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(const struct iphdr *iph, const struct tcphdr *th,
 164                               u16 *mssp)
 165 {
 166         int mssind;
 167         const __u16 mss = *mssp;
 168
 169         for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
 170                 if (mss >= msstab[mssind])
 171                         break;
 172         *mssp = msstab[mssind];
 173
 174         return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
 175                                      th->source, th->dest, ntohl(th->seq),
 176                                      jiffies / (HZ * 60), mssind);
 177 }
 178 EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
 179
 180 __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
 181 {
 182         const struct iphdr *iph = ip_hdr(skb);
 183         const struct tcphdr *th = tcp_hdr(skb);
 184
 185         tcp_synq_overflow(sk);
 186         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
 187
 188         return __cookie_v4_init_sequence(iph, th, mssp);
 189 }
 190
 191 /*
 192  * This (misnamed) value is the age of syncookie which is permitted.
 193  * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
 194  * sysctl_tcp_retries1. It's a rather complicated formula (exponential
 195  * backoff) to compute at runtime so it's currently hardcoded here.
 196  */
 197 #define COUNTER_TRIES 4
 198 /*
 199  * Check if a ack sequence number is a valid syncookie.
 200  * Return the decoded mss if it is, or 0 if not.
 201  */
 202 int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
 203                       u32 cookie)
 204 {
 205         __u32 seq = ntohl(th->seq) - 1;
 206         __u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
 207                                             th->source, th->dest, seq,
 208                                             jiffies / (HZ * 60),
 209                                             COUNTER_TRIES);
 210
 211         return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
 212 }
 213 EXPORT_SYMBOL_GPL(__cookie_v4_check);
 214
 215 static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
 216                                            struct request_sock *req,
 217                                            struct dst_entry *dst)
 218 {
 219         struct inet_connection_sock *icsk = inet_csk(sk);
 220         struct sock *child;
 221
 222         child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
 223         if (child)
 224                 inet_csk_reqsk_queue_add(sk, req, child);
 225         else
 226                 reqsk_free(req);
 227
 228         return child;
 229 }
 230
 231
 232 /*
 233  * when syncookies are in effect and tcp timestamps are enabled we stored
 234  * additional tcp options in the timestamp.
 235  * This extracts these options from the timestamp echo.
 236  *
 237  * The lowest 4 bits store snd_wscale.
 238  * next 2 bits indicate SACK and ECN support.
 239  *
 240  * return false if we decode an option that should not be.
 241  */
 242 bool cookie_check_timestamp(struct tcp_options_received *tcp_opt,
 243                         struct net *net, bool *ecn_ok)
 244 {
 245         /* echoed timestamp, lowest bits contain options */
 246         u32 options = tcp_opt->rcv_tsecr & TSMASK;
 247
 248         if (!tcp_opt->saw_tstamp)  {
 249                 tcp_clear_options(tcp_opt);
 250                 return true;
 251         }
 252
 253         if (!sysctl_tcp_timestamps)
 254                 return false;
 255
 256         tcp_opt->sack_ok = (options & (1 << 4)) ? TCP_SACK_SEEN : 0;
 257         *ecn_ok = (options >> 5) & 1;
 258         if (*ecn_ok && !net->ipv4.sysctl_tcp_ecn)
 259                 return false;
 260
 261         if (tcp_opt->sack_ok && !sysctl_tcp_sack)
 262                 return false;
 263
 264         if ((options & 0xf) == 0xf)
 265                 return true; /* no window scaling */
 266
 267         tcp_opt->wscale_ok = 1;
 268         tcp_opt->snd_wscale = options & 0xf;
 269         return sysctl_tcp_window_scaling != 0;
 270 }
 271 EXPORT_SYMBOL(cookie_check_timestamp);
 272
 273 struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 274                              struct ip_options *opt)
 275 {
 276         struct tcp_options_received tcp_opt;
 277         struct inet_request_sock *ireq;
 278         struct tcp_request_sock *treq;
 279         struct tcp_sock *tp = tcp_sk(sk);
 280         const struct tcphdr *th = tcp_hdr(skb);
 281         __u32 cookie = ntohl(th->ack_seq) - 1;
 282         struct sock *ret = sk;
 283         struct request_sock *req;
 284         int mss;
 285         struct rtable *rt;
 286         __u8 rcv_wscale;
 287         bool ecn_ok = false;
 288         struct flowi4 fl4;
 289
 290         if (!sysctl_tcp_syncookies || !th->ack || th->rst)
 291                 goto out;
 292
 293         if (tcp_synq_no_recent_overflow(sk) ||
 294             (mss = __cookie_v4_check(ip_hdr(skb), th, cookie)) == 0) {
 295                 NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
 296                 goto out;
 297         }
 298
 299         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
 300
 301         /* check for timestamp cookie support */
 302         memset(&tcp_opt, 0, sizeof(tcp_opt));
 303         tcp_parse_options(skb, &tcp_opt, 0, NULL);
 304
 305         if (!cookie_check_timestamp(&tcp_opt, sock_net(sk), &ecn_ok))
 306                 goto out;
 307
 308         ret = NULL;
 309         req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
 310         if (!req)
 311                 goto out;
 312
 313         ireq = inet_rsk(req);
 314         treq = tcp_rsk(req);
 315         treq->rcv_isn           = ntohl(th->seq) - 1;
 316         treq->snt_isn           = cookie;
 317         req->mss                = mss;
 318         ireq->loc_port          = th->dest;
 319         ireq->rmt_port          = th->source;
 320         ireq->loc_addr          = ip_hdr(skb)->daddr;
 321         ireq->rmt_addr          = ip_hdr(skb)->saddr;
 322         ireq->ecn_ok            = ecn_ok;
 323         ireq->snd_wscale        = tcp_opt.snd_wscale;
 324         ireq->sack_ok           = tcp_opt.sack_ok;
 325         ireq->wscale_ok         = tcp_opt.wscale_ok;
 326         ireq->tstamp_ok         = tcp_opt.saw_tstamp;
 327         req->ts_recent          = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 328         treq->snt_synack        = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
 329         treq->listener          = NULL;
 330
 331         /* We throwed the options of the initial SYN away, so we hope
 332          * the ACK carries the same options again (see RFC1122 4.2.3.8)
 333          */
 334         if (opt && opt->optlen) {
 335                 int opt_size = sizeof(struct ip_options_rcu) + opt->optlen;
 336
 337                 ireq->opt = kmalloc(opt_size, GFP_ATOMIC);
 338                 if (ireq->opt != NULL && ip_options_echo(&ireq->opt->opt, skb)) {
 339                         kfree(ireq->opt);
 340                         ireq->opt = NULL;
 341                 }
 342         }
 343
 344         if (security_inet_conn_request(sk, skb, req)) {
 345                 reqsk_free(req);
 346                 goto out;
 347         }
 348
 349         req->expires    = 0UL;
 350         req->num_retrans = 0;
 351
 352         /*
 353          * We need to lookup the route here to get at the correct
 354          * window size. We should better make sure that the window size
 355          * hasn't changed since we received the original syn, but I see
 356          * no easy way to do this.
 357          */
 358         flowi4_init_output(&fl4, sk->sk_bound_dev_if, sk->sk_mark,
 359                            RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
 360                            inet_sk_flowi_flags(sk),
 361                            (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
 362                            ireq->loc_addr, th->source, th->dest);
 363         security_req_classify_flow(req, flowi4_to_flowi(&fl4));
 364         rt = ip_route_output_key(sock_net(sk), &fl4);
 365         if (IS_ERR(rt)) {
 366                 reqsk_free(req);
 367                 goto out;
 368         }
 369
 370         /* Try to redo what tcp_v4_send_synack did. */
 371         req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
 372
 373         tcp_select_initial_window(tcp_full_space(sk), req->mss,
 374                                   &req->rcv_wnd, &req->window_clamp,
 375                                   ireq->wscale_ok, &rcv_wscale,
 376                                   dst_metric(&rt->dst, RTAX_INITRWND));
 377
 378         ireq->rcv_wscale  = rcv_wscale;
 379
 380         ret = get_cookie_sock(sk, skb, req, &rt->dst);
 381         /* ip_queue_xmit() depends on our flow being setup
 382          * Normal sockets get it right from inet_csk_route_child_sock()
 383          */
 384         if (ret)
 385                 inet_sk(ret)->cork.fl.u.ip4 = fl4;
 386 out:    return ret;
 387 }