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 /*      $OpenBSD: ip6_id.c,v 1.16 2021/03/10 10:21:49 jsg Exp $ */
    2 /*      $NetBSD: ip6_id.c,v 1.7 2003/09/13 21:32:59 itojun Exp $        */
    3 /*      $KAME: ip6_id.c,v 1.8 2003/09/06 13:41:06 itojun Exp $  */
    4 
    5 /*
    6  * Copyright (C) 2003 WIDE Project.
    7  * All rights reserved.
    8  *
    9  * Redistribution and use in source and binary forms, with or without
   10  * modification, are permitted provided that the following conditions
   11  * are met:
   12  * 1. Redistributions of source code must retain the above copyright
   13  *    notice, this list of conditions and the following disclaimer.
   14  * 2. Redistributions in binary form must reproduce the above copyright
   15  *    notice, this list of conditions and the following disclaimer in the
   16  *    documentation and/or other materials provided with the distribution.
   17  * 3. Neither the name of the project nor the names of its contributors
   18  *    may be used to endorse or promote products derived from this software
   19  *    without specific prior written permission.
   20  *
   21  * THIS SOFTWARE IS PROVIDED BY THE PROJECT AND CONTRIBUTORS ``AS IS'' AND
   22  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   23  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   24  * ARE DISCLAIMED.  IN NO EVENT SHALL THE PROJECT OR CONTRIBUTORS BE LIABLE
   25  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   26  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   27  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   28  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   29  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   30  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   31  * SUCH DAMAGE.
   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 
   64 /*
   65  * seed = random (bits - 1) bit
   66  * n = prime, g0 = generator to n,
   67  * j = random so that gcd(j,n-1) == 1
   68  * g = g0^j mod n will be a generator again.
   69  *
   70  * X[0] = random seed.
   71  * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
   72  * with a = 7^(even random) mod m,
   73  *      b = random with gcd(b,m) == 1
   74  *      m = constant and a maximal period of m-1.
   75  *
   76  * The transaction id is determined by:
   77  * id[n] = seed xor (g^X[n] mod n)
   78  *
   79  * Effectivly the id is restricted to the lower (bits - 1) bits, thus
   80  * yielding two different cycles by toggling the msb on and off.
   81  * This avoids reuse issues caused by reseeding.
   82  */
   83 
   84 #include <sys/param.h>
   85 #include <sys/kernel.h>
   86 #include <sys/mbuf.h>
   87 #include <sys/socket.h>
   88 #include <sys/systm.h>
   89 
   90 #include <netinet/in.h>
   91 #include <netinet/ip6.h>
   92 #include <netinet6/ip6_var.h>
   93 
   94 struct randomtab {
   95         const int       ru_bits; /* resulting bits */
   96         const long      ru_out; /* Time after which will be reseeded */
   97         const u_int32_t ru_max; /* Uniq cycle, avoid blackjack prediction */
   98         const u_int32_t ru_gen; /* Starting generator */
   99         const u_int32_t ru_n;   /* ru_n: prime, ru_n - 1: product of pfacts[] */
  100         const u_int32_t ru_agen; /* determine ru_a as ru_agen^(2*rand) */
  101         const u_int32_t ru_m;   /* ru_m = 2^x*3^y */
  102         const u_int32_t pfacts[4];      /* factors of ru_n */
  103 
  104         u_int32_t ru_counter;
  105         u_int32_t ru_msb;
  106 
  107         u_int32_t ru_x;
  108         u_int32_t ru_seed, ru_seed2;
  109         u_int32_t ru_a, ru_b;
  110         u_int32_t ru_g;
  111         long ru_reseed;
  112 };
  113 
  114 static struct randomtab randomtab_20 = {
  115         20,                     /* resulting bits */
  116         180,                    /* Time after which will be reseeded */
  117         200000,                 /* Uniq cycle, avoid blackjack prediction */
  118         2,                      /* Starting generator */
  119         524269,                 /* RU_N-1 = 2^2*3^2*14563 */
  120         7,                      /* determine ru_a as RU_AGEN^(2*rand) */
  121         279936,                 /* RU_M = 2^7*3^7 - don't change */
  122         { 2, 3, 14563, 0 },     /* factors of ru_n */
  123 };
  124 
  125 u_int32_t ip6id_pmod(u_int32_t, u_int32_t, u_int32_t);
  126 void ip6id_initid(struct randomtab *);
  127 u_int32_t ip6id_randomid(struct randomtab *);
  128 
  129 /*
  130  * Do a fast modular exponation, returned value will be in the range
  131  * of 0 - (mod-1)
  132  */
  133 
  134 u_int32_t
  135 ip6id_pmod(u_int32_t gen, u_int32_t expo, u_int32_t mod)
  136 {
  137         u_int64_t s, t, u;
  138 
  139         s = 1;
  140         t = gen;
  141         u = expo;
  142 
  143         while (u) {
  144                 if (u & 1)
  145                         s = (s * t) % mod;
  146                 u >>= 1;
  147                 t = (t * t) % mod;
  148         }
  149         return (s);
  150 }
  151 
  152 /*
  153  * Initializes the seed and chooses a suitable generator. Also toggles
  154  * the msb flag. The msb flag is used to generate two distinct
  155  * cycles of random numbers and thus avoiding reuse of ids.
  156  *
  157  * This function is called from id_randomid() when needed, an
  158  * application does not have to worry about it.
  159  */
  160 void
  161 ip6id_initid(struct randomtab *p)
  162 {
  163         u_int32_t j, i;
  164         int noprime = 1;
  165 
  166         p->ru_x = arc4random_uniform(p->ru_m);
  167 
  168         /* (bits - 1) bits of random seed */
  169         p->ru_seed = arc4random() & (~0U >> (32 - p->ru_bits + 1));
  170         p->ru_seed2 = arc4random() & (~0U >> (32 - p->ru_bits + 1));
  171 
  172         /* Determine the LCG we use */
  173         p->ru_b = (arc4random() & (~0U >> (32 - p->ru_bits))) | 1;
  174         p->ru_a = ip6id_pmod(p->ru_agen,
  175             (arc4random() & (~0U >> (32 - p->ru_bits))) & (~1U), p->ru_m);
  176         while (p->ru_b % 3 == 0)
  177                 p->ru_b += 2;
  178 
  179         j = arc4random_uniform(p->ru_n);
  180 
  181         /*
  182          * Do a fast gcd(j, RU_N - 1), so we can find a j with
  183          * gcd(j, RU_N - 1) == 1, giving a new generator for
  184          * RU_GEN^j mod RU_N
  185          */
  186         while (noprime) {
  187                 for (i = 0; p->pfacts[i] > 0; i++)
  188                         if (j % p->pfacts[i] == 0)
  189                                 break;
  190 
  191                 if (p->pfacts[i] == 0)
  192                         noprime = 0;
  193                 else
  194                         j = (j + 1) % p->ru_n;
  195         }
  196 
  197         p->ru_g = ip6id_pmod(p->ru_gen, j, p->ru_n);
  198         p->ru_counter = 0;
  199 
  200         p->ru_reseed = getuptime() + p->ru_out;
  201         p->ru_msb = p->ru_msb ? 0 : (1U << (p->ru_bits - 1));
  202 }
  203 
  204 u_int32_t
  205 ip6id_randomid(struct randomtab *p)
  206 {
  207         int i, n;
  208 
  209         if (p->ru_counter >= p->ru_max || getuptime() > p->ru_reseed)
  210                 ip6id_initid(p);
  211 
  212         /* Skip a random number of ids */
  213         n = arc4random() & 0x3;
  214         if (p->ru_counter + n >= p->ru_max)
  215                 ip6id_initid(p);
  216 
  217         for (i = 0; i <= n; i++) {
  218                 /* Linear Congruential Generator */
  219                 p->ru_x = (u_int32_t)((u_int64_t)p->ru_a * p->ru_x + p->ru_b) % p->ru_m;
  220         }
  221 
  222         p->ru_counter += i;
  223 
  224         return (p->ru_seed ^ ip6id_pmod(p->ru_g, p->ru_seed2 + p->ru_x, p->ru_n)) |
  225             p->ru_msb;
  226 }
  227 
  228 u_int32_t
  229 ip6_randomflowlabel(void)
  230 {
  231         return ip6id_randomid(&randomtab_20) & 0xfffff;
  232 }
  233 

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