1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * Copyright (c) 2013 Patrick McHardy <kaber@trash.net>
4  */
5 
6 #include <linux/netfilter_ipv4/ip_tables.h>
7 #include <linux/netfilter/x_tables.h>
8 #include <linux/netfilter/xt_SYNPROXY.h>
9 
10 #include <net/netfilter/nf_synproxy.h>
11 
12 static unsigned int
13 synproxy_tg4(struct sk_buff *skb, const struct xt_action_param *par)
14 {
15 	const struct xt_synproxy_info *info = par->targinfo;
16 	struct net *net = xt_net(par);
17 	struct synproxy_net *snet = synproxy_pernet(net);
18 	struct synproxy_options opts = {};
19 	struct tcphdr *th, _th;
20 
21 	if (nf_ip_checksum(skb, xt_hooknum(par), par->thoff, IPPROTO_TCP))
22 		return NF_DROP;
23 
24 	th = skb_header_pointer(skb, par->thoff, sizeof(_th), &_th);
25 	if (th == NULL)
26 		return NF_DROP;
27 
28 	if (!synproxy_parse_options(skb, par->thoff, th, &opts))
29 		return NF_DROP;
30 
31 	if (th->syn && !(th->ack || th->fin || th->rst)) {
32 		/* Initial SYN from client */
33 		this_cpu_inc(snet->stats->syn_received);
34 
35 		if (th->ece && th->cwr)
36 			opts.options |= XT_SYNPROXY_OPT_ECN;
37 
38 		opts.options &= info->options;
39 		opts.mss_encode = opts.mss_option;
40 		opts.mss_option = info->mss;
41 		if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
42 			synproxy_init_timestamp_cookie(info, &opts);
43 		else
44 			opts.options &= ~(XT_SYNPROXY_OPT_WSCALE |
45 					  XT_SYNPROXY_OPT_SACK_PERM |
46 					  XT_SYNPROXY_OPT_ECN);
47 
48 		synproxy_send_client_synack(net, skb, th, &opts);
49 		consume_skb(skb);
50 		return NF_STOLEN;
51 	} else if (th->ack && !(th->fin || th->rst || th->syn)) {
52 		/* ACK from client */
53 		if (synproxy_recv_client_ack(net, skb, th, &opts, ntohl(th->seq))) {
54 			consume_skb(skb);
55 			return NF_STOLEN;
56 		} else {
57 			return NF_DROP;
58 		}
59 	}
60 
61 	return XT_CONTINUE;
62 }
63 
64 static int synproxy_tg4_check(const struct xt_tgchk_param *par)
65 {
66 	struct synproxy_net *snet = synproxy_pernet(par->net);
67 	const struct ipt_entry *e = par->entryinfo;
68 	int err;
69 
70 	if (e->ip.proto != IPPROTO_TCP ||
71 	    e->ip.invflags & XT_INV_PROTO)
72 		return -EINVAL;
73 
74 	err = nf_ct_netns_get(par->net, par->family);
75 	if (err)
76 		return err;
77 
78 	err = nf_synproxy_ipv4_init(snet, par->net);
79 	if (err) {
80 		nf_ct_netns_put(par->net, par->family);
81 		return err;
82 	}
83 
84 	return err;
85 }
86 
87 static void synproxy_tg4_destroy(const struct xt_tgdtor_param *par)
88 {
89 	struct synproxy_net *snet = synproxy_pernet(par->net);
90 
91 	nf_synproxy_ipv4_fini(snet, par->net);
92 	nf_ct_netns_put(par->net, par->family);
93 }
94 
95 static struct xt_target synproxy_tg4_reg __read_mostly = {
96 	.name		= "SYNPROXY",
97 	.family		= NFPROTO_IPV4,
98 	.hooks		= (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD),
99 	.target		= synproxy_tg4,
100 	.targetsize	= sizeof(struct xt_synproxy_info),
101 	.checkentry	= synproxy_tg4_check,
102 	.destroy	= synproxy_tg4_destroy,
103 	.me		= THIS_MODULE,
104 };
105 
106 static int __init synproxy_tg4_init(void)
107 {
108 	return xt_register_target(&synproxy_tg4_reg);
109 }
110 
111 static void __exit synproxy_tg4_exit(void)
112 {
113 	xt_unregister_target(&synproxy_tg4_reg);
114 }
115 
116 module_init(synproxy_tg4_init);
117 module_exit(synproxy_tg4_exit);
118 
119 MODULE_LICENSE("GPL");
120 MODULE_AUTHOR("Patrick McHardy <kaber@trash.net>");
121 MODULE_DESCRIPTION("Intercept TCP connections and establish them using syncookies");
122