linux/net/ipv4/syncookies.c
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   1/*
   2 *  Syncookies implementation for the Linux kernel
   3 *
   4 *  Copyright (C) 1997 Andi Kleen
   5 *  Based on ideas by D.J.Bernstein and Eric Schenk.
   6 *
   7 *      This program is free software; you can redistribute it and/or
   8 *      modify it under the terms of the GNU General Public License
   9 *      as published by the Free Software Foundation; either version
  10 *      2 of the License, or (at your option) any later version.
  11 */
  12
  13#include <linux/tcp.h>
  14#include <linux/slab.h>
  15#include <linux/random.h>
  16#include <linux/cryptohash.h>
  17#include <linux/kernel.h>
  18#include <linux/export.h>
  19#include <net/tcp.h>
  20#include <net/route.h>
  21
  22/* Timestamps: lowest bits store TCP options */
  23#define TSBITS 6
  24#define TSMASK (((__u32)1 << TSBITS) - 1)
  25
  26extern int sysctl_tcp_syncookies;
  27
  28__u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
  29EXPORT_SYMBOL(syncookie_secret);
  30
  31static __init int init_syncookies(void)
  32{
  33        get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
  34        return 0;
  35}
  36__initcall(init_syncookies);
  37
  38#define COOKIEBITS 24   /* Upper bits store count */
  39#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
  40
  41static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
  42                      ipv4_cookie_scratch);
  43
  44static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
  45                       u32 count, int c)
  46{
  47        __u32 *tmp = __get_cpu_var(ipv4_cookie_scratch);
  48
  49        memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
  50        tmp[0] = (__force u32)saddr;
  51        tmp[1] = (__force u32)daddr;
  52        tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
  53        tmp[3] = count;
  54        sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
  55
  56        return tmp[17];
  57}
  58
  59
  60/*
  61 * when syncookies are in effect and tcp timestamps are enabled we encode
  62 * tcp options in the lower bits of the timestamp value that will be
  63 * sent in the syn-ack.
  64 * Since subsequent timestamps use the normal tcp_time_stamp value, we
  65 * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
  66 */
  67__u32 cookie_init_timestamp(struct request_sock *req)
  68{
  69        struct inet_request_sock *ireq;
  70        u32 ts, ts_now = tcp_time_stamp;
  71        u32 options = 0;
  72
  73        ireq = inet_rsk(req);
  74
  75        options = ireq->wscale_ok ? ireq->snd_wscale : 0xf;
  76        options |= ireq->sack_ok << 4;
  77        options |= ireq->ecn_ok << 5;
  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 ts;
  88}
  89
  90
  91static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
  92                                   __be16 dport, __u32 sseq, __u32 count,
  93                                   __u32 data)
  94{
  95        /*
  96         * Compute the secure sequence number.
  97         * The output should be:
  98         *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
  99         *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
 100         * Where sseq is their sequence number and count increases every
 101         * minute by 1.
 102         * As an extra hack, we add a small "data" value that encodes the
 103         * MSS into the second hash value.
 104         */
 105
 106        return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
 107                sseq + (count << COOKIEBITS) +
 108                ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
 109                 & COOKIEMASK));
 110}
 111
 112/*
 113 * This retrieves the small "data" value from the syncookie.
 114 * If the syncookie is bad, the data returned will be out of
 115 * range.  This must be checked by the caller.
 116 *
 117 * The count value used to generate the cookie must be within
 118 * "maxdiff" if the current (passed-in) "count".  The return value
 119 * is (__u32)-1 if this test fails.
 120 */
 121static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
 122                                  __be16 sport, __be16 dport, __u32 sseq,
 123                                  __u32 count, __u32 maxdiff)
 124{
 125        __u32 diff;
 126
 127        /* Strip away the layers from the cookie */
 128        cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
 129
 130        /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
 131        diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS);
 132        if (diff >= maxdiff)
 133                return (__u32)-1;
 134
 135        return (cookie -
 136                cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
 137                & COOKIEMASK;   /* Leaving the data behind */
 138}
 139
 140/*
 141 * MSS Values are taken from the 2009 paper
 142 * 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson:
 143 *  - values 1440 to 1460 accounted for 80% of observed mss values
 144 *  - values outside the 536-1460 range are rare (<0.2%).
 145 *
 146 * Table must be sorted.
 147 */
 148static __u16 const msstab[] = {
 149        64,
 150        512,
 151        536,
 152        1024,
 153        1440,
 154        1460,
 155        4312,
 156        8960,
 157};
 158
 159/*
 160 * Generate a syncookie.  mssp points to the mss, which is returned
 161 * rounded down to the value encoded in the cookie.
 162 */
 163__u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
 164{
 165        const struct iphdr *iph = ip_hdr(skb);
 166        const struct tcphdr *th = tcp_hdr(skb);
 167        int mssind;
 168        const __u16 mss = *mssp;
 169
 170        tcp_synq_overflow(sk);
 171
 172        for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
 173                if (mss >= msstab[mssind])
 174                        break;
 175        *mssp = msstab[mssind];
 176
 177        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
 178
 179        return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
 180                                     th->source, th->dest, ntohl(th->seq),
 181                                     jiffies / (HZ * 60), mssind);
 182}
 183
 184/*
 185 * This (misnamed) value is the age of syncookie which is permitted.
 186 * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
 187 * sysctl_tcp_retries1. It's a rather complicated formula (exponential
 188 * backoff) to compute at runtime so it's currently hardcoded here.
 189 */
 190#define COUNTER_TRIES 4
 191/*
 192 * Check if a ack sequence number is a valid syncookie.
 193 * Return the decoded mss if it is, or 0 if not.
 194 */
 195static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
 196{
 197        const struct iphdr *iph = ip_hdr(skb);
 198        const struct tcphdr *th = tcp_hdr(skb);
 199        __u32 seq = ntohl(th->seq) - 1;
 200        __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
 201                                            th->source, th->dest, seq,
 202                                            jiffies / (HZ * 60),
 203                                            COUNTER_TRIES);
 204
 205        return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 206}
 207
 208static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
 209                                           struct request_sock *req,
 210                                           struct dst_entry *dst)
 211{
 212        struct inet_connection_sock *icsk = inet_csk(sk);
 213        struct sock *child;
 214
 215        child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
 216        if (child)
 217                inet_csk_reqsk_queue_add(sk, req, child);
 218        else
 219                reqsk_free(req);
 220
 221        return child;
 222}
 223
 224
 225/*
 226 * when syncookies are in effect and tcp timestamps are enabled we stored
 227 * additional tcp options in the timestamp.
 228 * This extracts these options from the timestamp echo.
 229 *
 230 * The lowest 4 bits store snd_wscale.
 231 * next 2 bits indicate SACK and ECN support.
 232 *
 233 * return false if we decode an option that should not be.
 234 */
 235bool cookie_check_timestamp(struct tcp_options_received *tcp_opt, bool *ecn_ok)
 236{
 237        /* echoed timestamp, lowest bits contain options */
 238        u32 options = tcp_opt->rcv_tsecr & TSMASK;
 239
 240        if (!tcp_opt->saw_tstamp)  {
 241                tcp_clear_options(tcp_opt);
 242                return true;
 243        }
 244
 245        if (!sysctl_tcp_timestamps)
 246                return false;
 247
 248        tcp_opt->sack_ok = (options & (1 << 4)) ? TCP_SACK_SEEN : 0;
 249        *ecn_ok = (options >> 5) & 1;
 250        if (*ecn_ok && !sysctl_tcp_ecn)
 251                return false;
 252
 253        if (tcp_opt->sack_ok && !sysctl_tcp_sack)
 254                return false;
 255
 256        if ((options & 0xf) == 0xf)
 257                return true; /* no window scaling */
 258
 259        tcp_opt->wscale_ok = 1;
 260        tcp_opt->snd_wscale = options & 0xf;
 261        return sysctl_tcp_window_scaling != 0;
 262}
 263EXPORT_SYMBOL(cookie_check_timestamp);
 264
 265struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 266                             struct ip_options *opt)
 267{
 268        struct tcp_options_received tcp_opt;
 269        const u8 *hash_location;
 270        struct inet_request_sock *ireq;
 271        struct tcp_request_sock *treq;
 272        struct tcp_sock *tp = tcp_sk(sk);
 273        const struct tcphdr *th = tcp_hdr(skb);
 274        __u32 cookie = ntohl(th->ack_seq) - 1;
 275        struct sock *ret = sk;
 276        struct request_sock *req;
 277        int mss;
 278        struct rtable *rt;
 279        __u8 rcv_wscale;
 280        bool ecn_ok = false;
 281        struct flowi4 fl4;
 282
 283        if (!sysctl_tcp_syncookies || !th->ack || th->rst)
 284                goto out;
 285
 286        if (tcp_synq_no_recent_overflow(sk) ||
 287            (mss = cookie_check(skb, cookie)) == 0) {
 288                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 289                goto out;
 290        }
 291
 292        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 293
 294        /* check for timestamp cookie support */
 295        memset(&tcp_opt, 0, sizeof(tcp_opt));
 296        tcp_parse_options(skb, &tcp_opt, &hash_location, 0, NULL);
 297
 298        if (!cookie_check_timestamp(&tcp_opt, &ecn_ok))
 299                goto out;
 300
 301        ret = NULL;
 302        req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
 303        if (!req)
 304                goto out;
 305
 306        ireq = inet_rsk(req);
 307        treq = tcp_rsk(req);
 308        treq->rcv_isn           = ntohl(th->seq) - 1;
 309        treq->snt_isn           = cookie;
 310        req->mss                = mss;
 311        ireq->loc_port          = th->dest;
 312        ireq->rmt_port          = th->source;
 313        ireq->loc_addr          = ip_hdr(skb)->daddr;
 314        ireq->rmt_addr          = ip_hdr(skb)->saddr;
 315        ireq->ecn_ok            = ecn_ok;
 316        ireq->snd_wscale        = tcp_opt.snd_wscale;
 317        ireq->sack_ok           = tcp_opt.sack_ok;
 318        ireq->wscale_ok         = tcp_opt.wscale_ok;
 319        ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 320        req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 321        treq->snt_synack        = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
 322
 323        /* We throwed the options of the initial SYN away, so we hope
 324         * the ACK carries the same options again (see RFC1122 4.2.3.8)
 325         */
 326        if (opt && opt->optlen) {
 327                int opt_size = sizeof(struct ip_options_rcu) + opt->optlen;
 328
 329                ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
 330                if (ireq->opt != NULL && ip_options_echo(&ireq->opt->opt, skb)) {
 331                        kfree(ireq->opt);
 332                        ireq->opt = NULL;
 333                }
 334        }
 335
 336        if (security_inet_conn_request(sk, skb, req)) {
 337                reqsk_free(req);
 338                goto out;
 339        }
 340
 341        req->expires    = 0UL;
 342        req->retrans    = 0;
 343
 344        /*
 345         * We need to lookup the route here to get at the correct
 346         * window size. We should better make sure that the window size
 347         * hasn't changed since we received the original syn, but I see
 348         * no easy way to do this.
 349         */
 350        flowi4_init_output(&fl4, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
 351                           RT_SCOPE_UNIVERSE, IPPROTO_TCP,
 352                           inet_sk_flowi_flags(sk),
 353                           (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
 354                           ireq->loc_addr, th->source, th->dest);
 355        security_req_classify_flow(req, flowi4_to_flowi(&fl4));
 356        rt = ip_route_output_key(sock_net(sk), &fl4);
 357        if (IS_ERR(rt)) {
 358                reqsk_free(req);
 359                goto out;
 360        }
 361
 362        /* Try to redo what tcp_v4_send_synack did. */
 363        req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
 364
 365        tcp_select_initial_window(tcp_full_space(sk), req->mss,
 366                                  &req->rcv_wnd, &req->window_clamp,
 367                                  ireq->wscale_ok, &rcv_wscale,
 368                                  dst_metric(&rt->dst, RTAX_INITRWND));
 369
 370        ireq->rcv_wscale  = rcv_wscale;
 371
 372        ret = get_cookie_sock(sk, skb, req, &rt->dst);
 373        /* ip_queue_xmit() depends on our flow being setup
 374         * Normal sockets get it right from inet_csk_route_child_sock()
 375         */
 376        if (ret)
 377                inet_sk(ret)->cork.fl.u.ip4 = fl4;
 378out:    return ret;
 379}
 380
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