[PATCH linux dev-6.12 01/14] net: mctp: don't use source cb data when forwarding, ensure pkt_type is set

[Marc Olberding]

From: Jeremy Kerr <jk at codeconstruct.com.au>

In the output path, only check the skb->cb data when we know it's from
a local socket; input packets will have source address information there
instead.

In order to detect when we're forwarding, set skb->pkt_type on
input/output.

Signed-off-by: Jeremy Kerr <jk at codeconstruct.com.au>
Link: https://patch.msgid.link/20250702-dev-forwarding-v5-1-1468191da8a4@codeconstruct.com.au
Signed-off-by: Paolo Abeni <pabeni at redhat.com>

(cherry picked from commit e0f3c79cc0bbf4925de1afde5abf148b7f7f4398)
---
 net/mctp/route.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/net/mctp/route.c b/net/mctp/route.c
index d9c8e5a5f9ce9aefbf16730c65a1f54caa5592b9..128ac46dda5eb882994960b8c0eb671007ad8583 100644
--- a/net/mctp/route.c
+++ b/net/mctp/route.c
@@ -392,6 +392,9 @@ static int mctp_route_input(struct mctp_route *route, struct sk_buff *skb)
 	 */
 	skb_orphan(skb);
 
+	if (skb->pkt_type == PACKET_OUTGOING)
+		skb->pkt_type = PACKET_LOOPBACK;
+
 	/* ensure we have enough data for a header and a type */
 	if (skb->len < sizeof(struct mctp_hdr) + 1)
 		goto out;
@@ -578,7 +581,13 @@ static int mctp_route_output(struct mctp_route *route, struct sk_buff *skb)
 		return -EMSGSIZE;
 	}
 
-	if (cb->ifindex) {
+	/* If we're forwarding, we don't want to use the input path's cb,
+	 * as it holds the *source* hardware addressing information.
+	 *
+	 * We will have a PACKET_HOST skb from the dev, or PACKET_OUTGOING
+	 * from a socket; only use cb in the latter case.
+	 */
+	if (skb->pkt_type == PACKET_OUTGOING && cb->ifindex) {
 		/* direct route; use the hwaddr we stashed in sendmsg */
 		if (cb->halen != skb->dev->addr_len) {
 			/* sanity check, sendmsg should have already caught this */
@@ -587,6 +596,7 @@ static int mctp_route_output(struct mctp_route *route, struct sk_buff *skb)
 		}
 		daddr = cb->haddr;
 	} else {
+		skb->pkt_type = PACKET_OUTGOING;
 		/* If lookup fails let the device handle daddr==NULL */
 		if (mctp_neigh_lookup(route->dev, hdr->dest, daddr_buf) == 0)
 			daddr = daddr_buf;
@@ -1032,6 +1042,7 @@ int mctp_local_output(struct sock *sk, struct mctp_route *rt,
 		tag = req_tag & MCTP_TAG_MASK;
 	}
 
+	skb->pkt_type = PACKET_OUTGOING;
 	skb->protocol = htons(ETH_P_MCTP);
 	skb->priority = 0;
 	skb_reset_transport_header(skb);

-- 
2.34.1