The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/netinet6/ip6_id.c

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    1 /*-
    2  * SPDX-License-Identifier: (BSD-3-Clause AND BSD-2-Clause)
    3  *
    4  * Copyright (C) 2003 WIDE Project.
    5  * All rights reserved.
    6  *
    7  * Redistribution and use in source and binary forms, with or without
    8  * modification, are permitted provided that the following conditions
    9  * are met:
   10  * 1. Redistributions of source code must retain the above copyright
   11  *    notice, this list of conditions and the following disclaimer.
   12  * 2. Redistributions in binary form must reproduce the above copyright
   13  *    notice, this list of conditions and the following disclaimer in the
   14  *    documentation and/or other materials provided with the distribution.
   15  * 3. Neither the name of the project nor the names of its contributors
   16  *    may be used to endorse or promote products derived from this software
   17  *    without specific prior written permission.
   18  *
   19  * THIS SOFTWARE IS PROVIDED BY THE PROJECT AND CONTRIBUTORS ``AS IS'' AND
   20  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   21  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   22  * ARE DISCLAIMED.  IN NO EVENT SHALL THE PROJECT OR CONTRIBUTORS BE LIABLE
   23  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   24  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   25  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   26  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   27  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   28  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   29  * SUCH DAMAGE.
   30  *
   31  *      $KAME: ip6_id.c,v 1.13 2003/09/16 09:11:19 itojun Exp $
   32  */
   33 
   34 /*-
   35  * Copyright 1998 Niels Provos <provos@citi.umich.edu>
   36  * All rights reserved.
   37  *
   38  * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using
   39  * such a mathematical system to generate more random (yet non-repeating)
   40  * ids to solve the resolver/named problem.  But Niels designed the
   41  * actual system based on the constraints.
   42  *
   43  * Redistribution and use in source and binary forms, with or without
   44  * modification, are permitted provided that the following conditions
   45  * are met:
   46  * 1. Redistributions of source code must retain the above copyright
   47  *    notice, this list of conditions and the following disclaimer.
   48  * 2. Redistributions in binary form must reproduce the above copyright
   49  *    notice, this list of conditions and the following disclaimer in the
   50  *    documentation and/or other materials provided with the distribution.
   51  *
   52  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
   53  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
   54  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
   55  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
   56  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
   57  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   58  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   59  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   60  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   61  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   62  *
   63  * $OpenBSD: ip6_id.c,v 1.3 2003/12/12 06:57:12 itojun Exp $
   64  */
   65 
   66 #include <sys/cdefs.h>
   67 __FBSDID("$FreeBSD$");
   68 
   69 /*
   70  * seed = random (bits - 1) bit
   71  * n = prime, g0 = generator to n,
   72  * j = random so that gcd(j,n-1) == 1
   73  * g = g0^j mod n will be a generator again.
   74  *
   75  * X[0] = random seed.
   76  * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
   77  * with a = 7^(even random) mod m,
   78  *      b = random with gcd(b,m) == 1
   79  *      m = constant and a maximal period of m-1.
   80  *
   81  * The transaction id is determined by:
   82  * id[n] = seed xor (g^X[n] mod n)
   83  *
   84  * Effectively the id is restricted to the lower (bits - 1) bits, thus
   85  * yielding two different cycles by toggling the msb on and off.
   86  * This avoids reuse issues caused by reseeding.
   87  */
   88 
   89 #include <sys/types.h>
   90 #include <sys/param.h>
   91 #include <sys/kernel.h>
   92 #include <sys/random.h>
   93 #include <sys/socket.h>
   94 #include <sys/libkern.h>
   95 
   96 #include <net/if.h>
   97 #include <net/route.h>
   98 #include <net/vnet.h>
   99 #include <netinet/in.h>
  100 #include <netinet/ip6.h>
  101 #include <netinet6/ip6_var.h>
  102 
  103 #ifndef INT32_MAX
  104 #define INT32_MAX       0x7fffffffU
  105 #endif
  106 
  107 struct randomtab {
  108         const int       ru_bits; /* resulting bits */
  109         const long      ru_out; /* Time after which will be reseeded */
  110         const u_int32_t ru_max; /* Uniq cycle, avoid blackjack prediction */
  111         const u_int32_t ru_gen; /* Starting generator */
  112         const u_int32_t ru_n;   /* ru_n: prime, ru_n - 1: product of pfacts[] */
  113         const u_int32_t ru_agen; /* determine ru_a as ru_agen^(2*rand) */
  114         const u_int32_t ru_m;   /* ru_m = 2^x*3^y */
  115         const u_int32_t pfacts[4];      /* factors of ru_n */
  116 
  117         u_int32_t ru_counter;
  118         u_int32_t ru_msb;
  119 
  120         u_int32_t ru_x;
  121         u_int32_t ru_seed, ru_seed2;
  122         u_int32_t ru_a, ru_b;
  123         u_int32_t ru_g;
  124         long ru_reseed;
  125 };
  126 
  127 static struct randomtab randomtab_32 = {
  128         32,                     /* resulting bits */
  129         180,                    /* Time after which will be reseeded */
  130         1000000000,             /* Uniq cycle, avoid blackjack prediction */
  131         2,                      /* Starting generator */
  132         2147483629,             /* RU_N-1 = 2^2*3^2*59652323 */
  133         7,                      /* determine ru_a as RU_AGEN^(2*rand) */
  134         1836660096,             /* RU_M = 2^7*3^15 - don't change */
  135         { 2, 3, 59652323, 0 },  /* factors of ru_n */
  136 };
  137 
  138 static struct randomtab randomtab_20 = {
  139         20,                     /* resulting bits */
  140         180,                    /* Time after which will be reseeded */
  141         200000,                 /* Uniq cycle, avoid blackjack prediction */
  142         2,                      /* Starting generator */
  143         524269,                 /* RU_N-1 = 2^2*3^2*14563 */
  144         7,                      /* determine ru_a as RU_AGEN^(2*rand) */
  145         279936,                 /* RU_M = 2^7*3^7 - don't change */
  146         { 2, 3, 14563, 0 },     /* factors of ru_n */
  147 };
  148 
  149 static u_int32_t pmod(u_int32_t, u_int32_t, u_int32_t);
  150 static void initid(struct randomtab *);
  151 static u_int32_t randomid(struct randomtab *);
  152 
  153 /*
  154  * Do a fast modular exponation, returned value will be in the range
  155  * of 0 - (mod-1)
  156  */
  157 static u_int32_t
  158 pmod(u_int32_t gen, u_int32_t expo, u_int32_t mod)
  159 {
  160         u_int64_t s, t, u;
  161 
  162         s = 1;
  163         t = gen;
  164         u = expo;
  165 
  166         while (u) {
  167                 if (u & 1)
  168                         s = (s * t) % mod;
  169                 u >>= 1;
  170                 t = (t * t) % mod;
  171         }
  172         return (s);
  173 }
  174 
  175 /*
  176  * Initializes the seed and chooses a suitable generator. Also toggles
  177  * the msb flag. The msb flag is used to generate two distinct
  178  * cycles of random numbers and thus avoiding reuse of ids.
  179  *
  180  * This function is called from id_randomid() when needed, an
  181  * application does not have to worry about it.
  182  */
  183 static void
  184 initid(struct randomtab *p)
  185 {
  186         u_int32_t j, i;
  187         int noprime = 1;
  188 
  189         p->ru_x = arc4random() % p->ru_m;
  190 
  191         /* (bits - 1) bits of random seed */
  192         p->ru_seed = arc4random() & (~0U >> (32 - p->ru_bits + 1));
  193         p->ru_seed2 = arc4random() & (~0U >> (32 - p->ru_bits + 1));
  194 
  195         /* Determine the LCG we use */
  196         p->ru_b = (arc4random() & (~0U >> (32 - p->ru_bits))) | 1;
  197         p->ru_a = pmod(p->ru_agen,
  198             (arc4random() & (~0U >> (32 - p->ru_bits))) & (~1U), p->ru_m);
  199         while (p->ru_b % 3 == 0)
  200                 p->ru_b += 2;
  201 
  202         j = arc4random() % p->ru_n;
  203 
  204         /*
  205          * Do a fast gcd(j, RU_N - 1), so we can find a j with
  206          * gcd(j, RU_N - 1) == 1, giving a new generator for
  207          * RU_GEN^j mod RU_N
  208          */
  209         while (noprime) {
  210                 for (i = 0; p->pfacts[i] > 0; i++)
  211                         if (j % p->pfacts[i] == 0)
  212                                 break;
  213 
  214                 if (p->pfacts[i] == 0)
  215                         noprime = 0;
  216                 else
  217                         j = (j + 1) % p->ru_n;
  218         }
  219 
  220         p->ru_g = pmod(p->ru_gen, j, p->ru_n);
  221         p->ru_counter = 0;
  222 
  223         p->ru_reseed = time_uptime + p->ru_out;
  224         p->ru_msb = p->ru_msb ? 0 : (1U << (p->ru_bits - 1));
  225 }
  226 
  227 static u_int32_t
  228 randomid(struct randomtab *p)
  229 {
  230         int i, n;
  231 
  232         if (p->ru_counter >= p->ru_max || time_uptime > p->ru_reseed)
  233                 initid(p);
  234 
  235         /* Skip a random number of ids */
  236         n = arc4random() & 0x3;
  237         if (p->ru_counter + n >= p->ru_max)
  238                 initid(p);
  239 
  240         for (i = 0; i <= n; i++) {
  241                 /* Linear Congruential Generator */
  242                 p->ru_x = (u_int32_t)((u_int64_t)p->ru_a * p->ru_x + p->ru_b) % p->ru_m;
  243         }
  244 
  245         p->ru_counter += i;
  246 
  247         return (p->ru_seed ^ pmod(p->ru_g, p->ru_seed2 + p->ru_x, p->ru_n)) |
  248             p->ru_msb;
  249 }
  250 
  251 u_int32_t
  252 ip6_randomid(void)
  253 {
  254 
  255         return randomid(&randomtab_32);
  256 }
  257 
  258 u_int32_t
  259 ip6_randomflowlabel(void)
  260 {
  261 
  262         /*
  263          * It's ok to emit zero flow labels early, before random is available
  264          * (seeded).  RFC 6437:
  265          *
  266          * "A Flow Label of zero is used to indicate packets that have not been
  267          * labeled."
  268          */
  269         if (__predict_false(!is_random_seeded()))
  270                 return (0);
  271 
  272         return randomid(&randomtab_20) & 0xfffff;
  273 }

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