netfilter: nf_conntrack_expect: bail out on insert dead expectations

If the NF_CT_EXPECT_DEAD expectation flag is set on, bail out on
insertion. Moreover, add also DEBUG_NET_WARN_ON_ONCE() since this should
not ever happen. This is hardening commit b8b09dc2bf ("netfilter:
nf_conntrack_expect: use conntrack GC to reap expectations").

Signed-off-by: Pablo Neira Ayuso <[email protected]>

net/netfilter/nf_conntrack_expect.c

@@ -528,6 +528,12 @@ int nf_ct_expect_related_report(struct nf_conntrack_expect *expect,
 	int ret;
 
 	spin_lock_bh(&nf_conntrack_expect_lock);
+	if (expect->flags & NF_CT_EXPECT_DEAD) {
+		DEBUG_NET_WARN_ON_ONCE(1);
+		ret = -EINVAL;
+		goto out;
+	}
+
 	master_help = nfct_help(expect->master);
 	if (!master_help) {
 		ret = -ESHUTDOWN;