netfilter: connmark: ignore skbs with magic untracked conntrack objects

The (percpu) untracked conntrack entries can end up with nonzero connmarks.

The 'untracked' conntrack objects are merely a way to distinguish INVALID
(i.e. protocol connection tracker says payload doesn't meet some
requirements or packet was never seen by the connection tracking code)
from packets that are intentionally not tracked (some icmpv6 types such as
neigh solicitation, or by using 'iptables -j CT --notrack' option).

Untracked conntrack objects are implementation detail, we might as well use
invalid magic address instead to tell INVALID and UNTRACKED apart.

Check skb->nfct for untracked dummy and behave as if skb->nfct is NULL.
Reported-by: NXU Tianwen <evan.xu.tianwen@gmail.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_connmark.c

@@ -44,7 +44,7 @@ connmark_tg(struct sk_buff *skb, const struct xt_action_param *par)
 	u_int32_t newmark;
 
 	ct = nf_ct_get(skb, &ctinfo);
-	if (ct == NULL)
+	if (ct == NULL || nf_ct_is_untracked(ct))
 		return XT_CONTINUE;
 
 	switch (info->mode) {
@@ -97,7 +97,7 @@ connmark_mt(const struct sk_buff *skb, struct xt_action_param *par)
 	const struct nf_conn *ct;
 
 	ct = nf_ct_get(skb, &ctinfo);
-	if (ct == NULL)
+	if (ct == NULL || nf_ct_is_untracked(ct))
 		return false;
 
 	return ((ct->mark & info->mask) == info->mark) ^ info->invert;