netfilter: ip6t_SYNPROXY: fix NULL pointer dereference

This happens when networking namespaces are enabled.
Suggested-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NPhil Sutter <phil@nwl.cc>
Acked-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/ipv6/netfilter/ip6t_SYNPROXY.c

@@ -37,12 +37,13 @@ synproxy_build_ip(struct sk_buff *skb, const struct in6_addr *saddr,
 }
 
 static void
-synproxy_send_tcp(const struct sk_buff *skb, struct sk_buff *nskb,
+synproxy_send_tcp(const struct synproxy_net *snet,
+		  const struct sk_buff *skb, struct sk_buff *nskb,
 		  struct nf_conntrack *nfct, enum ip_conntrack_info ctinfo,
 		  struct ipv6hdr *niph, struct tcphdr *nth,
 		  unsigned int tcp_hdr_size)
 {
-	struct net *net = nf_ct_net((struct nf_conn *)nfct);
+	struct net *net = nf_ct_net(snet->tmpl);
 	struct dst_entry *dst;
 	struct flowi6 fl6;
 
@@ -83,7 +84,8 @@ synproxy_send_tcp(const struct sk_buff *skb, struct sk_buff *nskb,
 }
 
 static void
-synproxy_send_client_synack(const struct sk_buff *skb, const struct tcphdr *th,
+synproxy_send_client_synack(const struct synproxy_net *snet,
+			    const struct sk_buff *skb, const struct tcphdr *th,
 			    const struct synproxy_options *opts)
 {
 	struct sk_buff *nskb;
@@ -119,7 +121,7 @@ synproxy_send_client_synack(const struct sk_buff *skb, const struct tcphdr *th,
 
 	synproxy_build_options(nth, opts);
 
-	synproxy_send_tcp(skb, nskb, skb->nfct, IP_CT_ESTABLISHED_REPLY,
+	synproxy_send_tcp(snet, skb, nskb, skb->nfct, IP_CT_ESTABLISHED_REPLY,
 			  niph, nth, tcp_hdr_size);
 }
 
@@ -163,7 +165,7 @@ synproxy_send_server_syn(const struct synproxy_net *snet,
 
 	synproxy_build_options(nth, opts);
 
-	synproxy_send_tcp(skb, nskb, &snet->tmpl->ct_general, IP_CT_NEW,
+	synproxy_send_tcp(snet, skb, nskb, &snet->tmpl->ct_general, IP_CT_NEW,
 			  niph, nth, tcp_hdr_size);
 }
 
@@ -203,7 +205,7 @@ synproxy_send_server_ack(const struct synproxy_net *snet,
 
 	synproxy_build_options(nth, opts);
 
-	synproxy_send_tcp(skb, nskb, NULL, 0, niph, nth, tcp_hdr_size);
+	synproxy_send_tcp(snet, skb, nskb, NULL, 0, niph, nth, tcp_hdr_size);
 }
 
 static void
@@ -241,7 +243,7 @@ synproxy_send_client_ack(const struct synproxy_net *snet,
 
 	synproxy_build_options(nth, opts);
 
-	synproxy_send_tcp(skb, nskb, NULL, 0, niph, nth, tcp_hdr_size);
+	synproxy_send_tcp(snet, skb, nskb, NULL, 0, niph, nth, tcp_hdr_size);
 }
 
 static bool
@@ -301,7 +303,7 @@ synproxy_tg6(struct sk_buff *skb, const struct xt_action_param *par)
 					  XT_SYNPROXY_OPT_SACK_PERM |
 					  XT_SYNPROXY_OPT_ECN);
 
-		synproxy_send_client_synack(skb, th, &opts);
+		synproxy_send_client_synack(snet, skb, th, &opts);
 		return NF_DROP;
 
 	} else if (th->ack && !(th->fin || th->rst || th->syn)) {