linux/net/ipv4/syncookies.c
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   1// SPDX-License-Identifier: GPL-2.0-or-later
   2/*
   3 *  Syncookies implementation for the Linux kernel
   4 *
   5 *  Copyright (C) 1997 Andi Kleen
   6 *  Based on ideas by D.J.Bernstein and Eric Schenk.
   7 */
   8
   9#include <linux/tcp.h>
  10#include <linux/slab.h>
  11#include <linux/random.h>
  12#include <linux/siphash.h>
  13#include <linux/kernel.h>
  14#include <linux/export.h>
  15#include <net/secure_seq.h>
  16#include <net/tcp.h>
  17#include <net/route.h>
  18
  19static siphash_key_t syncookie_secret[2] __read_mostly;
  20
  21#define COOKIEBITS 24   /* Upper bits store count */
  22#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
  23
  24/* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
  25 * stores TCP options:
  26 *
  27 * MSB                               LSB
  28 * | 31 ...   6 |  5  |  4   | 3 2 1 0 |
  29 * |  Timestamp | ECN | SACK | WScale  |
  30 *
  31 * When we receive a valid cookie-ACK, we look at the echoed tsval (if
  32 * any) to figure out which TCP options we should use for the rebuilt
  33 * connection.
  34 *
  35 * A WScale setting of '0xf' (which is an invalid scaling value)
  36 * means that original syn did not include the TCP window scaling option.
  37 */
  38#define TS_OPT_WSCALE_MASK      0xf
  39#define TS_OPT_SACK             BIT(4)
  40#define TS_OPT_ECN              BIT(5)
  41/* There is no TS_OPT_TIMESTAMP:
  42 * if ACK contains timestamp option, we already know it was
  43 * requested/supported by the syn/synack exchange.
  44 */
  45#define TSBITS  6
  46#define TSMASK  (((__u32)1 << TSBITS) - 1)
  47
  48static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
  49                       u32 count, int c)
  50{
  51        net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
  52        return siphash_4u32((__force u32)saddr, (__force u32)daddr,
  53                            (__force u32)sport << 16 | (__force u32)dport,
  54                            count, &syncookie_secret[c]);
  55}
  56
  57
  58/*
  59 * when syncookies are in effect and tcp timestamps are enabled we encode
  60 * tcp options in the lower bits of the timestamp value that will be
  61 * sent in the syn-ack.
  62 * Since subsequent timestamps use the normal tcp_time_stamp value, we
  63 * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
  64 */
  65u64 cookie_init_timestamp(struct request_sock *req, u64 now)
  66{
  67        struct inet_request_sock *ireq;
  68        u32 ts, ts_now = tcp_ns_to_ts(now);
  69        u32 options = 0;
  70
  71        ireq = inet_rsk(req);
  72
  73        options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
  74        if (ireq->sack_ok)
  75                options |= TS_OPT_SACK;
  76        if (ireq->ecn_ok)
  77                options |= TS_OPT_ECN;
  78
  79        ts = ts_now & ~TSMASK;
  80        ts |= options;
  81        if (ts > ts_now) {
  82                ts >>= TSBITS;
  83                ts--;
  84                ts <<= TSBITS;
  85                ts |= options;
  86        }
  87        return (u64)ts * (NSEC_PER_SEC / TCP_TS_HZ);
  88}
  89
  90
  91static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
  92                                   __be16 dport, __u32 sseq, __u32 data)
  93{
  94        /*
  95         * Compute the secure sequence number.
  96         * The output should be:
  97         *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
  98         *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
  99         * Where sseq is their sequence number and count increases every
 100         * minute by 1.
 101         * As an extra hack, we add a small "data" value that encodes the
 102         * MSS into the second hash value.
 103         */
 104        u32 count = tcp_cookie_time();
 105        return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
 106                sseq + (count << COOKIEBITS) +
 107                ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
 108                 & COOKIEMASK));
 109}
 110
 111/*
 112 * This retrieves the small "data" value from the syncookie.
 113 * If the syncookie is bad, the data returned will be out of
 114 * range.  This must be checked by the caller.
 115 *
 116 * The count value used to generate the cookie must be less than
 117 * MAX_SYNCOOKIE_AGE minutes in the past.
 118 * The return value (__u32)-1 if this test fails.
 119 */
 120static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
 121                                  __be16 sport, __be16 dport, __u32 sseq)
 122{
 123        u32 diff, count = tcp_cookie_time();
 124
 125        /* Strip away the layers from the cookie */
 126        cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
 127
 128        /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
 129        diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
 130        if (diff >= MAX_SYNCOOKIE_AGE)
 131                return (__u32)-1;
 132
 133        return (cookie -
 134                cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
 135                & COOKIEMASK;   /* Leaving the data behind */
 136}
 137
 138/*
 139 * MSS Values are chosen based on the 2011 paper
 140 * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
 141 * Values ..
 142 *  .. lower than 536 are rare (< 0.2%)
 143 *  .. between 537 and 1299 account for less than < 1.5% of observed values
 144 *  .. in the 1300-1349 range account for about 15 to 20% of observed mss values
 145 *  .. exceeding 1460 are very rare (< 0.04%)
 146 *
 147 *  1460 is the single most frequently announced mss value (30 to 46% depending
 148 *  on monitor location).  Table must be sorted.
 149 */
 150static __u16 const msstab[] = {
 151        536,
 152        1300,
 153        1440,   /* 1440, 1452: PPPoE */
 154        1460,
 155};
 156
 157/*
 158 * Generate a syncookie.  mssp points to the mss, which is returned
 159 * rounded down to the value encoded in the cookie.
 160 */
 161u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
 162                              u16 *mssp)
 163{
 164        int mssind;
 165        const __u16 mss = *mssp;
 166
 167        for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
 168                if (mss >= msstab[mssind])
 169                        break;
 170        *mssp = msstab[mssind];
 171
 172        return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
 173                                     th->source, th->dest, ntohl(th->seq),
 174                                     mssind);
 175}
 176EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
 177
 178__u32 cookie_v4_init_sequence(const struct sk_buff *skb, __u16 *mssp)
 179{
 180        const struct iphdr *iph = ip_hdr(skb);
 181        const struct tcphdr *th = tcp_hdr(skb);
 182
 183        return __cookie_v4_init_sequence(iph, th, mssp);
 184}
 185
 186/*
 187 * Check if a ack sequence number is a valid syncookie.
 188 * Return the decoded mss if it is, or 0 if not.
 189 */
 190int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
 191                      u32 cookie)
 192{
 193        __u32 seq = ntohl(th->seq) - 1;
 194        __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
 195                                            th->source, th->dest, seq);
 196
 197        return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 198}
 199EXPORT_SYMBOL_GPL(__cookie_v4_check);
 200
 201struct sock *tcp_get_cookie_sock(struct sock *sk, struct sk_buff *skb,
 202                                 struct request_sock *req,
 203                                 struct dst_entry *dst, u32 tsoff)
 204{
 205        struct inet_connection_sock *icsk = inet_csk(sk);
 206        struct sock *child;
 207        bool own_req;
 208
 209        child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst,
 210                                                 NULL, &own_req);
 211        if (child) {
 212                refcount_set(&req->rsk_refcnt, 1);
 213                tcp_sk(child)->tsoffset = tsoff;
 214                sock_rps_save_rxhash(child, skb);
 215
 216                if (rsk_drop_req(req)) {
 217                        reqsk_put(req);
 218                        return child;
 219                }
 220
 221                if (inet_csk_reqsk_queue_add(sk, req, child))
 222                        return child;
 223
 224                bh_unlock_sock(child);
 225                sock_put(child);
 226        }
 227        __reqsk_free(req);
 228
 229        return NULL;
 230}
 231EXPORT_SYMBOL(tcp_get_cookie_sock);
 232
 233/*
 234 * when syncookies are in effect and tcp timestamps are enabled we stored
 235 * additional tcp options in the timestamp.
 236 * This extracts these options from the timestamp echo.
 237 *
 238 * return false if we decode a tcp option that is disabled
 239 * on the host.
 240 */
 241bool cookie_timestamp_decode(const struct net *net,
 242                             struct tcp_options_received *tcp_opt)
 243{
 244        /* echoed timestamp, lowest bits contain options */
 245        u32 options = tcp_opt->rcv_tsecr;
 246
 247        if (!tcp_opt->saw_tstamp)  {
 248                tcp_clear_options(tcp_opt);
 249                return true;
 250        }
 251
 252        if (!net->ipv4.sysctl_tcp_timestamps)
 253                return false;
 254
 255        tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;
 256
 257        if (tcp_opt->sack_ok && !net->ipv4.sysctl_tcp_sack)
 258                return false;
 259
 260        if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
 261                return true; /* no window scaling */
 262
 263        tcp_opt->wscale_ok = 1;
 264        tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;
 265
 266        return net->ipv4.sysctl_tcp_window_scaling != 0;
 267}
 268EXPORT_SYMBOL(cookie_timestamp_decode);
 269
 270bool cookie_ecn_ok(const struct tcp_options_received *tcp_opt,
 271                   const struct net *net, const struct dst_entry *dst)
 272{
 273        bool ecn_ok = tcp_opt->rcv_tsecr & TS_OPT_ECN;
 274
 275        if (!ecn_ok)
 276                return false;
 277
 278        if (net->ipv4.sysctl_tcp_ecn)
 279                return true;
 280
 281        return dst_feature(dst, RTAX_FEATURE_ECN);
 282}
 283EXPORT_SYMBOL(cookie_ecn_ok);
 284
 285struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
 286                                            struct sock *sk,
 287                                            struct sk_buff *skb)
 288{
 289        struct tcp_request_sock *treq;
 290        struct request_sock *req;
 291
 292#ifdef CONFIG_MPTCP
 293        if (sk_is_mptcp(sk))
 294                ops = &mptcp_subflow_request_sock_ops;
 295#endif
 296
 297        req = inet_reqsk_alloc(ops, sk, false);
 298        if (!req)
 299                return NULL;
 300
 301        treq = tcp_rsk(req);
 302        treq->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
 303#if IS_ENABLED(CONFIG_MPTCP)
 304        treq->is_mptcp = sk_is_mptcp(sk);
 305        if (treq->is_mptcp) {
 306                int err = mptcp_subflow_init_cookie_req(req, sk, skb);
 307
 308                if (err) {
 309                        reqsk_free(req);
 310                        return NULL;
 311                }
 312        }
 313#endif
 314
 315        return req;
 316}
 317EXPORT_SYMBOL_GPL(cookie_tcp_reqsk_alloc);
 318
 319/* On input, sk is a listener.
 320 * Output is listener if incoming packet would not create a child
 321 *           NULL if memory could not be allocated.
 322 */
 323struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
 324{
 325        struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt;
 326        struct tcp_options_received tcp_opt;
 327        struct inet_request_sock *ireq;
 328        struct tcp_request_sock *treq;
 329        struct tcp_sock *tp = tcp_sk(sk);
 330        const struct tcphdr *th = tcp_hdr(skb);
 331        __u32 cookie = ntohl(th->ack_seq) - 1;
 332        struct sock *ret = sk;
 333        struct request_sock *req;
 334        int full_space, mss;
 335        struct rtable *rt;
 336        __u8 rcv_wscale;
 337        struct flowi4 fl4;
 338        u32 tsoff = 0;
 339
 340        if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst)
 341                goto out;
 342
 343        if (tcp_synq_no_recent_overflow(sk))
 344                goto out;
 345
 346        mss = __cookie_v4_check(ip_hdr(skb), th, cookie);
 347        if (mss == 0) {
 348                __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 349                goto out;
 350        }
 351
 352        __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 353
 354        /* check for timestamp cookie support */
 355        memset(&tcp_opt, 0, sizeof(tcp_opt));
 356        tcp_parse_options(sock_net(sk), skb, &tcp_opt, 0, NULL);
 357
 358        if (tcp_opt.saw_tstamp && tcp_opt.rcv_tsecr) {
 359                tsoff = secure_tcp_ts_off(sock_net(sk),
 360                                          ip_hdr(skb)->daddr,
 361                                          ip_hdr(skb)->saddr);
 362                tcp_opt.rcv_tsecr -= tsoff;
 363        }
 364
 365        if (!cookie_timestamp_decode(sock_net(sk), &tcp_opt))
 366                goto out;
 367
 368        ret = NULL;
 369        req = cookie_tcp_reqsk_alloc(&tcp_request_sock_ops, sk, skb);
 370        if (!req)
 371                goto out;
 372
 373        ireq = inet_rsk(req);
 374        treq = tcp_rsk(req);
 375        treq->rcv_isn           = ntohl(th->seq) - 1;
 376        treq->snt_isn           = cookie;
 377        treq->ts_off            = 0;
 378        treq->txhash            = net_tx_rndhash();
 379        req->mss                = mss;
 380        ireq->ir_num            = ntohs(th->dest);
 381        ireq->ir_rmt_port       = th->source;
 382        sk_rcv_saddr_set(req_to_sk(req), ip_hdr(skb)->daddr);
 383        sk_daddr_set(req_to_sk(req), ip_hdr(skb)->saddr);
 384        ireq->ir_mark           = inet_request_mark(sk, skb);
 385        ireq->snd_wscale        = tcp_opt.snd_wscale;
 386        ireq->sack_ok           = tcp_opt.sack_ok;
 387        ireq->wscale_ok         = tcp_opt.wscale_ok;
 388        ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 389        req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 390        treq->snt_synack        = 0;
 391        treq->tfo_listener      = false;
 392
 393        if (IS_ENABLED(CONFIG_SMC))
 394                ireq->smc_ok = 0;
 395
 396        ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
 397
 398        /* We throwed the options of the initial SYN away, so we hope
 399         * the ACK carries the same options again (see RFC1122 4.2.3.8)
 400         */
 401        RCU_INIT_POINTER(ireq->ireq_opt, tcp_v4_save_options(sock_net(sk), skb));
 402
 403        if (security_inet_conn_request(sk, skb, req)) {
 404                reqsk_free(req);
 405                goto out;
 406        }
 407
 408        req->num_retrans = 0;
 409
 410        /*
 411         * We need to lookup the route here to get at the correct
 412         * window size. We should better make sure that the window size
 413         * hasn't changed since we received the original syn, but I see
 414         * no easy way to do this.
 415         */
 416        flowi4_init_output(&fl4, ireq->ir_iif, ireq->ir_mark,
 417                           RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
 418                           inet_sk_flowi_flags(sk),
 419                           opt->srr ? opt->faddr : ireq->ir_rmt_addr,
 420                           ireq->ir_loc_addr, th->source, th->dest, sk->sk_uid);
 421        security_req_classify_flow(req, flowi4_to_flowi_common(&fl4));
 422        rt = ip_route_output_key(sock_net(sk), &fl4);
 423        if (IS_ERR(rt)) {
 424                reqsk_free(req);
 425                goto out;
 426        }
 427
 428        /* Try to redo what tcp_v4_send_synack did. */
 429        req->rsk_window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
 430        /* limit the window selection if the user enforce a smaller rx buffer */
 431        full_space = tcp_full_space(sk);
 432        if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
 433            (req->rsk_window_clamp > full_space || req->rsk_window_clamp == 0))
 434                req->rsk_window_clamp = full_space;
 435
 436        tcp_select_initial_window(sk, full_space, req->mss,
 437                                  &req->rsk_rcv_wnd, &req->rsk_window_clamp,
 438                                  ireq->wscale_ok, &rcv_wscale,
 439                                  dst_metric(&rt->dst, RTAX_INITRWND));
 440
 441        ireq->rcv_wscale  = rcv_wscale;
 442        ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), &rt->dst);
 443
 444        ret = tcp_get_cookie_sock(sk, skb, req, &rt->dst, tsoff);
 445        /* ip_queue_xmit() depends on our flow being setup
 446         * Normal sockets get it right from inet_csk_route_child_sock()
 447         */
 448        if (ret)
 449                inet_sk(ret)->cork.fl.u.ip4 = fl4;
 450out:    return ret;
 451}
 452