net/ipv6/netfilter/ip6t_SYNPROXY.c

   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/ip6_checksum.h>
  12 #include <net/ip6_route.h>
  13 #include <net/tcp.h>
  14
  15 #include <linux/netfilter_ipv6/ip6_tables.h>
  16 #include <linux/netfilter/x_tables.h>
  17 #include <linux/netfilter/xt_SYNPROXY.h>
  18 #include <net/netfilter/nf_conntrack.h>
  19 #include <net/netfilter/nf_conntrack_seqadj.h>
  20 #include <net/netfilter/nf_conntrack_synproxy.h>
  21
  22 static struct ipv6hdr *
  23 synproxy_build_ip(struct net *net, struct sk_buff *skb,
  24                   const struct in6_addr *saddr,
  25                   const struct in6_addr *daddr)
  26 {
  27         struct ipv6hdr *iph;
  28
  29         skb_reset_network_header(skb);
  30         iph = (struct ipv6hdr *)skb_put(skb, sizeof(*iph));
  31         ip6_flow_hdr(iph, 0, 0);
  32         iph->hop_limit  = net->ipv6.devconf_all->hop_limit;
  33         iph->nexthdr    = IPPROTO_TCP;
  34         iph->saddr      = *saddr;
  35         iph->daddr      = *daddr;
  36
  37         return iph;
  38 }
  39
  40 static void
  41 synproxy_send_tcp(struct net *net,
  42                   const struct sk_buff *skb, struct sk_buff *nskb,
  43                   struct nf_conntrack *nfct, enum ip_conntrack_info ctinfo,
  44                   struct ipv6hdr *niph, struct tcphdr *nth,
  45                   unsigned int tcp_hdr_size)
  46 {
  47         struct dst_entry *dst;
  48         struct flowi6 fl6;
  49
  50         nth->check = ~tcp_v6_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         memset(&fl6, 0, sizeof(fl6));
  56         fl6.flowi6_proto = IPPROTO_TCP;
  57         fl6.saddr = niph->saddr;
  58         fl6.daddr = niph->daddr;
  59         fl6.fl6_sport = nth->source;
  60         fl6.fl6_dport = nth->dest;
  61         security_skb_classify_flow((struct sk_buff *)skb, flowi6_to_flowi(&fl6));
  62         dst = ip6_route_output(net, NULL, &fl6);
  63         if (dst->error) {
  64                 dst_release(dst);
  65                 goto free_nskb;
  66         }
  67         dst = xfrm_lookup(net, dst, flowi6_to_flowi(&fl6), NULL, 0);
  68         if (IS_ERR(dst))
  69                 goto free_nskb;
  70
  71         skb_dst_set(nskb, dst);
  72
  73         if (nfct) {
  74                 nskb->nfct = nfct;
  75                 nskb->nfctinfo = ctinfo;
  76                 nf_conntrack_get(nfct);
  77         }
  78
  79         ip6_local_out(net, nskb->sk, nskb);
  80         return;
  81
  82 free_nskb:
  83         kfree_skb(nskb);
  84 }
  85
  86 static void
  87 synproxy_send_client_synack(struct net *net,
  88                             const struct sk_buff *skb, const struct tcphdr *th,
  89                             const struct synproxy_options *opts)
  90 {
  91         struct sk_buff *nskb;
  92         struct ipv6hdr *iph, *niph;
  93         struct tcphdr *nth;
  94         unsigned int tcp_hdr_size;
  95         u16 mss = opts->mss;
  96
  97         iph = ipv6_hdr(skb);
  98
  99         tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts);
 100         nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER,
 101                          GFP_ATOMIC);
 102         if (nskb == NULL)
 103                 return;
 104         skb_reserve(nskb, MAX_TCP_HEADER);
 105
 106         niph = synproxy_build_ip(net, nskb, &iph->daddr, &iph->saddr);
 107
 108         skb_reset_transport_header(nskb);
 109         nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size);
 110         nth->source     = th->dest;
 111         nth->dest       = th->source;
 112         nth->seq        = htonl(__cookie_v6_init_sequence(iph, th, &mss));
 113         nth->ack_seq    = htonl(ntohl(th->seq) + 1);
 114         tcp_flag_word(nth) = TCP_FLAG_SYN | TCP_FLAG_ACK;
 115         if (opts->options & XT_SYNPROXY_OPT_ECN)
 116                 tcp_flag_word(nth) |= TCP_FLAG_ECE;
 117         nth->doff       = tcp_hdr_size / 4;
 118         nth->window     = 0;
 119         nth->check      = 0;
 120         nth->urg_ptr    = 0;
 121
 122         synproxy_build_options(nth, opts);
 123
 124         synproxy_send_tcp(net, skb, nskb, skb->nfct, IP_CT_ESTABLISHED_REPLY,
 125                           niph, nth, tcp_hdr_size);
 126 }
 127
 128 static void
 129 synproxy_send_server_syn(struct net *net,
 130                          const struct sk_buff *skb, const struct tcphdr *th,
 131                          const struct synproxy_options *opts, u32 recv_seq)
 132 {
 133         struct synproxy_net *snet = synproxy_pernet(net);
 134         struct sk_buff *nskb;
 135         struct ipv6hdr *iph, *niph;
 136         struct tcphdr *nth;
 137         unsigned int tcp_hdr_size;
 138
 139         iph = ipv6_hdr(skb);
 140
 141         tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts);
 142         nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER,
 143                          GFP_ATOMIC);
 144         if (nskb == NULL)
 145                 return;
 146         skb_reserve(nskb, MAX_TCP_HEADER);
 147
 148         niph = synproxy_build_ip(net, nskb, &iph->saddr, &iph->daddr);
 149
 150         skb_reset_transport_header(nskb);
 151         nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size);
 152         nth->source     = th->source;
 153         nth->dest       = th->dest;
 154         nth->seq        = htonl(recv_seq - 1);
 155         /* ack_seq is used to relay our ISN to the synproxy hook to initialize
 156          * sequence number translation once a connection tracking entry exists.
 157          */
 158         nth->ack_seq    = htonl(ntohl(th->ack_seq) - 1);
 159         tcp_flag_word(nth) = TCP_FLAG_SYN;
 160         if (opts->options & XT_SYNPROXY_OPT_ECN)
 161                 tcp_flag_word(nth) |= TCP_FLAG_ECE | TCP_FLAG_CWR;
 162         nth->doff       = tcp_hdr_size / 4;
 163         nth->window     = th->window;
 164         nth->check      = 0;
 165         nth->urg_ptr    = 0;
 166
 167         synproxy_build_options(nth, opts);
 168
 169         synproxy_send_tcp(net, skb, nskb, &snet->tmpl->ct_general, IP_CT_NEW,
 170                           niph, nth, tcp_hdr_size);
 171 }
 172
 173 static void
 174 synproxy_send_server_ack(struct net *net,
 175                          const struct ip_ct_tcp *state,
 176                          const struct sk_buff *skb, const struct tcphdr *th,
 177                          const struct synproxy_options *opts)
 178 {
 179         struct sk_buff *nskb;
 180         struct ipv6hdr *iph, *niph;
 181         struct tcphdr *nth;
 182         unsigned int tcp_hdr_size;
 183
 184         iph = ipv6_hdr(skb);
 185
 186         tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts);
 187         nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER,
 188                          GFP_ATOMIC);
 189         if (nskb == NULL)
 190                 return;
 191         skb_reserve(nskb, MAX_TCP_HEADER);
 192
 193         niph = synproxy_build_ip(net, nskb, &iph->daddr, &iph->saddr);
 194
 195         skb_reset_transport_header(nskb);
 196         nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size);
 197         nth->source     = th->dest;
 198         nth->dest       = th->source;
 199         nth->seq        = htonl(ntohl(th->ack_seq));
 200         nth->ack_seq    = htonl(ntohl(th->seq) + 1);
 201         tcp_flag_word(nth) = TCP_FLAG_ACK;
 202         nth->doff       = tcp_hdr_size / 4;
 203         nth->window     = htons(state->seen[IP_CT_DIR_ORIGINAL].td_maxwin);
 204         nth->check      = 0;
 205         nth->urg_ptr    = 0;
 206
 207         synproxy_build_options(nth, opts);
 208
 209         synproxy_send_tcp(net, skb, nskb, NULL, 0, niph, nth, tcp_hdr_size);
 210 }
 211
 212 static void
 213 synproxy_send_client_ack(struct net *net,
 214                          const struct sk_buff *skb, const struct tcphdr *th,
 215                          const struct synproxy_options *opts)
 216 {
 217         struct sk_buff *nskb;
 218         struct ipv6hdr *iph, *niph;
 219         struct tcphdr *nth;
 220         unsigned int tcp_hdr_size;
 221
 222         iph = ipv6_hdr(skb);
 223
 224         tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts);
 225         nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER,
 226                          GFP_ATOMIC);
 227         if (nskb == NULL)
 228                 return;
 229         skb_reserve(nskb, MAX_TCP_HEADER);
 230
 231         niph = synproxy_build_ip(net, nskb, &iph->saddr, &iph->daddr);
 232
 233         skb_reset_transport_header(nskb);
 234         nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size);
 235         nth->source     = th->source;
 236         nth->dest       = th->dest;
 237         nth->seq        = htonl(ntohl(th->seq) + 1);
 238         nth->ack_seq    = th->ack_seq;
 239         tcp_flag_word(nth) = TCP_FLAG_ACK;
 240         nth->doff       = tcp_hdr_size / 4;
 241         nth->window     = htons(ntohs(th->window) >> opts->wscale);
 242         nth->check      = 0;
 243         nth->urg_ptr    = 0;
 244
 245         synproxy_build_options(nth, opts);
 246
 247         synproxy_send_tcp(net, skb, nskb, skb->nfct, IP_CT_ESTABLISHED_REPLY,
 248                           niph, nth, tcp_hdr_size);
 249 }
 250
 251 static bool
 252 synproxy_recv_client_ack(struct net *net,
 253                          const struct sk_buff *skb, const struct tcphdr *th,
 254                          struct synproxy_options *opts, u32 recv_seq)
 255 {
 256         struct synproxy_net *snet = synproxy_pernet(net);
 257         int mss;
 258
 259         mss = __cookie_v6_check(ipv6_hdr(skb), th, ntohl(th->ack_seq) - 1);
 260         if (mss == 0) {
 261                 this_cpu_inc(snet->stats->cookie_invalid);
 262                 return false;
 263         }
 264
 265         this_cpu_inc(snet->stats->cookie_valid);
 266         opts->mss = mss;
 267         opts->options |= XT_SYNPROXY_OPT_MSS;
 268
 269         if (opts->options & XT_SYNPROXY_OPT_TIMESTAMP)
 270                 synproxy_check_timestamp_cookie(opts);
 271
 272         synproxy_send_server_syn(net, skb, th, opts, recv_seq);
 273         return true;
 274 }
 275
 276 static unsigned int
 277 synproxy_tg6(struct sk_buff *skb, const struct xt_action_param *par)
 278 {
 279         const struct xt_synproxy_info *info = par->targinfo;
 280         struct net *net = par->net;
 281         struct synproxy_net *snet = synproxy_pernet(net);
 282         struct synproxy_options opts = {};
 283         struct tcphdr *th, _th;
 284
 285         if (nf_ip6_checksum(skb, par->hooknum, par->thoff, IPPROTO_TCP))
 286                 return NF_DROP;
 287
 288         th = skb_header_pointer(skb, par->thoff, sizeof(_th), &_th);
 289         if (th == NULL)
 290                 return NF_DROP;
 291
 292         if (!synproxy_parse_options(skb, par->thoff, th, &opts))
 293                 return NF_DROP;
 294
 295         if (th->syn && !(th->ack || th->fin || th->rst)) {
 296                 /* Initial SYN from client */
 297                 this_cpu_inc(snet->stats->syn_received);
 298
 299                 if (th->ece && th->cwr)
 300                         opts.options |= XT_SYNPROXY_OPT_ECN;
 301
 302                 opts.options &= info->options;
 303                 if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
 304                         synproxy_init_timestamp_cookie(info, &opts);
 305                 else
 306                         opts.options &= ~(XT_SYNPROXY_OPT_WSCALE |
 307                                           XT_SYNPROXY_OPT_SACK_PERM |
 308                                           XT_SYNPROXY_OPT_ECN);
 309
 310                 synproxy_send_client_synack(net, skb, th, &opts);
 311                 return NF_DROP;
 312
 313         } else if (th->ack && !(th->fin || th->rst || th->syn)) {
 314                 /* ACK from client */
 315                 synproxy_recv_client_ack(net, skb, th, &opts, ntohl(th->seq));
 316                 return NF_DROP;
 317         }
 318
 319         return XT_CONTINUE;
 320 }
 321
 322 static unsigned int ipv6_synproxy_hook(void *priv,
 323                                        struct sk_buff *skb,
 324                                        const struct nf_hook_state *nhs)
 325 {
 326         struct net *net = nhs->net;
 327         struct synproxy_net *snet = synproxy_pernet(net);
 328         enum ip_conntrack_info ctinfo;
 329         struct nf_conn *ct;
 330         struct nf_conn_synproxy *synproxy;
 331         struct synproxy_options opts = {};
 332         const struct ip_ct_tcp *state;
 333         struct tcphdr *th, _th;
 334         __be16 frag_off;
 335         u8 nexthdr;
 336         int thoff;
 337
 338         ct = nf_ct_get(skb, &ctinfo);
 339         if (ct == NULL)
 340                 return NF_ACCEPT;
 341
 342         synproxy = nfct_synproxy(ct);
 343         if (synproxy == NULL)
 344                 return NF_ACCEPT;
 345
 346         if (nf_is_loopback_packet(skb))
 347                 return NF_ACCEPT;
 348
 349         nexthdr = ipv6_hdr(skb)->nexthdr;
 350         thoff = ipv6_skip_exthdr(skb, sizeof(struct ipv6hdr), &nexthdr,
 351                                  &frag_off);
 352         if (thoff < 0)
 353                 return NF_ACCEPT;
 354
 355         th = skb_header_pointer(skb, thoff, sizeof(_th), &_th);
 356         if (th == NULL)
 357                 return NF_DROP;
 358
 359         state = &ct->proto.tcp;
 360         switch (state->state) {
 361         case TCP_CONNTRACK_CLOSE:
 362                 if (th->rst && !test_bit(IPS_SEEN_REPLY_BIT, &ct->status)) {
 363                         nf_ct_seqadj_init(ct, ctinfo, synproxy->isn -
 364                                                       ntohl(th->seq) + 1);
 365                         break;
 366                 }
 367
 368                 if (!th->syn || th->ack ||
 369                     CTINFO2DIR(ctinfo) != IP_CT_DIR_ORIGINAL)
 370                         break;
 371
 372                 /* Reopened connection - reset the sequence number and timestamp
 373                  * adjustments, they will get initialized once the connection is
 374                  * reestablished.
 375                  */
 376                 nf_ct_seqadj_init(ct, ctinfo, 0);
 377                 synproxy->tsoff = 0;
 378                 this_cpu_inc(snet->stats->conn_reopened);
 379
 380                 /* fall through */
 381         case TCP_CONNTRACK_SYN_SENT:
 382                 if (!synproxy_parse_options(skb, thoff, th, &opts))
 383                         return NF_DROP;
 384
 385                 if (!th->syn && th->ack &&
 386                     CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL) {
 387                         /* Keep-Alives are sent with SEG.SEQ = SND.NXT-1,
 388                          * therefore we need to add 1 to make the SYN sequence
 389                          * number match the one of first SYN.
 390                          */
 391                         if (synproxy_recv_client_ack(net, skb, th, &opts,
 392                                                      ntohl(th->seq) + 1))
 393                                 this_cpu_inc(snet->stats->cookie_retrans);
 394
 395                         return NF_DROP;
 396                 }
 397
 398                 synproxy->isn = ntohl(th->ack_seq);
 399                 if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
 400                         synproxy->its = opts.tsecr;
 401                 break;
 402         case TCP_CONNTRACK_SYN_RECV:
 403                 if (!th->syn || !th->ack)
 404                         break;
 405
 406                 if (!synproxy_parse_options(skb, thoff, th, &opts))
 407                         return NF_DROP;
 408
 409                 if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
 410                         synproxy->tsoff = opts.tsval - synproxy->its;
 411
 412                 opts.options &= ~(XT_SYNPROXY_OPT_MSS |
 413                                   XT_SYNPROXY_OPT_WSCALE |
 414                                   XT_SYNPROXY_OPT_SACK_PERM);
 415
 416                 swap(opts.tsval, opts.tsecr);
 417                 synproxy_send_server_ack(net, state, skb, th, &opts);
 418
 419                 nf_ct_seqadj_init(ct, ctinfo, synproxy->isn - ntohl(th->seq));
 420
 421                 swap(opts.tsval, opts.tsecr);
 422                 synproxy_send_client_ack(net, skb, th, &opts);
 423
 424                 consume_skb(skb);
 425                 return NF_STOLEN;
 426         default:
 427                 break;
 428         }
 429
 430         synproxy_tstamp_adjust(skb, thoff, th, ct, ctinfo, synproxy);
 431         return NF_ACCEPT;
 432 }
 433
 434 static int synproxy_tg6_check(const struct xt_tgchk_param *par)
 435 {
 436         const struct ip6t_entry *e = par->entryinfo;
 437
 438         if (!(e->ipv6.flags & IP6T_F_PROTO) ||
 439             e->ipv6.proto != IPPROTO_TCP ||
 440             e->ipv6.invflags & XT_INV_PROTO)
 441                 return -EINVAL;
 442
 443         return nf_ct_l3proto_try_module_get(par->family);
 444 }
 445
 446 static void synproxy_tg6_destroy(const struct xt_tgdtor_param *par)
 447 {
 448         nf_ct_l3proto_module_put(par->family);
 449 }
 450
 451 static struct xt_target synproxy_tg6_reg __read_mostly = {
 452         .name           = "SYNPROXY",
 453         .family         = NFPROTO_IPV6,
 454         .hooks          = (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD),
 455         .target         = synproxy_tg6,
 456         .targetsize     = sizeof(struct xt_synproxy_info),
 457         .checkentry     = synproxy_tg6_check,
 458         .destroy        = synproxy_tg6_destroy,
 459         .me             = THIS_MODULE,
 460 };
 461
 462 static struct nf_hook_ops ipv6_synproxy_ops[] __read_mostly = {
 463         {
 464                 .hook           = ipv6_synproxy_hook,
 465                 .pf             = NFPROTO_IPV6,
 466                 .hooknum        = NF_INET_LOCAL_IN,
 467                 .priority       = NF_IP_PRI_CONNTRACK_CONFIRM - 1,
 468         },
 469         {
 470                 .hook           = ipv6_synproxy_hook,
 471                 .pf             = NFPROTO_IPV6,
 472                 .hooknum        = NF_INET_POST_ROUTING,
 473                 .priority       = NF_IP_PRI_CONNTRACK_CONFIRM - 1,
 474         },
 475 };
 476
 477 static int __init synproxy_tg6_init(void)
 478 {
 479         int err;
 480
 481         err = nf_register_hooks(ipv6_synproxy_ops,
 482                                 ARRAY_SIZE(ipv6_synproxy_ops));
 483         if (err < 0)
 484                 goto err1;
 485
 486         err = xt_register_target(&synproxy_tg6_reg);
 487         if (err < 0)
 488                 goto err2;
 489
 490         return 0;
 491
 492 err2:
 493         nf_unregister_hooks(ipv6_synproxy_ops, ARRAY_SIZE(ipv6_synproxy_ops));
 494 err1:
 495         return err;
 496 }
 497
 498 static void __exit synproxy_tg6_exit(void)
 499 {
 500         xt_unregister_target(&synproxy_tg6_reg);
 501         nf_unregister_hooks(ipv6_synproxy_ops, ARRAY_SIZE(ipv6_synproxy_ops));
 502 }
 503
 504 module_init(synproxy_tg6_init);
 505 module_exit(synproxy_tg6_exit);
 506
 507 MODULE_LICENSE("GPL");
 508 MODULE_AUTHOR("Patrick McHardy <kaber@trash.net>");