net: mctp: copy skb ext data when fragmenting

If we're fragmenting on local output, the original packet may contain
ext data for the MCTP flows. We'll want this in the resulting fragment
skbs too.

So, do a skb_ext_copy() in the fragmentation path, and implement the
MCTP-specific parts of an ext copy operation.

Fixes: 67737c45 ("mctp: Pass flow data & flow release events to drivers")
Reported-by: Jian Zhang <zhangjian.3032@bytedance.com>
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/core/skbuff.c

@@ -6849,6 +6849,14 @@ static struct skb_ext *skb_ext_maybe_cow(struct skb_ext *old,
 		for (i = 0; i < sp->len; i++)
 			xfrm_state_hold(sp->xvec[i]);
 	}
+#endif
+#ifdef CONFIG_MCTP_FLOWS
+	if (old_active & (1 << SKB_EXT_MCTP)) {
+		struct mctp_flow *flow = skb_ext_get_ptr(old, SKB_EXT_MCTP);
+
+		if (flow->key)
+			refcount_inc(&flow->key->refs);
+	}
 #endif
 	__skb_ext_put(old);
 	return new;

net/mctp/route.c

@@ -905,6 +905,9 @@ static int mctp_do_fragment_route(struct mctp_route *rt, struct sk_buff *skb,
 		/* copy message payload */
 		skb_copy_bits(skb, pos, skb_transport_header(skb2), size);
 
+		/* we need to copy the extensions, for MCTP flow data */
+		skb_ext_copy(skb2, skb);
+
 		/* do route */
 		rc = rt->output(rt, skb2);
 		if (rc)