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/netinet/ip_id.c

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    1 
    2 /*-
    3  * Copyright (c) 2008 Michael J. Silbersack.
    4  * All rights reserved.
    5  *
    6  * Redistribution and use in source and binary forms, with or without
    7  * modification, are permitted provided that the following conditions
    8  * are met:
    9  * 1. Redistributions of source code must retain the above copyright
   10  *    notice unmodified, this list of conditions, and the following
   11  *    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  *
   16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
   17  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
   18  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
   19  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
   20  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
   21  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   22  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   23  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   24  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   25  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   26  */
   27 
   28 #include <sys/cdefs.h>
   29 __FBSDID("$FreeBSD: releng/11.0/sys/netinet/ip_id.c 302372 2016-07-06 14:09:49Z nwhitehorn $");
   30 
   31 /*
   32  * IP ID generation is a fascinating topic.
   33  *
   34  * In order to avoid ID collisions during packet reassembly, common sense
   35  * dictates that the period between reuse of IDs be as large as possible.
   36  * This leads to the classic implementation of a system-wide counter, thereby
   37  * ensuring that IDs repeat only once every 2^16 packets.
   38  *
   39  * Subsequent security researchers have pointed out that using a global
   40  * counter makes ID values predictable.  This predictability allows traffic
   41  * analysis, idle scanning, and even packet injection in specific cases.
   42  * These results suggest that IP IDs should be as random as possible.
   43  *
   44  * The "searchable queues" algorithm used in this IP ID implementation was
   45  * proposed by Amit Klein.  It is a compromise between the above two
   46  * viewpoints that has provable behavior that can be tuned to the user's
   47  * requirements.
   48  *
   49  * The basic concept is that we supplement a standard random number generator
   50  * with a queue of the last L IDs that we have handed out to ensure that all
   51  * IDs have a period of at least L.
   52  *
   53  * To efficiently implement this idea, we keep two data structures: a
   54  * circular array of IDs of size L and a bitstring of 65536 bits.
   55  *
   56  * To start, we ask the RNG for a new ID.  A quick index into the bitstring
   57  * is used to determine if this is a recently used value.  The process is
   58  * repeated until a value is returned that is not in the bitstring.
   59  *
   60  * Having found a usable ID, we remove the ID stored at the current position
   61  * in the queue from the bitstring and replace it with our new ID.  Our new
   62  * ID is then added to the bitstring and the queue pointer is incremented.
   63  *
   64  * The lower limit of 512 was chosen because there doesn't seem to be much
   65  * point to having a smaller value.  The upper limit of 32768 was chosen for
   66  * two reasons.  First, every step above 32768 decreases the entropy.  Taken
   67  * to an extreme, 65533 would offer 1 bit of entropy.  Second, the number of
   68  * attempts it takes the algorithm to find an unused ID drastically
   69  * increases, killing performance.  The default value of 8192 was chosen
   70  * because it provides a good tradeoff between randomness and non-repetition.
   71  *
   72  * With L=8192, the queue will use 16K of memory.  The bitstring always
   73  * uses 8K of memory.  No memory is allocated until the use of random ids is
   74  * enabled.
   75  */
   76 
   77 #include <sys/param.h>
   78 #include <sys/systm.h>
   79 #include <sys/counter.h>
   80 #include <sys/kernel.h>
   81 #include <sys/malloc.h>
   82 #include <sys/lock.h>
   83 #include <sys/mutex.h>
   84 #include <sys/random.h>
   85 #include <sys/smp.h>
   86 #include <sys/sysctl.h>
   87 #include <sys/bitstring.h>
   88 
   89 #include <net/vnet.h>
   90 
   91 #include <netinet/in.h>
   92 #include <netinet/ip.h>
   93 #include <netinet/ip_var.h>
   94 
   95 /*
   96  * By default we generate IP ID only for non-atomic datagrams, as
   97  * suggested by RFC6864.  We use per-CPU counter for that, or if
   98  * user wants to, we can turn on random ID generation.
   99  */
  100 static VNET_DEFINE(int, ip_rfc6864) = 1;
  101 static VNET_DEFINE(int, ip_do_randomid) = 0;
  102 #define V_ip_rfc6864            VNET(ip_rfc6864)
  103 #define V_ip_do_randomid        VNET(ip_do_randomid)
  104 
  105 /*
  106  * Random ID state engine.
  107  */
  108 static MALLOC_DEFINE(M_IPID, "ipid", "randomized ip id state");
  109 static VNET_DEFINE(uint16_t *, id_array);
  110 static VNET_DEFINE(bitstr_t *, id_bits);
  111 static VNET_DEFINE(int, array_ptr);
  112 static VNET_DEFINE(int, array_size);
  113 static VNET_DEFINE(int, random_id_collisions);
  114 static VNET_DEFINE(int, random_id_total);
  115 static VNET_DEFINE(struct mtx, ip_id_mtx);
  116 #define V_id_array      VNET(id_array)
  117 #define V_id_bits       VNET(id_bits)
  118 #define V_array_ptr     VNET(array_ptr)
  119 #define V_array_size    VNET(array_size)
  120 #define V_random_id_collisions  VNET(random_id_collisions)
  121 #define V_random_id_total       VNET(random_id_total)
  122 #define V_ip_id_mtx     VNET(ip_id_mtx)
  123 
  124 /*
  125  * Non-random ID state engine is simply a per-cpu counter.
  126  */
  127 static VNET_DEFINE(counter_u64_t, ip_id);
  128 #define V_ip_id         VNET(ip_id)
  129 
  130 static int      sysctl_ip_randomid(SYSCTL_HANDLER_ARGS);
  131 static int      sysctl_ip_id_change(SYSCTL_HANDLER_ARGS);
  132 static void     ip_initid(int);
  133 static uint16_t ip_randomid(void);
  134 static void     ipid_sysinit(void);
  135 static void     ipid_sysuninit(void);
  136 
  137 SYSCTL_DECL(_net_inet_ip);
  138 SYSCTL_PROC(_net_inet_ip, OID_AUTO, random_id,
  139     CTLTYPE_INT | CTLFLAG_VNET | CTLFLAG_RW,
  140     &VNET_NAME(ip_do_randomid), 0, sysctl_ip_randomid, "IU",
  141     "Assign random ip_id values");
  142 SYSCTL_INT(_net_inet_ip, OID_AUTO, rfc6864, CTLFLAG_VNET | CTLFLAG_RW,
  143     &VNET_NAME(ip_rfc6864), 0,
  144     "Use constant IP ID for atomic datagrams");
  145 SYSCTL_PROC(_net_inet_ip, OID_AUTO, random_id_period,
  146     CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_VNET,
  147     &VNET_NAME(array_size), 0, sysctl_ip_id_change, "IU", "IP ID Array size");
  148 SYSCTL_INT(_net_inet_ip, OID_AUTO, random_id_collisions,
  149     CTLFLAG_RD | CTLFLAG_VNET,
  150     &VNET_NAME(random_id_collisions), 0, "Count of IP ID collisions");
  151 SYSCTL_INT(_net_inet_ip, OID_AUTO, random_id_total, CTLFLAG_RD | CTLFLAG_VNET,
  152     &VNET_NAME(random_id_total), 0, "Count of IP IDs created");
  153 
  154 static int
  155 sysctl_ip_randomid(SYSCTL_HANDLER_ARGS)
  156 {
  157         int error, new;
  158 
  159         new = V_ip_do_randomid;
  160         error = sysctl_handle_int(oidp, &new, 0, req);
  161         if (error || req->newptr == NULL)
  162                 return (error);
  163         if (new != 0 && new != 1)
  164                 return (EINVAL);
  165         if (new == V_ip_do_randomid)
  166                 return (0);
  167         if (new == 1 && V_ip_do_randomid == 0)
  168                 ip_initid(8192);
  169         /* We don't free memory when turning random ID off, due to race. */
  170         V_ip_do_randomid = new;
  171         return (0);
  172 }
  173 
  174 static int
  175 sysctl_ip_id_change(SYSCTL_HANDLER_ARGS)
  176 {
  177         int error, new;
  178 
  179         new = V_array_size;
  180         error = sysctl_handle_int(oidp, &new, 0, req);
  181         if (error == 0 && req->newptr) {
  182                 if (new >= 512 && new <= 32768)
  183                         ip_initid(new);
  184                 else
  185                         error = EINVAL;
  186         }
  187         return (error);
  188 }
  189 
  190 static void
  191 ip_initid(int new_size)
  192 {
  193         uint16_t *new_array;
  194         bitstr_t *new_bits;
  195 
  196         new_array = malloc(new_size * sizeof(uint16_t), M_IPID,
  197             M_WAITOK | M_ZERO);
  198         new_bits = malloc(bitstr_size(65536), M_IPID, M_WAITOK | M_ZERO);
  199 
  200         mtx_lock(&V_ip_id_mtx);
  201         if (V_id_array != NULL) {
  202                 free(V_id_array, M_IPID);
  203                 free(V_id_bits, M_IPID);
  204         }
  205         V_id_array = new_array;
  206         V_id_bits = new_bits;
  207         V_array_size = new_size;
  208         V_array_ptr = 0;
  209         V_random_id_collisions = 0;
  210         V_random_id_total = 0;
  211         mtx_unlock(&V_ip_id_mtx);
  212 }
  213 
  214 static uint16_t
  215 ip_randomid(void)
  216 {
  217         uint16_t new_id;
  218 
  219         mtx_lock(&V_ip_id_mtx);
  220         /*
  221          * To avoid a conflict with the zeros that the array is initially
  222          * filled with, we never hand out an id of zero.
  223          */
  224         new_id = 0;
  225         do {
  226                 if (new_id != 0)
  227                         V_random_id_collisions++;
  228                 arc4rand(&new_id, sizeof(new_id), 0);
  229         } while (bit_test(V_id_bits, new_id) || new_id == 0);
  230         bit_clear(V_id_bits, V_id_array[V_array_ptr]);
  231         bit_set(V_id_bits, new_id);
  232         V_id_array[V_array_ptr] = new_id;
  233         V_array_ptr++;
  234         if (V_array_ptr == V_array_size)
  235                 V_array_ptr = 0;
  236         V_random_id_total++;
  237         mtx_unlock(&V_ip_id_mtx);
  238         return (new_id);
  239 }
  240 
  241 void
  242 ip_fillid(struct ip *ip)
  243 {
  244 
  245         /*
  246          * Per RFC6864 Section 4
  247          *
  248          * o  Atomic datagrams: (DF==1) && (MF==0) && (frag_offset==0)
  249          * o  Non-atomic datagrams: (DF==0) || (MF==1) || (frag_offset>0)
  250          */
  251         if (V_ip_rfc6864 && (ip->ip_off & htons(IP_DF)) == htons(IP_DF))
  252                 ip->ip_id = 0;
  253         else if (V_ip_do_randomid)
  254                 ip->ip_id = ip_randomid();
  255         else {
  256                 counter_u64_add(V_ip_id, 1);
  257                 /*
  258                  * There are two issues about this trick, to be kept in mind.
  259                  * 1) We can migrate between counter_u64_add() and next
  260                  *    line, and grab counter from other CPU, resulting in too
  261                  *    quick ID reuse. This is tolerable in our particular case,
  262                  *    since probability of such event is much lower then reuse
  263                  *    of ID due to legitimate overflow, that at modern Internet
  264                  *    speeds happens all the time.
  265                  * 2) We are relying on the fact that counter(9) is based on
  266                  *    UMA_ZONE_PCPU uma(9) zone. We also take only last
  267                  *    sixteen bits of a counter, so we don't care about the
  268                  *    fact that machines with 32-bit word update their counters
  269                  *    not atomically.
  270                  */
  271                 ip->ip_id = htons((*(uint64_t *)zpcpu_get(V_ip_id)) & 0xffff);
  272         }
  273 }
  274 
  275 static void
  276 ipid_sysinit(void)
  277 {
  278         int i;
  279 
  280         mtx_init(&V_ip_id_mtx, "ip_id_mtx", NULL, MTX_DEF);
  281         V_ip_id = counter_u64_alloc(M_WAITOK);
  282         
  283         CPU_FOREACH(i)
  284                 arc4rand(zpcpu_get_cpu(V_ip_id, i), sizeof(uint64_t), 0);
  285 }
  286 VNET_SYSINIT(ip_id, SI_SUB_PROTO_DOMAIN, SI_ORDER_ANY, ipid_sysinit, NULL);
  287 
  288 static void
  289 ipid_sysuninit(void)
  290 {
  291 
  292         if (V_id_array != NULL) {
  293                 free(V_id_array, M_IPID);
  294                 free(V_id_bits, M_IPID);
  295         }
  296         counter_u64_free(V_ip_id);
  297         mtx_destroy(&V_ip_id_mtx);
  298 }
  299 VNET_SYSUNINIT(ip_id, SI_SUB_PROTO_DOMAIN, SI_ORDER_THIRD, ipid_sysuninit, NULL);

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