netfilter: nf_tables: reject nat hook registration if prio is before conntrack

No problem for iptables as priorities are fixed values defined in the
nat modules, but in nftables the priority its coming from userspace.

Reject in case we see that such a hook would not work.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -1264,7 +1264,7 @@ static void nf_tables_chain_destroy(struct nft_chain *chain)
 
 struct nft_chain_hook {
 	u32				num;
-	u32				priority;
+	s32				priority;
 	const struct nf_chain_type	*type;
 	struct net_device		*dev;
 };
@@ -1303,6 +1303,11 @@ static int nft_chain_parse_hook(struct net *net,
 	}
 	if (!(type->hook_mask & (1 << hook->num)))
 		return -EOPNOTSUPP;
+
+	if (type->type == NFT_CHAIN_T_NAT &&
+	    hook->priority <= NF_IP_PRI_CONNTRACK)
+		return -EOPNOTSUPP;
+
 	if (!try_module_get(type->owner))
 		return -ENOENT;