[PATCH net v3 5/6] usbnet: ipheth: refactor NCM datagram loop, fix DPE OoB read

[Foster Snowhill <forst@xxxxxx>]

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.

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: a2d274c62e44 ("usbnet: ipheth: add CDC NCM support")
Signed-off-by: Foster Snowhill <forst@xxxxxx>
---
v3:
    Split out from a monolithic patch in v2 as an atomic change.
v2: https://lore.kernel.org/netdev/20240912211817.1707844-1-forst@xxxxxx/
    No code changes. Update commit message to further clarify that
    `ipheth` is not and does not aim to be a complete or spec-compliant
    CDC NCM implementation.
v1: https://lore.kernel.org/netdev/20240907230108.978355-1-forst@xxxxxx/
---
 drivers/net/usb/ipheth.c | 44 +++++++++++++++++++++-------------------
 1 file changed, 23 insertions(+), 21 deletions(-)

diff --git a/drivers/net/usb/ipheth.c b/drivers/net/usb/ipheth.c
index 3f9ea6546720..122557b5e5a9 100644
--- a/drivers/net/usb/ipheth.c
+++ b/drivers/net/usb/ipheth.c
@@ -224,9 +224,9 @@ static int ipheth_rcvbulk_callback_ncm(struct urb *urb)
 	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;
 
@@ -236,40 +236,42 @@ static int ipheth_rcvbulk_callback_ncm(struct urb *urb)
 	}
 
 	ncmh = urb->transfer_buffer;
-	if (ncmh->dwSignature != cpu_to_le32(USB_CDC_NCM_NTH16_SIGN)) {
-		dev->net->stats.rx_errors++;
-		return retval;
-	}
+	if (ncmh->dwSignature != cpu_to_le32(USB_CDC_NCM_NTH16_SIGN))
+		goto rx_error;
 
 	/* On iOS, NDP16 directly follows NTH16 */
 	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)) {
+	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);
+
+		/* 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)
-- 
2.45.1