buf;
-	int len;
 
 	dev = urb->context;
 
···
 	ncmh = urb->transfer_buffer;
 	if (ncmh->dwSignature != cpu_to_le32(USB_CDC_NCM_NTH16_SIGN) ||
 	    /* On iOS, NDP16 directly follows NTH16 */
-	    ncmh->wNdpIndex != cpu_to_le16(sizeof(struct usb_cdc_ncm_nth16))) {
-		dev->net->stats.rx_errors++;
-		return retval;
-	}
+	    ncmh->wNdpIndex != cpu_to_le16(sizeof(struct usb_cdc_ncm_nth16)))
+		goto rx_error;
 
 	ncm0 = urb->transfer_buffer + sizeof(struct usb_cdc_ncm_nth16);
-	if (ncm0->dwSignature != cpu_to_le32(USB_CDC_NCM_NDP16_NOCRC_SIGN)) {
-		dev->net->stats.rx_errors++;
-		return retval;
-	}
+	if (ncm0->dwSignature != cpu_to_le32(USB_CDC_NCM_NDP16_NOCRC_SIGN))
+		goto rx_error;
 
 	dpe = ncm0->dpe16;
-	while (le16_to_cpu(dpe->wDatagramIndex) != 0 &&
-	       le16_to_cpu(dpe->wDatagramLength) != 0) {
-		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)) {
+	while (true) {
+		dg_idx = le16_to_cpu(dpe->wDatagramIndex);
+		dg_len = le16_to_cpu(dpe->wDatagramLength);
+
+		/* Null DPE must be present after last datagram pointer entry
+		 * (3.3.1 USB CDC NCM spec v1.0)
+		 */
+		if (dg_idx == 0 && dg_len == 0)
+			return 0;
+
+		if (dg_idx < IPHETH_NCM_HEADER_SIZE ||
+		    dg_idx >= urb->actual_length ||
+		    dg_len > urb->actual_length - dg_idx) {
 			dev->net->stats.rx_length_errors++;
 			return retval;
 		}
 
-		buf = urb->transfer_buffer + le16_to_cpu(dpe->wDatagramIndex);
-		len = le16_to_cpu(dpe->wDatagramLength);
+		buf = urb->transfer_buffer + dg_idx;
 
-		retval = ipheth_consume_skb(buf, len, dev);
+		retval = ipheth_consume_skb(buf, dg_len, dev);
 		if (retval != 0)
 			return retval;
 
 		dpe++;
 	}
 
-	return 0;
+rx_error:
+	dev->net->stats.rx_errors++;
+	return retval;
 }
 
 static void ipheth_rcvbulk_callback(struct urb *urb)