net/ipv4/syncookies.c

   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
  26 extern int sysctl_tcp_syncookies;
  27
  28 __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
  29 EXPORT_SYMBOL(syncookie_secret);
  30
  31 static __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
  41 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
  42                       ipv4_cookie_scratch);
  43
  44 static 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
  91 static __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  */
 121 static __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  */
 148 static __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  */
 195 static 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
 208 static 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  */
 235 bool 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 >> 4) & 0x1;
 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 }
 263 EXPORT_SYMBOL(cookie_check_timestamp);
 264
 265 struct 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
 282         if (!sysctl_tcp_syncookies || !th->ack || th->rst)
 283                 goto out;
 284
 285         if (tcp_synq_no_recent_overflow(sk) ||
 286             (mss = cookie_check(skb, cookie)) == 0) {
 287                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 288                 goto out;
 289         }
 290
 291         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 292
 293         /* check for timestamp cookie support */
 294         memset(&tcp_opt, 0, sizeof(tcp_opt));
 295         tcp_parse_options(skb, &tcp_opt, &hash_location, 0);
 296
 297         if (!cookie_check_timestamp(&tcp_opt, &ecn_ok))
 298                 goto out;
 299
 300         ret = NULL;
 301         req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
 302         if (!req)
 303                 goto out;
 304
 305         ireq = inet_rsk(req);
 306         treq = tcp_rsk(req);
 307         treq->rcv_isn           = ntohl(th->seq) - 1;
 308         treq->snt_isn           = cookie;
 309         req->mss                = mss;
 310         ireq->loc_port          = th->dest;
 311         ireq->rmt_port          = th->source;
 312         ireq->loc_addr          = ip_hdr(skb)->daddr;
 313         ireq->rmt_addr          = ip_hdr(skb)->saddr;
 314         ireq->ecn_ok            = ecn_ok;
 315         ireq->snd_wscale        = tcp_opt.snd_wscale;
 316         ireq->sack_ok           = tcp_opt.sack_ok;
 317         ireq->wscale_ok         = tcp_opt.wscale_ok;
 318         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 319         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 320         treq->snt_synack        = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
 321
 322         /* We throwed the options of the initial SYN away, so we hope
 323          * the ACK carries the same options again (see RFC1122 4.2.3.8)
 324          */
 325         if (opt && opt->optlen) {
 326                 int opt_size = sizeof(struct ip_options_rcu) + opt->optlen;
 327
 328                 ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
 329                 if (ireq->opt != NULL && ip_options_echo(&ireq->opt->opt, skb)) {
 330                         kfree(ireq->opt);
 331                         ireq->opt = NULL;
 332                 }
 333         }
 334
 335         if (security_inet_conn_request(sk, skb, req)) {
 336                 reqsk_free(req);
 337                 goto out;
 338         }
 339
 340         req->expires    = 0UL;
 341         req->retrans    = 0;
 342
 343         /*
 344          * We need to lookup the route here to get at the correct
 345          * window size. We should better make sure that the window size
 346          * hasn't changed since we received the original syn, but I see
 347          * no easy way to do this.
 348          */
 349         {
 350                 struct flowi4 fl4;
 351
 352                 flowi4_init_output(&fl4, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
 353                                    RT_SCOPE_UNIVERSE, IPPROTO_TCP,
 354                                    inet_sk_flowi_flags(sk),
 355                                    (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
 356                                    ireq->loc_addr, th->source, th->dest);
 357                 security_req_classify_flow(req, flowi4_to_flowi(&fl4));
 358                 rt = ip_route_output_key(sock_net(sk), &fl4);
 359                 if (IS_ERR(rt)) {
 360                         reqsk_free(req);
 361                         goto out;
 362                 }
 363         }
 364
 365         /* Try to redo what tcp_v4_send_synack did. */
 366         req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
 367
 368         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 369                                   &req->rcv_wnd, &req->window_clamp,
 370                                   ireq->wscale_ok, &rcv_wscale,
 371                                   dst_metric(&rt->dst, RTAX_INITRWND));
 372
 373         ireq->rcv_wscale  = rcv_wscale;
 374
 375         ret = get_cookie_sock(sk, skb, req, &rt->dst);
 376 out:    return ret;
 377 }