[PATCH net-next 05/11] can: isotp: check the frame type, not just the length

From: Kaixuan Li <redacted>

isotp_rcv() separates Classic CAN from CAN FD by skb->len alone:

	if (skb->len != so->ll.mtu)
		return;

	cf = (struct canfd_frame *)skb->data;

A CAN XL frame with cxl->len 4 is CAN_MTU bytes, so it passes, and is then
read as a canfd_frame whose len comes out of canxl_frame.flags: at least
0x80.

Of the paths that follow, only the flow control one uses that length
without bounding it first, so check_pad() walks to 255 over a 16-byte
frame and the caller reports EBADMSG on an unrelated socket.

bcm_rx_handler(), j1939_can_recv(), can_can_gw_rcv() and raw_rcv() check
the frame type here, and can_dropped_invalid_skb() switches on
skb->protocol on the transmit side. isotp_rcv() is the gap.

Fixes: fb08cba12b52 ("can: canxl: update CAN infrastructure for CAN XL frames")
Signed-off-by: Kaixuan Li <redacted>
Reviewed-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Reviewed-by: Quchaosheng <redacted>
Link: https://patch.msgid.link/20260920035626.2581040-1-kaixuanli0131@gmail.com (local)
Cc: <redacted>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
---
 net/can/isotp.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/can/isotp.c b/net/can/isotp.c
index f5dc9d04bd68..d3e43ab48425 100644
--- a/net/can/isotp.c
+++ b/net/can/isotp.c

@@ -755,6 +755,14 @@ static void isotp_rcv(struct sk_buff *skb, void *data)
 	if (skb->len != so->ll.mtu)
 		return;
 
+	/* check for correct CAN CC/FD frame content */
+	if (so->ll.mtu == CAN_MTU) {
+		if (!can_is_can_skb(skb))
+			return;
+	} else if (!can_is_canfd_skb(skb)) {
+		return;
+	}
+
 	cf = (struct canfd_frame *)skb->data;
 
 	/* if enabled: check reception of my configured extended address */

-- 
2.53.0