-/* $OpenBSD: ip_id.c,v 1.2 1999/08/26 13:37:01 provos Exp $ */
-
/*
- * Copyright 1998 Niels Provos <provos@citi.umich.edu>
- * All rights reserved.
+ * Copyright (c) 2002-2013 Apple Inc. All rights reserved.
+ *
+ * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
+ *
+ * This file contains Original Code and/or Modifications of Original Code
+ * as defined in and that are subject to the Apple Public Source License
+ * Version 2.0 (the 'License'). You may not use this file except in
+ * compliance with the License. The rights granted to you under the License
+ * may not be used to create, or enable the creation or redistribution of,
+ * unlawful or unlicensed copies of an Apple operating system, or to
+ * circumvent, violate, or enable the circumvention or violation of, any
+ * terms of an Apple operating system software license agreement.
+ *
+ * Please obtain a copy of the License at
+ * http://www.opensource.apple.com/apsl/ and read it before using this file.
*
- * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using
- * such a mathematical system to generate more random (yet non-repeating)
- * ids to solve the resolver/named problem. But Niels designed the
- * actual system based on the constraints.
+ * The Original Code and all software distributed under the License are
+ * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
+ * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
+ * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
+ * Please see the License for the specific language governing rights and
+ * limitations under the License.
+ *
+ * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
+ */
+
+/*-
+ * Copyright (c) 2008 Michael J. Silbersack.
+ * All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
- * notice, this list of conditions and the following disclaimer.
+ * notice unmodified, this list of conditions, and the following
+ * disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
- * 3. All advertising materials mentioning features or use of this software
- * must display the following acknowledgement:
- * This product includes software developed by Niels Provos.
- * 4. The name of the author may not be used to endorse or promote products
- * derived from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
- *
- * $FreeBSD: src/sys/netinet/ip_id.c,v 1.1.2.1 2001/07/19 06:37:26 kris Exp $
*/
-/*
- * seed = random 15bit
- * n = prime, g0 = generator to n,
- * j = random so that gcd(j,n-1) == 1
- * g = g0^j mod n will be a generator again.
+/*
+ * IP ID generation is a fascinating topic.
+ *
+ * In order to avoid ID collisions during packet reassembly, common sense
+ * dictates that the period between reuse of IDs be as large as possible.
+ * This leads to the classic implementation of a system-wide counter, thereby
+ * ensuring that IDs repeat only once every 2^16 packets.
+ *
+ * Subsequent security researchers have pointed out that using a global
+ * counter makes ID values predictable. This predictability allows traffic
+ * analysis, idle scanning, and even packet injection in specific cases.
+ * These results suggest that IP IDs should be as random as possible.
+ *
+ * The "searchable queues" algorithm used in this IP ID implementation was
+ * proposed by Amit Klein. It is a compromise between the above two
+ * viewpoints that has provable behavior that can be tuned to the user's
+ * requirements.
+ *
+ * The basic concept is that we supplement a standard random number generator
+ * with a queue of the last L IDs that we have handed out to ensure that all
+ * IDs have a period of at least L.
*
- * X[0] = random seed.
- * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
- * with a = 7^(even random) mod m,
- * b = random with gcd(b,m) == 1
- * m = 31104 and a maximal period of m-1.
+ * To efficiently implement this idea, we keep two data structures: a
+ * circular array of IDs of size L and a bitstring of 65536 bits.
*
- * The transaction id is determined by:
- * id[n] = seed xor (g^X[n] mod n)
+ * To start, we ask the RNG for a new ID. A quick index into the bitstring
+ * is used to determine if this is a recently used value. The process is
+ * repeated until a value is returned that is not in the bitstring.
*
- * Effectivly the id is restricted to the lower 15 bits, thus
- * yielding two different cycles by toggling the msb on and off.
- * This avoids reuse issues caused by reseeding.
+ * Having found a usable ID, we remove the ID stored at the current position
+ * in the queue from the bitstring and replace it with our new ID. Our new
+ * ID is then added to the bitstring and the queue pointer is incremented.
+ *
+ * The lower limit of 512 was chosen because there doesn't seem to be much
+ * point to having a smaller value. The upper limit of 32768 was chosen for
+ * two reasons. First, every step above 32768 decreases the entropy. Taken
+ * to an extreme, 65533 would offer 1 bit of entropy. Second, the number of
+ * attempts it takes the algorithm to find an unused ID drastically
+ * increases, killing performance. The default value of 4096 was chosen
+ * because it provides a good tradeoff between randomness and non-repetition,
+ * while taking performance into account.
+ *
+ * With L=4096, the queue will use 8K of memory. The bitstring always uses
+ * 8K of memory (2^16/8). This yields to around 7% ID collisions. No memory
+ * is allocated until the use of random ids is enabled.
*/
#include <sys/param.h>
#include <sys/time.h>
#include <sys/kernel.h>
#include <sys/random.h>
+#include <sys/protosw.h>
+#include <sys/bitstring.h>
+#include <kern/locks.h>
+#include <net/if_var.h>
+#include <netinet/in.h>
+#include <netinet/in_var.h>
+#include <netinet/ip_var.h>
+#include <dev/random/randomdev.h>
-#if RANDOM_IP_ID
-#define RU_OUT 180 /* Time after wich will be reseeded */
-#define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */
-#define RU_GEN 2 /* Starting generator */
-#define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */
-#define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */
-#define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */
-
-#define PFAC_N 3
-static u_int16_t pfacts[PFAC_N] = {
- 2,
- 3,
- 2729
-};
-
-static u_int16_t ru_x;
-static u_int16_t ru_seed, ru_seed2;
-static u_int16_t ru_a, ru_b;
-static u_int16_t ru_g;
-static u_int16_t ru_counter = 0;
-static u_int16_t ru_msb = 0;
-static time_t ru_reseed;
-static u_int32_t tmp; /* Storage for unused random */
-
-static u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t);
-static void ip_initid(void);
-u_int16_t ip_randomid(void);
-
-extern u_short ip_id;
-extern int ip_use_randomid;
/*
- * Do a fast modular exponation, returned value will be in the range
- * of 0 - (mod-1)
+ * Size of L (see comments above on the lower and upper limits.)
*/
+#define ARRAY_SIZE (4096)
+
+static uint16_t *id_array = NULL;
+static bitstr_t *id_bits = NULL;
+static uint32_t array_ptr = 0;
+static uint32_t random_id_statistics = 0;
+static uint64_t random_id_collisions = 0;
+static uint64_t random_id_total = 0;
+
+decl_lck_mtx_data(static, ipid_lock);
+static lck_attr_t *ipid_lock_attr;
+static lck_grp_t *ipid_lock_grp;
+static lck_grp_attr_t *ipid_lock_grp_attr;
+
+SYSCTL_UINT(_net_inet_ip, OID_AUTO, random_id_statistics,
+ CTLFLAG_RW | CTLFLAG_LOCKED, &random_id_statistics, 0,
+ "Enable IP ID statistics");
+SYSCTL_QUAD(_net_inet_ip, OID_AUTO, random_id_collisions,
+ CTLFLAG_RD | CTLFLAG_LOCKED, &random_id_collisions,
+ "Count of IP ID collisions");
+SYSCTL_QUAD(_net_inet_ip, OID_AUTO, random_id_total,
+ CTLFLAG_RD | CTLFLAG_LOCKED, &random_id_total,
+ "Count of IP IDs created");
-#ifdef __STDC__
-static u_int16_t
-pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod)
-#else
-static u_int16_t
-pmod(gen, exp, mod)
- u_int16_t gen, exp, mod;
-#endif
-{
- u_int16_t s, t, u;
-
- s = 1;
- t = gen;
- u = exp;
-
- while (u) {
- if (u & 1)
- s = (s*t) % mod;
- u >>= 1;
- t = (t*t) % mod;
- }
- return (s);
-}
-
-/*
- * Initalizes the seed and chooses a suitable generator. Also toggles
- * the msb flag. The msb flag is used to generate two distinct
- * cycles of random numbers and thus avoiding reuse of ids.
- *
- * This function is called from id_randomid() when needed, an
- * application does not have to worry about it.
+/*
+ * Called once from ip_init().
*/
-static void
+void
ip_initid(void)
{
- u_int16_t j, i;
- int noprime = 1;
- struct timeval timenow;
-
- getmicrotime(&timenow);
- ru_x = (tmp & 0xFFFF) % RU_M;
-
- /* 15 bits of random seed */
- ru_seed = (tmp >> 16) & 0x7FFF;
- read_random((void *) &tmp, sizeof(tmp));
- ru_seed2 = tmp & 0x7FFF;
-
- read_random((void *) &tmp, sizeof(tmp));
-
- /* Determine the LCG we use */
- ru_b = (tmp & 0xfffe) | 1;
- ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
- while (ru_b % 3 == 0)
- ru_b += 2;
-
- read_random((void *) &tmp, sizeof(tmp));
- j = tmp % RU_N;
- tmp = tmp >> 16;
-
- /*
- * Do a fast gcd(j,RU_N-1), so we can find a j with
- * gcd(j, RU_N-1) == 1, giving a new generator for
- * RU_GEN^j mod RU_N
- */
-
- while (noprime) {
- for (i=0; i<PFAC_N; i++)
- if (j%pfacts[i] == 0)
- break;
-
- if (i>=PFAC_N)
- noprime = 0;
- else
- j = (j+1) % RU_N;
+ VERIFY(id_array == NULL);
+ VERIFY(id_bits == NULL);
+
+ _CASSERT(ARRAY_SIZE >= 512 && ARRAY_SIZE <= 32768);
+
+ ipid_lock_grp_attr = lck_grp_attr_alloc_init();
+ ipid_lock_grp = lck_grp_alloc_init("ipid", ipid_lock_grp_attr);
+ ipid_lock_attr = lck_attr_alloc_init();
+ lck_mtx_init(&ipid_lock, ipid_lock_grp, ipid_lock_attr);
+
+ id_array = (uint16_t *)_MALLOC(ARRAY_SIZE * sizeof (uint16_t),
+ M_TEMP, M_WAITOK | M_ZERO);
+ id_bits = (bitstr_t *)_MALLOC(bitstr_size(65536), M_TEMP,
+ M_WAITOK | M_ZERO);
+ if (id_array == NULL || id_bits == NULL) {
+ /* Just in case; neither or both. */
+ if (id_array != NULL) {
+ _FREE(id_array, M_TEMP);
+ id_array = NULL;
+ }
+ if (id_bits != NULL) {
+ _FREE(id_bits, M_TEMP);
+ id_bits = NULL;
+ }
}
-
- ru_g = pmod(RU_GEN,j,RU_N);
- ru_counter = 0;
-
- ru_reseed = timenow.tv_sec + RU_OUT;
- ru_msb = ru_msb == 0x8000 ? 0 : 0x8000;
}
-u_int16_t
+uint16_t
ip_randomid(void)
{
- int i, n;
- struct timeval timenow;
+ uint16_t new_id;
- /* if net.inet.ip.random_id is disabled,
- * reverts to the incrementing ip_id
+ /*
+ * If net.inet.ip.random_id is disabled, revert to incrementing ip_id.
+ * Given that we don't allow the size of the array to change, accessing
+ * id_array and id_bits prior to acquiring the lock below is safe.
*/
+ if (id_array == NULL || ip_use_randomid == 0)
+ return (htons(ip_id++));
- if (ip_use_randomid == 0)
- return htons(ip_id++);
-
-
- getmicrotime(&timenow);
- if (ru_counter >= RU_MAX || timenow.tv_sec > ru_reseed)
- ip_initid();
-
- if (!tmp)
- read_random((void *) &tmp, sizeof(tmp));
-
- /* Skip a random number of ids */
- n = tmp & 0x3; tmp = tmp >> 2;
- if (ru_counter + n >= RU_MAX)
- ip_initid();
-
- for (i = 0; i <= n; i++)
- /* Linear Congruential Generator */
- ru_x = (ru_a*ru_x + ru_b) % RU_M;
-
- ru_counter += i;
-
- return (ru_seed ^ pmod(ru_g,ru_seed2 ^ ru_x,RU_N)) | ru_msb;
+ /*
+ * To avoid a conflict with the zeros that the array is initially
+ * filled with, we never hand out an id of zero. bit_test() below
+ * uses single memory access, therefore no lock is needed.
+ */
+ new_id = 0;
+ do {
+ if (random_id_statistics && new_id != 0)
+ random_id_collisions++;
+ read_random(&new_id, sizeof (new_id));
+ } while (bit_test(id_bits, new_id) || new_id == 0);
+
+ /*
+ * These require serialization to maintain correctness.
+ */
+ lck_mtx_lock_spin(&ipid_lock);
+ bit_clear(id_bits, id_array[array_ptr]);
+ bit_set(id_bits, new_id);
+ id_array[array_ptr] = new_id;
+ if (++array_ptr == ARRAY_SIZE)
+ array_ptr = 0;
+ lck_mtx_unlock(&ipid_lock);
+
+ if (random_id_statistics)
+ random_id_total++;
+
+ return (new_id);
}
-
-#endif /* RANDOM_IP_ID */
projects / apple / xnu.git / blobdiff