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 #include <net/netfilter/nf_conntrack_ecache.h> 20 21 static struct iphdr * 22 synproxy_build_ip(struct net *net, struct sk_buff *skb, __be32 saddr, 23 __be32 daddr) 24 { 25 struct iphdr *iph; 26 27 skb_reset_network_header(skb); 28 iph = skb_put(skb, sizeof(*iph)); 29 iph->version = 4; 30 iph->ihl = sizeof(*iph) / 4; 31 iph->tos = 0; 32 iph->id = 0; 33 iph->frag_off = htons(IP_DF); 34 iph->ttl = net->ipv4.sysctl_ip_default_ttl; 35 iph->protocol = IPPROTO_TCP; 36 iph->check = 0; 37 iph->saddr = saddr; 38 iph->daddr = daddr; 39 40 return iph; 41 } 42 43 static void 44 synproxy_send_tcp(struct net *net, 45 const struct sk_buff *skb, struct sk_buff *nskb, 46 struct nf_conntrack *nfct, enum ip_conntrack_info ctinfo, 47 struct iphdr *niph, struct tcphdr *nth, 48 unsigned int tcp_hdr_size) 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 nf_ct_set(nskb, (struct nf_conn *)nfct, ctinfo); 62 nf_conntrack_get(nfct); 63 } 64 65 ip_local_out(net, nskb->sk, nskb); 66 return; 67 68 free_nskb: 69 kfree_skb(nskb); 70 } 71 72 static void 73 synproxy_send_client_synack(struct net *net, 74 const struct sk_buff *skb, const struct tcphdr *th, 75 const struct synproxy_options *opts) 76 { 77 struct sk_buff *nskb; 78 struct iphdr *iph, *niph; 79 struct tcphdr *nth; 80 unsigned int tcp_hdr_size; 81 u16 mss = opts->mss; 82 83 iph = ip_hdr(skb); 84 85 tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts); 86 nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER, 87 GFP_ATOMIC); 88 if (nskb == NULL) 89 return; 90 skb_reserve(nskb, MAX_TCP_HEADER); 91 92 niph = synproxy_build_ip(net, nskb, iph->daddr, iph->saddr); 93 94 skb_reset_transport_header(nskb); 95 nth = skb_put(nskb, tcp_hdr_size); 96 nth->source = th->dest; 97 nth->dest = th->source; 98 nth->seq = htonl(__cookie_v4_init_sequence(iph, th, &mss)); 99 nth->ack_seq = htonl(ntohl(th->seq) + 1); 100 tcp_flag_word(nth) = TCP_FLAG_SYN | TCP_FLAG_ACK; 101 if (opts->options & XT_SYNPROXY_OPT_ECN) 102 tcp_flag_word(nth) |= TCP_FLAG_ECE; 103 nth->doff = tcp_hdr_size / 4; 104 nth->window = 0; 105 nth->check = 0; 106 nth->urg_ptr = 0; 107 108 synproxy_build_options(nth, opts); 109 110 synproxy_send_tcp(net, skb, nskb, skb_nfct(skb), 111 IP_CT_ESTABLISHED_REPLY, niph, nth, tcp_hdr_size); 112 } 113 114 static void 115 synproxy_send_server_syn(struct net *net, 116 const struct sk_buff *skb, const struct tcphdr *th, 117 const struct synproxy_options *opts, u32 recv_seq) 118 { 119 struct synproxy_net *snet = synproxy_pernet(net); 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(net, nskb, iph->saddr, iph->daddr); 135 136 skb_reset_transport_header(nskb); 137 nth = 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(net, skb, nskb, &snet->tmpl->ct_general, IP_CT_NEW, 156 niph, nth, tcp_hdr_size); 157 } 158 159 static void 160 synproxy_send_server_ack(struct net *net, 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(net, nskb, iph->daddr, iph->saddr); 180 181 skb_reset_transport_header(nskb); 182 nth = 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(net, skb, nskb, NULL, 0, niph, nth, tcp_hdr_size); 196 } 197 198 static void 199 synproxy_send_client_ack(struct net *net, 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(net, nskb, iph->saddr, iph->daddr); 218 219 skb_reset_transport_header(nskb); 220 nth = 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(net, skb, nskb, skb_nfct(skb), 234 IP_CT_ESTABLISHED_REPLY, niph, nth, tcp_hdr_size); 235 } 236 237 static bool 238 synproxy_recv_client_ack(struct net *net, 239 const struct sk_buff *skb, const struct tcphdr *th, 240 struct synproxy_options *opts, u32 recv_seq) 241 { 242 struct synproxy_net *snet = synproxy_pernet(net); 243 int mss; 244 245 mss = __cookie_v4_check(ip_hdr(skb), th, ntohl(th->ack_seq) - 1); 246 if (mss == 0) { 247 this_cpu_inc(snet->stats->cookie_invalid); 248 return false; 249 } 250 251 this_cpu_inc(snet->stats->cookie_valid); 252 opts->mss = mss; 253 opts->options |= XT_SYNPROXY_OPT_MSS; 254 255 if (opts->options & XT_SYNPROXY_OPT_TIMESTAMP) 256 synproxy_check_timestamp_cookie(opts); 257 258 synproxy_send_server_syn(net, skb, th, opts, recv_seq); 259 return true; 260 } 261 262 static unsigned int 263 synproxy_tg4(struct sk_buff *skb, const struct xt_action_param *par) 264 { 265 const struct xt_synproxy_info *info = par->targinfo; 266 struct net *net = xt_net(par); 267 struct synproxy_net *snet = synproxy_pernet(net); 268 struct synproxy_options opts = {}; 269 struct tcphdr *th, _th; 270 271 if (nf_ip_checksum(skb, xt_hooknum(par), par->thoff, IPPROTO_TCP)) 272 return NF_DROP; 273 274 th = skb_header_pointer(skb, par->thoff, sizeof(_th), &_th); 275 if (th == NULL) 276 return NF_DROP; 277 278 if (!synproxy_parse_options(skb, par->thoff, th, &opts)) 279 return NF_DROP; 280 281 if (th->syn && !(th->ack || th->fin || th->rst)) { 282 /* Initial SYN from client */ 283 this_cpu_inc(snet->stats->syn_received); 284 285 if (th->ece && th->cwr) 286 opts.options |= XT_SYNPROXY_OPT_ECN; 287 288 opts.options &= info->options; 289 if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP) 290 synproxy_init_timestamp_cookie(info, &opts); 291 else 292 opts.options &= ~(XT_SYNPROXY_OPT_WSCALE | 293 XT_SYNPROXY_OPT_SACK_PERM | 294 XT_SYNPROXY_OPT_ECN); 295 296 synproxy_send_client_synack(net, skb, th, &opts); 297 consume_skb(skb); 298 return NF_STOLEN; 299 } else if (th->ack && !(th->fin || th->rst || th->syn)) { 300 /* ACK from client */ 301 if (synproxy_recv_client_ack(net, skb, th, &opts, ntohl(th->seq))) { 302 consume_skb(skb); 303 return NF_STOLEN; 304 } else { 305 return NF_DROP; 306 } 307 } 308 309 return XT_CONTINUE; 310 } 311 312 static unsigned int ipv4_synproxy_hook(void *priv, 313 struct sk_buff *skb, 314 const struct nf_hook_state *nhs) 315 { 316 struct net *net = nhs->net; 317 struct synproxy_net *snet = synproxy_pernet(net); 318 enum ip_conntrack_info ctinfo; 319 struct nf_conn *ct; 320 struct nf_conn_synproxy *synproxy; 321 struct synproxy_options opts = {}; 322 const struct ip_ct_tcp *state; 323 struct tcphdr *th, _th; 324 unsigned int thoff; 325 326 ct = nf_ct_get(skb, &ctinfo); 327 if (ct == NULL) 328 return NF_ACCEPT; 329 330 synproxy = nfct_synproxy(ct); 331 if (synproxy == NULL) 332 return NF_ACCEPT; 333 334 if (nf_is_loopback_packet(skb) || 335 ip_hdr(skb)->protocol != IPPROTO_TCP) 336 return NF_ACCEPT; 337 338 thoff = ip_hdrlen(skb); 339 th = skb_header_pointer(skb, thoff, sizeof(_th), &_th); 340 if (th == NULL) 341 return NF_DROP; 342 343 state = &ct->proto.tcp; 344 switch (state->state) { 345 case TCP_CONNTRACK_CLOSE: 346 if (th->rst && !test_bit(IPS_SEEN_REPLY_BIT, &ct->status)) { 347 nf_ct_seqadj_init(ct, ctinfo, synproxy->isn - 348 ntohl(th->seq) + 1); 349 break; 350 } 351 352 if (!th->syn || th->ack || 353 CTINFO2DIR(ctinfo) != IP_CT_DIR_ORIGINAL) 354 break; 355 356 /* Reopened connection - reset the sequence number and timestamp 357 * adjustments, they will get initialized once the connection is 358 * reestablished. 359 */ 360 nf_ct_seqadj_init(ct, ctinfo, 0); 361 synproxy->tsoff = 0; 362 this_cpu_inc(snet->stats->conn_reopened); 363 364 /* fall through */ 365 case TCP_CONNTRACK_SYN_SENT: 366 if (!synproxy_parse_options(skb, thoff, th, &opts)) 367 return NF_DROP; 368 369 if (!th->syn && th->ack && 370 CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL) { 371 /* Keep-Alives are sent with SEG.SEQ = SND.NXT-1, 372 * therefore we need to add 1 to make the SYN sequence 373 * number match the one of first SYN. 374 */ 375 if (synproxy_recv_client_ack(net, skb, th, &opts, 376 ntohl(th->seq) + 1)) { 377 this_cpu_inc(snet->stats->cookie_retrans); 378 consume_skb(skb); 379 return NF_STOLEN; 380 } else { 381 return NF_DROP; 382 } 383 } 384 385 synproxy->isn = ntohl(th->ack_seq); 386 if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP) 387 synproxy->its = opts.tsecr; 388 389 nf_conntrack_event_cache(IPCT_SYNPROXY, ct); 390 break; 391 case TCP_CONNTRACK_SYN_RECV: 392 if (!th->syn || !th->ack) 393 break; 394 395 if (!synproxy_parse_options(skb, thoff, th, &opts)) 396 return NF_DROP; 397 398 if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP) { 399 synproxy->tsoff = opts.tsval - synproxy->its; 400 nf_conntrack_event_cache(IPCT_SYNPROXY, ct); 401 } 402 403 opts.options &= ~(XT_SYNPROXY_OPT_MSS | 404 XT_SYNPROXY_OPT_WSCALE | 405 XT_SYNPROXY_OPT_SACK_PERM); 406 407 swap(opts.tsval, opts.tsecr); 408 synproxy_send_server_ack(net, state, skb, th, &opts); 409 410 nf_ct_seqadj_init(ct, ctinfo, synproxy->isn - ntohl(th->seq)); 411 nf_conntrack_event_cache(IPCT_SEQADJ, ct); 412 413 swap(opts.tsval, opts.tsecr); 414 synproxy_send_client_ack(net, skb, th, &opts); 415 416 consume_skb(skb); 417 return NF_STOLEN; 418 default: 419 break; 420 } 421 422 synproxy_tstamp_adjust(skb, thoff, th, ct, ctinfo, synproxy); 423 return NF_ACCEPT; 424 } 425 426 static const struct nf_hook_ops ipv4_synproxy_ops[] = { 427 { 428 .hook = ipv4_synproxy_hook, 429 .pf = NFPROTO_IPV4, 430 .hooknum = NF_INET_LOCAL_IN, 431 .priority = NF_IP_PRI_CONNTRACK_CONFIRM - 1, 432 }, 433 { 434 .hook = ipv4_synproxy_hook, 435 .pf = NFPROTO_IPV4, 436 .hooknum = NF_INET_POST_ROUTING, 437 .priority = NF_IP_PRI_CONNTRACK_CONFIRM - 1, 438 }, 439 }; 440 441 static int synproxy_tg4_check(const struct xt_tgchk_param *par) 442 { 443 struct synproxy_net *snet = synproxy_pernet(par->net); 444 const struct ipt_entry *e = par->entryinfo; 445 int err; 446 447 if (e->ip.proto != IPPROTO_TCP || 448 e->ip.invflags & XT_INV_PROTO) 449 return -EINVAL; 450 451 err = nf_ct_netns_get(par->net, par->family); 452 if (err) 453 return err; 454 455 if (snet->hook_ref4 == 0) { 456 err = nf_register_net_hooks(par->net, ipv4_synproxy_ops, 457 ARRAY_SIZE(ipv4_synproxy_ops)); 458 if (err) { 459 nf_ct_netns_put(par->net, par->family); 460 return err; 461 } 462 } 463 464 snet->hook_ref4++; 465 return err; 466 } 467 468 static void synproxy_tg4_destroy(const struct xt_tgdtor_param *par) 469 { 470 struct synproxy_net *snet = synproxy_pernet(par->net); 471 472 snet->hook_ref4--; 473 if (snet->hook_ref4 == 0) 474 nf_unregister_net_hooks(par->net, ipv4_synproxy_ops, 475 ARRAY_SIZE(ipv4_synproxy_ops)); 476 nf_ct_netns_put(par->net, par->family); 477 } 478 479 static struct xt_target synproxy_tg4_reg __read_mostly = { 480 .name = "SYNPROXY", 481 .family = NFPROTO_IPV4, 482 .hooks = (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD), 483 .target = synproxy_tg4, 484 .targetsize = sizeof(struct xt_synproxy_info), 485 .checkentry = synproxy_tg4_check, 486 .destroy = synproxy_tg4_destroy, 487 .me = THIS_MODULE, 488 }; 489 490 static int __init synproxy_tg4_init(void) 491 { 492 return xt_register_target(&synproxy_tg4_reg); 493 } 494 495 static void __exit synproxy_tg4_exit(void) 496 { 497 xt_unregister_target(&synproxy_tg4_reg); 498 } 499 500 module_init(synproxy_tg4_init); 501 module_exit(synproxy_tg4_exit); 502 503 MODULE_LICENSE("GPL"); 504 MODULE_AUTHOR("Patrick McHardy <kaber@trash.net>"); 505