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 <net/tcp.h>
  19
  20 /* Timestamps: lowest 9 bits store TCP options */
  21 #define TSBITS 9
  22 #define TSMASK (((__u32)1 << TSBITS) - 1)
  23
  24 extern int sysctl_tcp_syncookies;
  25
  26 __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
  27 EXPORT_SYMBOL(syncookie_secret);
  28
  29 static __init int init_syncookies(void)
  30 {
  31         get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
  32         return 0;
  33 }
  34 __initcall(init_syncookies);
  35
  36 #define COOKIEBITS 24   /* Upper bits store count */
  37 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
  38
  39 static DEFINE_PER_CPU(__u32, cookie_scratch)[16 + 5 + SHA_WORKSPACE_WORDS];
  40
  41 static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
  42                        u32 count, int c)
  43 {
  44         __u32 *tmp = __get_cpu_var(cookie_scratch);
  45
  46         memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
  47         tmp[0] = (__force u32)saddr;
  48         tmp[1] = (__force u32)daddr;
  49         tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
  50         tmp[3] = count;
  51         sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
  52
  53         return tmp[17];
  54 }
  55
  56
  57 /*
  58  * when syncookies are in effect and tcp timestamps are enabled we encode
  59  * tcp options in the lowest 9 bits of the timestamp value that will be
  60  * sent in the syn-ack.
  61  * Since subsequent timestamps use the normal tcp_time_stamp value, we
  62  * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
  63  */
  64 __u32 cookie_init_timestamp(struct request_sock *req)
  65 {
  66         struct inet_request_sock *ireq;
  67         u32 ts, ts_now = tcp_time_stamp;
  68         u32 options = 0;
  69
  70         ireq = inet_rsk(req);
  71         if (ireq->wscale_ok) {
  72                 options = ireq->snd_wscale;
  73                 options |= ireq->rcv_wscale << 4;
  74         }
  75         options |= ireq->sack_ok << 8;
  76
  77         ts = ts_now & ~TSMASK;
  78         ts |= options;
  79         if (ts > ts_now) {
  80                 ts >>= TSBITS;
  81                 ts--;
  82                 ts <<= TSBITS;
  83                 ts |= options;
  84         }
  85         return ts;
  86 }
  87
  88
  89 static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
  90                                    __be16 dport, __u32 sseq, __u32 count,
  91                                    __u32 data)
  92 {
  93         /*
  94          * Compute the secure sequence number.
  95          * The output should be:
  96          *   HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
  97          *      + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
  98          * Where sseq is their sequence number and count increases every
  99          * minute by 1.
 100          * As an extra hack, we add a small "data" value that encodes the
 101          * MSS into the second hash value.
 102          */
 103
 104         return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
 105                 sseq + (count << COOKIEBITS) +
 106                 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
 107                  & COOKIEMASK));
 108 }
 109
 110 /*
 111  * This retrieves the small "data" value from the syncookie.
 112  * If the syncookie is bad, the data returned will be out of
 113  * range.  This must be checked by the caller.
 114  *
 115  * The count value used to generate the cookie must be within
 116  * "maxdiff" if the current (passed-in) "count".  The return value
 117  * is (__u32)-1 if this test fails.
 118  */
 119 static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
 120                                   __be16 sport, __be16 dport, __u32 sseq,
 121                                   __u32 count, __u32 maxdiff)
 122 {
 123         __u32 diff;
 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 >= maxdiff)
 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  * This table has to be sorted and terminated with (__u16)-1.
 140  * XXX generate a better table.
 141  * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
 142  */
 143 static __u16 const msstab[] = {
 144         64 - 1,
 145         256 - 1,
 146         512 - 1,
 147         536 - 1,
 148         1024 - 1,
 149         1440 - 1,
 150         1460 - 1,
 151         4312 - 1,
 152         (__u16)-1
 153 };
 154 /* The number doesn't include the -1 terminator */
 155 #define NUM_MSS (ARRAY_SIZE(msstab) - 1)
 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  */
 161 __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
 162 {
 163         struct tcp_sock *tp = tcp_sk(sk);
 164         const struct iphdr *iph = ip_hdr(skb);
 165         const struct tcphdr *th = tcp_hdr(skb);
 166         int mssind;
 167         const __u16 mss = *mssp;
 168
 169         tp->last_synq_overflow = jiffies;
 170
 171         /* XXX sort msstab[] by probability?  Binary search? */
 172         for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
 173                 ;
 174         *mssp = msstab[mssind] + 1;
 175
 176         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
 177
 178         return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
 179                                      th->source, th->dest, ntohl(th->seq),
 180                                      jiffies / (HZ * 60), mssind);
 181 }
 182
 183 /*
 184  * This (misnamed) value is the age of syncookie which is permitted.
 185  * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
 186  * sysctl_tcp_retries1. It's a rather complicated formula (exponential
 187  * backoff) to compute at runtime so it's currently hardcoded here.
 188  */
 189 #define COUNTER_TRIES 4
 190 /*
 191  * Check if a ack sequence number is a valid syncookie.
 192  * Return the decoded mss if it is, or 0 if not.
 193  */
 194 static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
 195 {
 196         const struct iphdr *iph = ip_hdr(skb);
 197         const struct tcphdr *th = tcp_hdr(skb);
 198         __u32 seq = ntohl(th->seq) - 1;
 199         __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
 200                                             th->source, th->dest, seq,
 201                                             jiffies / (HZ * 60),
 202                                             COUNTER_TRIES);
 203
 204         return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
 205 }
 206
 207 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
 208                                            struct request_sock *req,
 209                                            struct dst_entry *dst)
 210 {
 211         struct inet_connection_sock *icsk = inet_csk(sk);
 212         struct sock *child;
 213
 214         child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
 215         if (child)
 216                 inet_csk_reqsk_queue_add(sk, req, child);
 217         else
 218                 reqsk_free(req);
 219
 220         return child;
 221 }
 222
 223
 224 /*
 225  * when syncookies are in effect and tcp timestamps are enabled we stored
 226  * additional tcp options in the timestamp.
 227  * This extracts these options from the timestamp echo.
 228  *
 229  * The lowest 4 bits are for snd_wscale
 230  * The next 4 lsb are for rcv_wscale
 231  * The next lsb is for sack_ok
 232  */
 233 void cookie_check_timestamp(struct tcp_options_received *tcp_opt)
 234 {
 235         /* echoed timestamp, 9 lowest bits contain options */
 236         u32 options = tcp_opt->rcv_tsecr & TSMASK;
 237
 238         tcp_opt->snd_wscale = options & 0xf;
 239         options >>= 4;
 240         tcp_opt->rcv_wscale = options & 0xf;
 241
 242         tcp_opt->sack_ok = (options >> 4) & 0x1;
 243
 244         if (tcp_opt->sack_ok)
 245                 tcp_sack_reset(tcp_opt);
 246
 247         if (tcp_opt->snd_wscale || tcp_opt->rcv_wscale)
 248                 tcp_opt->wscale_ok = 1;
 249 }
 250 EXPORT_SYMBOL(cookie_check_timestamp);
 251
 252 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 253                              struct ip_options *opt)
 254 {
 255         struct inet_request_sock *ireq;
 256         struct tcp_request_sock *treq;
 257         struct tcp_sock *tp = tcp_sk(sk);
 258         const struct tcphdr *th = tcp_hdr(skb);
 259         __u32 cookie = ntohl(th->ack_seq) - 1;
 260         struct sock *ret = sk;
 261         struct request_sock *req;
 262         int mss;
 263         struct rtable *rt;
 264         __u8 rcv_wscale;
 265         struct tcp_options_received tcp_opt;
 266
 267         if (!sysctl_tcp_syncookies || !th->ack)
 268                 goto out;
 269
 270         if (time_after(jiffies, tp->last_synq_overflow + TCP_TIMEOUT_INIT) ||
 271             (mss = cookie_check(skb, cookie)) == 0) {
 272                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 273                 goto out;
 274         }
 275
 276         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 277
 278         /* check for timestamp cookie support */
 279         memset(&tcp_opt, 0, sizeof(tcp_opt));
 280         tcp_parse_options(skb, &tcp_opt, 0);
 281
 282         if (tcp_opt.saw_tstamp)
 283                 cookie_check_timestamp(&tcp_opt);
 284
 285         ret = NULL;
 286         req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
 287         if (!req)
 288                 goto out;
 289
 290         if (security_inet_conn_request(sk, skb, req)) {
 291                 reqsk_free(req);
 292                 goto out;
 293         }
 294         ireq = inet_rsk(req);
 295         treq = tcp_rsk(req);
 296         treq->rcv_isn           = ntohl(th->seq) - 1;
 297         treq->snt_isn           = cookie;
 298         req->mss                = mss;
 299         ireq->rmt_port          = th->source;
 300         ireq->loc_addr          = ip_hdr(skb)->daddr;
 301         ireq->rmt_addr          = ip_hdr(skb)->saddr;
 302         ireq->ecn_ok            = 0;
 303         ireq->snd_wscale        = tcp_opt.snd_wscale;
 304         ireq->rcv_wscale        = tcp_opt.rcv_wscale;
 305         ireq->sack_ok           = tcp_opt.sack_ok;
 306         ireq->wscale_ok         = tcp_opt.wscale_ok;
 307         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 308         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 309
 310         /* We throwed the options of the initial SYN away, so we hope
 311          * the ACK carries the same options again (see RFC1122 4.2.3.8)
 312          */
 313         if (opt && opt->optlen) {
 314                 int opt_size = sizeof(struct ip_options) + opt->optlen;
 315
 316                 ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
 317                 if (ireq->opt != NULL && ip_options_echo(ireq->opt, skb)) {
 318                         kfree(ireq->opt);
 319                         ireq->opt = NULL;
 320                 }
 321         }
 322
 323         req->expires    = 0UL;
 324         req->retrans    = 0;
 325
 326         /*
 327          * We need to lookup the route here to get at the correct
 328          * window size. We should better make sure that the window size
 329          * hasn't changed since we received the original syn, but I see
 330          * no easy way to do this.
 331          */
 332         {
 333                 struct flowi fl = { .nl_u = { .ip4_u =
 334                                               { .daddr = ((opt && opt->srr) ?
 335                                                           opt->faddr :
 336                                                           ireq->rmt_addr),
 337                                                 .saddr = ireq->loc_addr,
 338                                                 .tos = RT_CONN_FLAGS(sk) } },
 339                                     .proto = IPPROTO_TCP,
 340                                     .uli_u = { .ports =
 341                                                { .sport = th->dest,
 342                                                  .dport = th->source } } };
 343                 security_req_classify_flow(req, &fl);
 344                 if (ip_route_output_key(&init_net, &rt, &fl)) {
 345                         reqsk_free(req);
 346                         goto out;
 347                 }
 348         }
 349
 350         /* Try to redo what tcp_v4_send_synack did. */
 351         req->window_clamp = tp->window_clamp ? :dst_metric(&rt->u.dst, RTAX_WINDOW);
 352
 353         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 354                                   &req->rcv_wnd, &req->window_clamp,
 355                                   ireq->wscale_ok, &rcv_wscale);
 356
 357         ireq->rcv_wscale  = rcv_wscale;
 358
 359         ret = get_cookie_sock(sk, skb, req, &rt->u.dst);
 360 out:    return ret;
 361 }