1 /*
2 * Copyright (c) 2013 Patrick McHardy <kaber@trash.net>
3 *
4 * This program is free software; you can redistribute it and/or modify
5 * it under the terms of the GNU General Public License version 2 as
6 * published by the Free Software Foundation.
7 */
8
9 #include <linux/module.h>
10 #include <linux/skbuff.h>
11 #include <net/tcp.h>
12
13 #include <linux/netfilter_ipv4/ip_tables.h>
14 #include <linux/netfilter/x_tables.h>
15 #include <linux/netfilter/xt_SYNPROXY.h>
16 #include <net/netfilter/nf_conntrack.h>
17 #include <net/netfilter/nf_conntrack_seqadj.h>
18 #include <net/netfilter/nf_conntrack_synproxy.h>
19
20 static struct iphdr *
synproxy_build_ip(struct sk_buff * skb,__be32 saddr,__be32 daddr)21 synproxy_build_ip(struct sk_buff *skb, __be32 saddr, __be32 daddr)
22 {
23 struct iphdr *iph;
24
25 skb_reset_network_header(skb);
26 iph = (struct iphdr *)skb_put(skb, sizeof(*iph));
27 iph->version = 4;
28 iph->ihl = sizeof(*iph) / 4;
29 iph->tos = 0;
30 iph->id = 0;
31 iph->frag_off = htons(IP_DF);
32 iph->ttl = sysctl_ip_default_ttl;
33 iph->protocol = IPPROTO_TCP;
34 iph->check = 0;
35 iph->saddr = saddr;
36 iph->daddr = daddr;
37
38 return iph;
39 }
40
41 static void
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 iphdr * niph,struct tcphdr * nth,unsigned int tcp_hdr_size)42 synproxy_send_tcp(const struct synproxy_net *snet,
43 const struct sk_buff *skb, struct sk_buff *nskb,
44 struct nf_conntrack *nfct, enum ip_conntrack_info ctinfo,
45 struct iphdr *niph, struct tcphdr *nth,
46 unsigned int tcp_hdr_size)
47 {
48 struct net *net = nf_ct_net(snet->tmpl);
49
50 nth->check = ~tcp_v4_check(tcp_hdr_size, niph->saddr, niph->daddr, 0);
51 nskb->ip_summed = CHECKSUM_PARTIAL;
52 nskb->csum_start = (unsigned char *)nth - nskb->head;
53 nskb->csum_offset = offsetof(struct tcphdr, check);
54
55 skb_dst_set_noref(nskb, skb_dst(skb));
56 nskb->protocol = htons(ETH_P_IP);
57 if (ip_route_me_harder(net, nskb, RTN_UNSPEC))
58 goto free_nskb;
59
60 if (nfct) {
61 nskb->nfct = nfct;
62 nskb->nfctinfo = ctinfo;
63 nf_conntrack_get(nfct);
64 }
65
66 ip_local_out(net, nskb->sk, nskb);
67 return;
68
69 free_nskb:
70 kfree_skb(nskb);
71 }
72
73 static void
synproxy_send_client_synack(const struct synproxy_net * snet,const struct sk_buff * skb,const struct tcphdr * th,const struct synproxy_options * opts)74 synproxy_send_client_synack(const struct synproxy_net *snet,
75 const struct sk_buff *skb, const struct tcphdr *th,
76 const struct synproxy_options *opts)
77 {
78 struct sk_buff *nskb;
79 struct iphdr *iph, *niph;
80 struct tcphdr *nth;
81 unsigned int tcp_hdr_size;
82 u16 mss = opts->mss;
83
84 iph = ip_hdr(skb);
85
86 tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts);
87 nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER,
88 GFP_ATOMIC);
89 if (nskb == NULL)
90 return;
91 skb_reserve(nskb, MAX_TCP_HEADER);
92
93 niph = synproxy_build_ip(nskb, iph->daddr, iph->saddr);
94
95 skb_reset_transport_header(nskb);
96 nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size);
97 nth->source = th->dest;
98 nth->dest = th->source;
99 nth->seq = htonl(__cookie_v4_init_sequence(iph, th, &mss));
100 nth->ack_seq = htonl(ntohl(th->seq) + 1);
101 tcp_flag_word(nth) = TCP_FLAG_SYN | TCP_FLAG_ACK;
102 if (opts->options & XT_SYNPROXY_OPT_ECN)
103 tcp_flag_word(nth) |= TCP_FLAG_ECE;
104 nth->doff = tcp_hdr_size / 4;
105 nth->window = 0;
106 nth->check = 0;
107 nth->urg_ptr = 0;
108
109 synproxy_build_options(nth, opts);
110
111 synproxy_send_tcp(snet, skb, nskb, skb->nfct, IP_CT_ESTABLISHED_REPLY,
112 niph, nth, tcp_hdr_size);
113 }
114
115 static void
synproxy_send_server_syn(const struct synproxy_net * snet,const struct sk_buff * skb,const struct tcphdr * th,const struct synproxy_options * opts,u32 recv_seq)116 synproxy_send_server_syn(const struct synproxy_net *snet,
117 const struct sk_buff *skb, const struct tcphdr *th,
118 const struct synproxy_options *opts, u32 recv_seq)
119 {
120 struct sk_buff *nskb;
121 struct iphdr *iph, *niph;
122 struct tcphdr *nth;
123 unsigned int tcp_hdr_size;
124
125 iph = ip_hdr(skb);
126
127 tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts);
128 nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER,
129 GFP_ATOMIC);
130 if (nskb == NULL)
131 return;
132 skb_reserve(nskb, MAX_TCP_HEADER);
133
134 niph = synproxy_build_ip(nskb, iph->saddr, iph->daddr);
135
136 skb_reset_transport_header(nskb);
137 nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size);
138 nth->source = th->source;
139 nth->dest = th->dest;
140 nth->seq = htonl(recv_seq - 1);
141 /* ack_seq is used to relay our ISN to the synproxy hook to initialize
142 * sequence number translation once a connection tracking entry exists.
143 */
144 nth->ack_seq = htonl(ntohl(th->ack_seq) - 1);
145 tcp_flag_word(nth) = TCP_FLAG_SYN;
146 if (opts->options & XT_SYNPROXY_OPT_ECN)
147 tcp_flag_word(nth) |= TCP_FLAG_ECE | TCP_FLAG_CWR;
148 nth->doff = tcp_hdr_size / 4;
149 nth->window = th->window;
150 nth->check = 0;
151 nth->urg_ptr = 0;
152
153 synproxy_build_options(nth, opts);
154
155 synproxy_send_tcp(snet, skb, nskb, &snet->tmpl->ct_general, IP_CT_NEW,
156 niph, nth, tcp_hdr_size);
157 }
158
159 static void
synproxy_send_server_ack(const struct synproxy_net * snet,const struct ip_ct_tcp * state,const struct sk_buff * skb,const struct tcphdr * th,const struct synproxy_options * opts)160 synproxy_send_server_ack(const struct synproxy_net *snet,
161 const struct ip_ct_tcp *state,
162 const struct sk_buff *skb, const struct tcphdr *th,
163 const struct synproxy_options *opts)
164 {
165 struct sk_buff *nskb;
166 struct iphdr *iph, *niph;
167 struct tcphdr *nth;
168 unsigned int tcp_hdr_size;
169
170 iph = ip_hdr(skb);
171
172 tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts);
173 nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER,
174 GFP_ATOMIC);
175 if (nskb == NULL)
176 return;
177 skb_reserve(nskb, MAX_TCP_HEADER);
178
179 niph = synproxy_build_ip(nskb, iph->daddr, iph->saddr);
180
181 skb_reset_transport_header(nskb);
182 nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size);
183 nth->source = th->dest;
184 nth->dest = th->source;
185 nth->seq = htonl(ntohl(th->ack_seq));
186 nth->ack_seq = htonl(ntohl(th->seq) + 1);
187 tcp_flag_word(nth) = TCP_FLAG_ACK;
188 nth->doff = tcp_hdr_size / 4;
189 nth->window = htons(state->seen[IP_CT_DIR_ORIGINAL].td_maxwin);
190 nth->check = 0;
191 nth->urg_ptr = 0;
192
193 synproxy_build_options(nth, opts);
194
195 synproxy_send_tcp(snet, skb, nskb, NULL, 0, niph, nth, tcp_hdr_size);
196 }
197
198 static void
synproxy_send_client_ack(const struct synproxy_net * snet,const struct sk_buff * skb,const struct tcphdr * th,const struct synproxy_options * opts)199 synproxy_send_client_ack(const struct synproxy_net *snet,
200 const struct sk_buff *skb, const struct tcphdr *th,
201 const struct synproxy_options *opts)
202 {
203 struct sk_buff *nskb;
204 struct iphdr *iph, *niph;
205 struct tcphdr *nth;
206 unsigned int tcp_hdr_size;
207
208 iph = ip_hdr(skb);
209
210 tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts);
211 nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER,
212 GFP_ATOMIC);
213 if (nskb == NULL)
214 return;
215 skb_reserve(nskb, MAX_TCP_HEADER);
216
217 niph = synproxy_build_ip(nskb, iph->saddr, iph->daddr);
218
219 skb_reset_transport_header(nskb);
220 nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size);
221 nth->source = th->source;
222 nth->dest = th->dest;
223 nth->seq = htonl(ntohl(th->seq) + 1);
224 nth->ack_seq = th->ack_seq;
225 tcp_flag_word(nth) = TCP_FLAG_ACK;
226 nth->doff = tcp_hdr_size / 4;
227 nth->window = htons(ntohs(th->window) >> opts->wscale);
228 nth->check = 0;
229 nth->urg_ptr = 0;
230
231 synproxy_build_options(nth, opts);
232
233 synproxy_send_tcp(snet, skb, nskb, skb->nfct, IP_CT_ESTABLISHED_REPLY,
234 niph, nth, tcp_hdr_size);
235 }
236
237 static bool
synproxy_recv_client_ack(const struct synproxy_net * snet,const struct sk_buff * skb,const struct tcphdr * th,struct synproxy_options * opts,u32 recv_seq)238 synproxy_recv_client_ack(const struct synproxy_net *snet,
239 const struct sk_buff *skb, const struct tcphdr *th,
240 struct synproxy_options *opts, u32 recv_seq)
241 {
242 int mss;
243
244 mss = __cookie_v4_check(ip_hdr(skb), th, ntohl(th->ack_seq) - 1);
245 if (mss == 0) {
246 this_cpu_inc(snet->stats->cookie_invalid);
247 return false;
248 }
249
250 this_cpu_inc(snet->stats->cookie_valid);
251 opts->mss = mss;
252 opts->options |= XT_SYNPROXY_OPT_MSS;
253
254 if (opts->options & XT_SYNPROXY_OPT_TIMESTAMP)
255 synproxy_check_timestamp_cookie(opts);
256
257 synproxy_send_server_syn(snet, skb, th, opts, recv_seq);
258 return true;
259 }
260
261 static unsigned int
synproxy_tg4(struct sk_buff * skb,const struct xt_action_param * par)262 synproxy_tg4(struct sk_buff *skb, const struct xt_action_param *par)
263 {
264 const struct xt_synproxy_info *info = par->targinfo;
265 struct synproxy_net *snet = synproxy_pernet(par->net);
266 struct synproxy_options opts = {};
267 struct tcphdr *th, _th;
268
269 if (nf_ip_checksum(skb, par->hooknum, par->thoff, IPPROTO_TCP))
270 return NF_DROP;
271
272 th = skb_header_pointer(skb, par->thoff, sizeof(_th), &_th);
273 if (th == NULL)
274 return NF_DROP;
275
276 if (!synproxy_parse_options(skb, par->thoff, th, &opts))
277 return NF_DROP;
278
279 if (th->syn && !(th->ack || th->fin || th->rst)) {
280 /* Initial SYN from client */
281 this_cpu_inc(snet->stats->syn_received);
282
283 if (th->ece && th->cwr)
284 opts.options |= XT_SYNPROXY_OPT_ECN;
285
286 opts.options &= info->options;
287 if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
288 synproxy_init_timestamp_cookie(info, &opts);
289 else
290 opts.options &= ~(XT_SYNPROXY_OPT_WSCALE |
291 XT_SYNPROXY_OPT_SACK_PERM |
292 XT_SYNPROXY_OPT_ECN);
293
294 synproxy_send_client_synack(snet, skb, th, &opts);
295 return NF_DROP;
296
297 } else if (th->ack && !(th->fin || th->rst || th->syn)) {
298 /* ACK from client */
299 synproxy_recv_client_ack(snet, skb, th, &opts, ntohl(th->seq));
300 return NF_DROP;
301 }
302
303 return XT_CONTINUE;
304 }
305
ipv4_synproxy_hook(void * priv,struct sk_buff * skb,const struct nf_hook_state * nhs)306 static unsigned int ipv4_synproxy_hook(void *priv,
307 struct sk_buff *skb,
308 const struct nf_hook_state *nhs)
309 {
310 struct synproxy_net *snet = synproxy_pernet(nhs->net);
311 enum ip_conntrack_info ctinfo;
312 struct nf_conn *ct;
313 struct nf_conn_synproxy *synproxy;
314 struct synproxy_options opts = {};
315 const struct ip_ct_tcp *state;
316 struct tcphdr *th, _th;
317 unsigned int thoff;
318
319 ct = nf_ct_get(skb, &ctinfo);
320 if (ct == NULL)
321 return NF_ACCEPT;
322
323 synproxy = nfct_synproxy(ct);
324 if (synproxy == NULL)
325 return NF_ACCEPT;
326
327 if (nf_is_loopback_packet(skb))
328 return NF_ACCEPT;
329
330 thoff = ip_hdrlen(skb);
331 th = skb_header_pointer(skb, thoff, sizeof(_th), &_th);
332 if (th == NULL)
333 return NF_DROP;
334
335 state = &ct->proto.tcp;
336 switch (state->state) {
337 case TCP_CONNTRACK_CLOSE:
338 if (th->rst && !test_bit(IPS_SEEN_REPLY_BIT, &ct->status)) {
339 nf_ct_seqadj_init(ct, ctinfo, synproxy->isn -
340 ntohl(th->seq) + 1);
341 break;
342 }
343
344 if (!th->syn || th->ack ||
345 CTINFO2DIR(ctinfo) != IP_CT_DIR_ORIGINAL)
346 break;
347
348 /* Reopened connection - reset the sequence number and timestamp
349 * adjustments, they will get initialized once the connection is
350 * reestablished.
351 */
352 nf_ct_seqadj_init(ct, ctinfo, 0);
353 synproxy->tsoff = 0;
354 this_cpu_inc(snet->stats->conn_reopened);
355
356 /* fall through */
357 case TCP_CONNTRACK_SYN_SENT:
358 if (!synproxy_parse_options(skb, thoff, th, &opts))
359 return NF_DROP;
360
361 if (!th->syn && th->ack &&
362 CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL) {
363 /* Keep-Alives are sent with SEG.SEQ = SND.NXT-1,
364 * therefore we need to add 1 to make the SYN sequence
365 * number match the one of first SYN.
366 */
367 if (synproxy_recv_client_ack(snet, skb, th, &opts,
368 ntohl(th->seq) + 1))
369 this_cpu_inc(snet->stats->cookie_retrans);
370
371 return NF_DROP;
372 }
373
374 synproxy->isn = ntohl(th->ack_seq);
375 if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
376 synproxy->its = opts.tsecr;
377 break;
378 case TCP_CONNTRACK_SYN_RECV:
379 if (!th->syn || !th->ack)
380 break;
381
382 if (!synproxy_parse_options(skb, thoff, th, &opts))
383 return NF_DROP;
384
385 if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
386 synproxy->tsoff = opts.tsval - synproxy->its;
387
388 opts.options &= ~(XT_SYNPROXY_OPT_MSS |
389 XT_SYNPROXY_OPT_WSCALE |
390 XT_SYNPROXY_OPT_SACK_PERM);
391
392 swap(opts.tsval, opts.tsecr);
393 synproxy_send_server_ack(snet, state, skb, th, &opts);
394
395 nf_ct_seqadj_init(ct, ctinfo, synproxy->isn - ntohl(th->seq));
396
397 swap(opts.tsval, opts.tsecr);
398 synproxy_send_client_ack(snet, skb, th, &opts);
399
400 consume_skb(skb);
401 return NF_STOLEN;
402 default:
403 break;
404 }
405
406 synproxy_tstamp_adjust(skb, thoff, th, ct, ctinfo, synproxy);
407 return NF_ACCEPT;
408 }
409
synproxy_tg4_check(const struct xt_tgchk_param * par)410 static int synproxy_tg4_check(const struct xt_tgchk_param *par)
411 {
412 const struct ipt_entry *e = par->entryinfo;
413
414 if (e->ip.proto != IPPROTO_TCP ||
415 e->ip.invflags & XT_INV_PROTO)
416 return -EINVAL;
417
418 return nf_ct_l3proto_try_module_get(par->family);
419 }
420
synproxy_tg4_destroy(const struct xt_tgdtor_param * par)421 static void synproxy_tg4_destroy(const struct xt_tgdtor_param *par)
422 {
423 nf_ct_l3proto_module_put(par->family);
424 }
425
426 static struct xt_target synproxy_tg4_reg __read_mostly = {
427 .name = "SYNPROXY",
428 .family = NFPROTO_IPV4,
429 .hooks = (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD),
430 .target = synproxy_tg4,
431 .targetsize = sizeof(struct xt_synproxy_info),
432 .checkentry = synproxy_tg4_check,
433 .destroy = synproxy_tg4_destroy,
434 .me = THIS_MODULE,
435 };
436
437 static struct nf_hook_ops ipv4_synproxy_ops[] __read_mostly = {
438 {
439 .hook = ipv4_synproxy_hook,
440 .pf = NFPROTO_IPV4,
441 .hooknum = NF_INET_LOCAL_IN,
442 .priority = NF_IP_PRI_CONNTRACK_CONFIRM - 1,
443 },
444 {
445 .hook = ipv4_synproxy_hook,
446 .pf = NFPROTO_IPV4,
447 .hooknum = NF_INET_POST_ROUTING,
448 .priority = NF_IP_PRI_CONNTRACK_CONFIRM - 1,
449 },
450 };
451
synproxy_tg4_init(void)452 static int __init synproxy_tg4_init(void)
453 {
454 int err;
455
456 err = nf_register_hooks(ipv4_synproxy_ops,
457 ARRAY_SIZE(ipv4_synproxy_ops));
458 if (err < 0)
459 goto err1;
460
461 err = xt_register_target(&synproxy_tg4_reg);
462 if (err < 0)
463 goto err2;
464
465 return 0;
466
467 err2:
468 nf_unregister_hooks(ipv4_synproxy_ops, ARRAY_SIZE(ipv4_synproxy_ops));
469 err1:
470 return err;
471 }
472
synproxy_tg4_exit(void)473 static void __exit synproxy_tg4_exit(void)
474 {
475 xt_unregister_target(&synproxy_tg4_reg);
476 nf_unregister_hooks(ipv4_synproxy_ops, ARRAY_SIZE(ipv4_synproxy_ops));
477 }
478
479 module_init(synproxy_tg4_init);
480 module_exit(synproxy_tg4_exit);
481
482 MODULE_LICENSE("GPL");
483 MODULE_AUTHOR("Patrick McHardy <kaber@trash.net>");
484