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 #include <net/route.h>
  20
  21 /* Timestamps: lowest bits store TCP options */
  22 #define TSBITS 6
  23 #define TSMASK (((__u32)1 << TSBITS) - 1)
  24
  25 extern int sysctl_tcp_syncookies;
  26
  27 __u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS];
  28 EXPORT_SYMBOL(syncookie_secret);
  29
  30 static __init int init_syncookies(void)
  31 {
  32         get_random_bytes(syncookie_secret, sizeof(syncookie_secret));
  33         return 0;
  34 }
  35 __initcall(init_syncookies);
  36
  37 #define COOKIEBITS 24   /* Upper bits store count */
  38 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
  39
  40 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
  41                       ipv4_cookie_scratch);
  42
  43 static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
  44                        u32 count, int c)
  45 {
  46         __u32 *tmp = __get_cpu_var(ipv4_cookie_scratch);
  47
  48         memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
  49         tmp[0] = (__force u32)saddr;
  50         tmp[1] = (__force u32)daddr;
  51         tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
  52         tmp[3] = count;
  53         sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
  54
  55         return tmp[17];
  56 }
  57
  58
  59 /*
  60  * when syncookies are in effect and tcp timestamps are enabled we encode
  61  * tcp options in the lower bits of the timestamp value that will be
  62  * sent in the syn-ack.
  63  * Since subsequent timestamps use the normal tcp_time_stamp value, we
  64  * must make sure that the resulting initial timestamp is <= tcp_time_stamp.
  65  */
  66 __u32 cookie_init_timestamp(struct request_sock *req)
  67 {
  68         struct inet_request_sock *ireq;
  69         u32 ts, ts_now = tcp_time_stamp;
  70         u32 options = 0;
  71
  72         ireq = inet_rsk(req);
  73
  74         options = ireq->wscale_ok ? ireq->snd_wscale : 0xf;
  75         options |= ireq->sack_ok << 4;
  76         options |= ireq->ecn_ok << 5;
  77
  78         ts = ts_now & ~TSMASK;
  79         ts |= options;
  80         if (ts > ts_now) {
  81                 ts >>= TSBITS;
  82                 ts--;
  83                 ts <<= TSBITS;
  84                 ts |= options;
  85         }
  86         return ts;
  87 }
  88
  89
  90 static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
  91                                    __be16 dport, __u32 sseq, __u32 count,
  92                                    __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
 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 within
 117  * "maxdiff" if the current (passed-in) "count".  The return value
 118  * is (__u32)-1 if this test fails.
 119  */
 120 static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
 121                                   __be16 sport, __be16 dport, __u32 sseq,
 122                                   __u32 count, __u32 maxdiff)
 123 {
 124         __u32 diff;
 125
 126         /* Strip away the layers from the cookie */
 127         cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
 128
 129         /* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
 130         diff = (count - (cookie >> COOKIEBITS)) & ((__u32) - 1 >> COOKIEBITS);
 131         if (diff >= maxdiff)
 132                 return (__u32)-1;
 133
 134         return (cookie -
 135                 cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
 136                 & COOKIEMASK;   /* Leaving the data behind */
 137 }
 138
 139 /*
 140  * MSS Values are taken from the 2009 paper
 141  * 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson:
 142  *  - values 1440 to 1460 accounted for 80% of observed mss values
 143  *  - values outside the 536-1460 range are rare (<0.2%).
 144  *
 145  * Table must be sorted.
 146  */
 147 static __u16 const msstab[] = {
 148         64,
 149         512,
 150         536,
 151         1024,
 152         1440,
 153         1460,
 154         4312,
 155         8960,
 156 };
 157
 158 /*
 159  * Generate a syncookie.  mssp points to the mss, which is returned
 160  * rounded down to the value encoded in the cookie.
 161  */
 162 __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
 163 {
 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         tcp_synq_overflow(sk);
 170
 171         for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
 172                 if (mss >= msstab[mssind])
 173                         break;
 174         *mssp = msstab[mssind];
 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 < ARRAY_SIZE(msstab) ? msstab[mssind] : 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 store snd_wscale.
 230  * next 2 bits indicate SACK and ECN support.
 231  *
 232  * return false if we decode an option that should not be.
 233  */
 234 bool cookie_check_timestamp(struct tcp_options_received *tcp_opt, bool *ecn_ok)
 235 {
 236         /* echoed timestamp, lowest bits contain options */
 237         u32 options = tcp_opt->rcv_tsecr & TSMASK;
 238
 239         if (!tcp_opt->saw_tstamp)  {
 240                 tcp_clear_options(tcp_opt);
 241                 return true;
 242         }
 243
 244         if (!sysctl_tcp_timestamps)
 245                 return false;
 246
 247         tcp_opt->sack_ok = (options >> 4) & 0x1;
 248         *ecn_ok = (options >> 5) & 1;
 249         if (*ecn_ok && !sysctl_tcp_ecn)
 250                 return false;
 251
 252         if (tcp_opt->sack_ok && !sysctl_tcp_sack)
 253                 return false;
 254
 255         if ((options & 0xf) == 0xf)
 256                 return true; /* no window scaling */
 257
 258         tcp_opt->wscale_ok = 1;
 259         tcp_opt->snd_wscale = options & 0xf;
 260         return sysctl_tcp_window_scaling != 0;
 261 }
 262 EXPORT_SYMBOL(cookie_check_timestamp);
 263
 264 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 265                              struct ip_options *opt)
 266 {
 267         struct tcp_options_received tcp_opt;
 268         u8 *hash_location;
 269         struct inet_request_sock *ireq;
 270         struct tcp_request_sock *treq;
 271         struct tcp_sock *tp = tcp_sk(sk);
 272         const struct tcphdr *th = tcp_hdr(skb);
 273         __u32 cookie = ntohl(th->ack_seq) - 1;
 274         struct sock *ret = sk;
 275         struct request_sock *req;
 276         int mss;
 277         struct rtable *rt;
 278         __u8 rcv_wscale;
 279         bool ecn_ok = false;
 280
 281         if (!sysctl_tcp_syncookies || !th->ack || th->rst)
 282                 goto out;
 283
 284         if (tcp_synq_no_recent_overflow(sk) ||
 285             (mss = cookie_check(skb, cookie)) == 0) {
 286                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 287                 goto out;
 288         }
 289
 290         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 291
 292         /* check for timestamp cookie support */
 293         memset(&tcp_opt, 0, sizeof(tcp_opt));
 294         tcp_parse_options(skb, &tcp_opt, &hash_location, 0);
 295
 296         if (!cookie_check_timestamp(&tcp_opt, &ecn_ok))
 297                 goto out;
 298
 299         ret = NULL;
 300         req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
 301         if (!req)
 302                 goto out;
 303
 304         ireq = inet_rsk(req);
 305         treq = tcp_rsk(req);
 306         treq->rcv_isn           = ntohl(th->seq) - 1;
 307         treq->snt_isn           = cookie;
 308         req->mss                = mss;
 309         ireq->loc_port          = th->dest;
 310         ireq->rmt_port          = th->source;
 311         ireq->loc_addr          = ip_hdr(skb)->daddr;
 312         ireq->rmt_addr          = ip_hdr(skb)->saddr;
 313         ireq->ecn_ok            = ecn_ok;
 314         ireq->snd_wscale        = tcp_opt.snd_wscale;
 315         ireq->sack_ok           = tcp_opt.sack_ok;
 316         ireq->wscale_ok         = tcp_opt.wscale_ok;
 317         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 318         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 319         treq->snt_synack        = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
 320
 321         /* We throwed the options of the initial SYN away, so we hope
 322          * the ACK carries the same options again (see RFC1122 4.2.3.8)
 323          */
 324         if (opt && opt->optlen) {
 325                 int opt_size = sizeof(struct ip_options_rcu) + opt->optlen;
 326
 327                 ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
 328                 if (ireq->opt != NULL && ip_options_echo(&ireq->opt->opt, skb)) {
 329                         kfree(ireq->opt);
 330                         ireq->opt = NULL;
 331                 }
 332         }
 333
 334         if (security_inet_conn_request(sk, skb, req)) {
 335                 reqsk_free(req);
 336                 goto out;
 337         }
 338
 339         req->expires    = 0UL;
 340         req->retrans    = 0;
 341
 342         /*
 343          * We need to lookup the route here to get at the correct
 344          * window size. We should better make sure that the window size
 345          * hasn't changed since we received the original syn, but I see
 346          * no easy way to do this.
 347          */
 348         {
 349                 struct flowi4 fl4;
 350
 351                 flowi4_init_output(&fl4, 0, sk->sk_mark, RT_CONN_FLAGS(sk),
 352                                    RT_SCOPE_UNIVERSE, IPPROTO_TCP,
 353                                    inet_sk_flowi_flags(sk),
 354                                    (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
 355                                    ireq->loc_addr, th->source, th->dest);
 356                 security_req_classify_flow(req, flowi4_to_flowi(&fl4));
 357                 rt = ip_route_output_key(sock_net(sk), &fl4);
 358                 if (IS_ERR(rt)) {
 359                         reqsk_free(req);
 360                         goto out;
 361                 }
 362         }
 363
 364         /* Try to redo what tcp_v4_send_synack did. */
 365         req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
 366
 367         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 368                                   &req->rcv_wnd, &req->window_clamp,
 369                                   ireq->wscale_ok, &rcv_wscale,
 370                                   dst_metric(&rt->dst, RTAX_INITRWND));
 371
 372         ireq->rcv_wscale  = rcv_wscale;
 373
 374         ret = get_cookie_sock(sk, skb, req, &rt->dst);
 375 out:    return ret;
 376 }