netfilter: synproxy: fix erroneous tcp mss option

Now synproxy sends the mss value set by the user on client syn-ack packet
instead of the mss value that client announced.

Fixes: 48b1de4c ("netfilter: add SYNPROXY core/target")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_conntrack_synproxy.h

@@ -68,6 +68,7 @@ struct synproxy_options {
 	u8				options;
 	u8				wscale;
 	u16				mss;
+	u16				mss_encode;
 	u32				tsval;
 	u32				tsecr;
 };

net/ipv4/netfilter/ipt_SYNPROXY.c

@@ -36,6 +36,8 @@ synproxy_tg4(struct sk_buff *skb, const struct xt_action_param *par)
 			opts.options |= XT_SYNPROXY_OPT_ECN;
 
 		opts.options &= info->options;
+		opts.mss_encode = opts.mss;
+		opts.mss = info->mss;
 		if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
 			synproxy_init_timestamp_cookie(info, &opts);
 		else

net/ipv6/netfilter/ip6t_SYNPROXY.c

@@ -36,6 +36,8 @@ synproxy_tg6(struct sk_buff *skb, const struct xt_action_param *par)
 			opts.options |= XT_SYNPROXY_OPT_ECN;
 
 		opts.options &= info->options;
+		opts.mss_encode = opts.mss;
+		opts.mss = info->mss;
 		if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
 			synproxy_init_timestamp_cookie(info, &opts);
 		else

net/netfilter/nf_synproxy_core.c

@@ -470,7 +470,7 @@ synproxy_send_client_synack(struct net *net,
 	struct iphdr *iph, *niph;
 	struct tcphdr *nth;
 	unsigned int tcp_hdr_size;
-	u16 mss = opts->mss;
+	u16 mss = opts->mss_encode;
 
 	iph = ip_hdr(skb);
 
@@ -884,7 +884,7 @@ synproxy_send_client_synack_ipv6(struct net *net,
 	struct ipv6hdr *iph, *niph;
 	struct tcphdr *nth;
 	unsigned int tcp_hdr_size;
-	u16 mss = opts->mss;
+	u16 mss = opts->mss_encode;
 
 	iph = ipv6_hdr(skb);
 

net/netfilter/nft_synproxy.c

@@ -31,6 +31,8 @@ static void nft_synproxy_tcp_options(struct synproxy_options *opts,
 		opts->options |= NF_SYNPROXY_OPT_ECN;
 
 	opts->options &= priv->info.options;
+	opts->mss_encode = opts->mss;
+	opts->mss = info->mss;
 	if (opts->options & NF_SYNPROXY_OPT_TIMESTAMP)
 		synproxy_init_timestamp_cookie(info, opts);
 	else