[PATCH v2] can: isotp: check the frame type, not just the length

[Kaixuan Li <kaixuanli0131@gmail.com>]

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 <kaixuanli0131@gmail.com>
Reviewed-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
---
v2: shorten the comment above the new check to say what it does; the
    reasoning stays in the description (Oliver Hartkopp). Add Oliver's
    Reviewed-by and Acked-by. No code change.

v1: https://lore.kernel.org/linux-can/20260919122852.1868961-1-kaixuanli0131@gmail.com/

Reproduced on v7.2.4 over vcan, one isotp socket per case bound rx 0x123
with RX_PADDING|CHK_PAD_DATA and rxpad_content 0xAA, a first frame in
flight, and one frame injected from a CAN_RAW socket.

	case                            stock     patched
	A CAN XL, cxl->len 4, flags ff  EBADMSG   none
	B Classic FC, padded 0xAA       none      none
	C Classic FC, padded 0x00       EBADMSG   EBADMSG
	D as A, with CHK_PAD_LEN on     EBADMSG   none

C bounds the impact: a malformed Classic FC frame from any sender on the
bus gives the same EBADMSG, so nothing becomes reachable that was not
already. D differs only in which branch of check_pad() returns.

No memory safety issue. KASAN was on for all eight runs and reported
nothing.
---
 net/can/isotp.c | 8 ++++++++
 1 file changed, 8 insertions(+)

--- a/net/can/isotp.c
+++ b/net/can/isotp.c
@@ -754,8 +754,16 @@ 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 */
 	if (ae && cf->data[0] != so->opt.rx_ext_address)