netfilter: nftables: Do not run chains in the wrong network namespace

Currenlty nf_tables chains added in one network namespace are being
run in all network namespace.  The issues are myriad with the simplest
being an unprivileged user can cause any network packets to be dropped.

Address this by simply not running nf_tables chains in the wrong
network namespace.

Cc: stable@vger.kernel.org
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: NPablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netfilter/nf_tables_core.c

@@ -114,7 +114,8 @@ unsigned int
 nft_do_chain(struct nft_pktinfo *pkt, const struct nf_hook_ops *ops)
 {
 	const struct nft_chain *chain = ops->priv, *basechain = chain;
-	const struct net *net = read_pnet(&nft_base_chain(basechain)->pnet);
+	const struct net *chain_net = read_pnet(&nft_base_chain(basechain)->pnet);
+	const struct net *net = dev_net(pkt->in ? pkt->in : pkt->out);
 	const struct nft_rule *rule;
 	const struct nft_expr *expr, *last;
 	struct nft_regs regs;
@@ -124,6 +125,10 @@ nft_do_chain(struct nft_pktinfo *pkt, const struct nf_hook_ops *ops)
 	int rulenum;
 	unsigned int gencursor = nft_genmask_cur(net);
 
+	/* Ignore chains that are not for the current network namespace */
+	if (!net_eq(net, chain_net))
+		return NF_ACCEPT;
+
 do_chain:
 	rulenum = 0;
 	rule = list_entry(&chain->rules, struct nft_rule, list);