1 /*- 2 * SPDX-License-Identifier: BSD-2-Clause 3 * 4 * Copyright (c) 2008 Michael J. Silbersack. 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 unmodified, this list of conditions, and the following 12 * disclaimer. 13 * 2. Redistributions in binary form must reproduce the above copyright 14 * notice, this list of conditions and the following disclaimer in the 15 * documentation and/or other materials provided with the distribution. 16 * 17 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 18 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 19 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 20 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 21 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 22 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 23 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 24 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 26 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 27 */ 28 29 /* 30 * IP ID generation is a fascinating topic. 31 * 32 * In order to avoid ID collisions during packet reassembly, common sense 33 * dictates that the period between reuse of IDs be as large as possible. 34 * This leads to the classic implementation of a system-wide counter, thereby 35 * ensuring that IDs repeat only once every 2^16 packets. 36 * 37 * Subsequent security researchers have pointed out that using a global 38 * counter makes ID values predictable. This predictability allows traffic 39 * analysis, idle scanning, and even packet injection in specific cases. 40 * These results suggest that IP IDs should be as random as possible. 41 * 42 * The "searchable queues" algorithm used in this IP ID implementation was 43 * proposed by Amit Klein. It is a compromise between the above two 44 * viewpoints that has provable behavior that can be tuned to the user's 45 * requirements. 46 * 47 * The basic concept is that we supplement a standard random number generator 48 * with a queue of the last L IDs that we have handed out to ensure that all 49 * IDs have a period of at least L. 50 * 51 * To efficiently implement this idea, we keep two data structures: a 52 * circular array of IDs of size L and a bitstring of 65536 bits. 53 * 54 * To start, we ask the RNG for a new ID. A quick index into the bitstring 55 * is used to determine if this is a recently used value. The process is 56 * repeated until a value is returned that is not in the bitstring. 57 * 58 * Having found a usable ID, we remove the ID stored at the current position 59 * in the queue from the bitstring and replace it with our new ID. Our new 60 * ID is then added to the bitstring and the queue pointer is incremented. 61 * 62 * The lower limit of 512 was chosen because there doesn't seem to be much 63 * point to having a smaller value. The upper limit of 32768 was chosen for 64 * two reasons. First, every step above 32768 decreases the entropy. Taken 65 * to an extreme, 65533 would offer 1 bit of entropy. Second, the number of 66 * attempts it takes the algorithm to find an unused ID drastically 67 * increases, killing performance. The default value of 8192 was chosen 68 * because it provides a good tradeoff between randomness and non-repetition. 69 * 70 * With L=8192, the queue will use 16K of memory. The bitstring always 71 * uses 8K of memory. No memory is allocated until the use of random ids is 72 * enabled. 73 */ 74 75 #include <sys/param.h> 76 #include <sys/systm.h> 77 #include <sys/counter.h> 78 #include <sys/kernel.h> 79 #include <sys/malloc.h> 80 #include <sys/lock.h> 81 #include <sys/mutex.h> 82 #include <sys/random.h> 83 #include <sys/smp.h> 84 #include <sys/sysctl.h> 85 #include <sys/bitstring.h> 86 87 #include <net/vnet.h> 88 89 #include <netinet/in.h> 90 #include <netinet/ip.h> 91 #include <netinet/ip_var.h> 92 93 /* 94 * By default we generate IP ID only for non-atomic datagrams, as 95 * suggested by RFC6864. We use per-CPU counter for that, or if 96 * user wants to, we can turn on random ID generation. 97 */ 98 VNET_DEFINE_STATIC(int, ip_rfc6864) = 1; 99 #define V_ip_rfc6864 VNET(ip_rfc6864) 100 101 VNET_DEFINE(int, ip_random_id) = 0; 102 103 /* 104 * Random ID state engine. 105 */ 106 static MALLOC_DEFINE(M_IPID, "ipid", "randomized ip id state"); 107 VNET_DEFINE_STATIC(uint16_t *, id_array); 108 VNET_DEFINE_STATIC(bitstr_t *, id_bits); 109 VNET_DEFINE_STATIC(int, array_ptr); 110 VNET_DEFINE_STATIC(int, array_size); 111 VNET_DEFINE_STATIC(int, random_id_collisions); 112 VNET_DEFINE_STATIC(int, random_id_total); 113 VNET_DEFINE_STATIC(struct mtx, ip_id_mtx); 114 #define V_id_array VNET(id_array) 115 #define V_id_bits VNET(id_bits) 116 #define V_array_ptr VNET(array_ptr) 117 #define V_array_size VNET(array_size) 118 #define V_random_id_collisions VNET(random_id_collisions) 119 #define V_random_id_total VNET(random_id_total) 120 #define V_ip_id_mtx VNET(ip_id_mtx) 121 122 /* 123 * Non-random ID state engine is simply a per-cpu counter. 124 */ 125 VNET_DEFINE_STATIC(counter_u64_t, ip_id); 126 #define V_ip_id VNET(ip_id) 127 128 static int sysctl_ip_random_id(SYSCTL_HANDLER_ARGS); 129 static int sysctl_ip_id_change(SYSCTL_HANDLER_ARGS); 130 static void ip_initid(int); 131 static uint16_t ip_randomid(void); 132 static void ipid_sysinit(void); 133 static void ipid_sysuninit(void); 134 135 SYSCTL_DECL(_net_inet_ip); 136 SYSCTL_PROC(_net_inet_ip, OID_AUTO, random_id, 137 CTLTYPE_INT | CTLFLAG_VNET | CTLFLAG_RW | CTLFLAG_MPSAFE, 138 &VNET_NAME(ip_random_id), 0, sysctl_ip_random_id, "IU", 139 "Assign random ip_id values"); 140 SYSCTL_INT(_net_inet_ip, OID_AUTO, rfc6864, CTLFLAG_VNET | CTLFLAG_RW, 141 &VNET_NAME(ip_rfc6864), 0, 142 "Use constant IP ID for atomic datagrams"); 143 SYSCTL_PROC(_net_inet_ip, OID_AUTO, random_id_period, 144 CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_VNET | CTLFLAG_MPSAFE, 145 &VNET_NAME(array_size), 0, sysctl_ip_id_change, "IU", "IP ID Array size"); 146 SYSCTL_INT(_net_inet_ip, OID_AUTO, random_id_collisions, 147 CTLFLAG_RD | CTLFLAG_VNET, 148 &VNET_NAME(random_id_collisions), 0, "Count of IP ID collisions"); 149 SYSCTL_INT(_net_inet_ip, OID_AUTO, random_id_total, CTLFLAG_RD | CTLFLAG_VNET, 150 &VNET_NAME(random_id_total), 0, "Count of IP IDs created"); 151 152 static int 153 sysctl_ip_random_id(SYSCTL_HANDLER_ARGS) 154 { 155 int error, new; 156 157 new = V_ip_random_id; 158 error = sysctl_handle_int(oidp, &new, 0, req); 159 if (error || req->newptr == NULL) 160 return (error); 161 if (new != 0 && new != 1) 162 return (EINVAL); 163 if (new == V_ip_random_id) 164 return (0); 165 if (new == 1 && V_ip_random_id == 0) 166 ip_initid(8192); 167 /* We don't free memory when turning random ID off, due to race. */ 168 V_ip_random_id = new; 169 return (0); 170 } 171 172 static int 173 sysctl_ip_id_change(SYSCTL_HANDLER_ARGS) 174 { 175 int error, new; 176 177 new = V_array_size; 178 error = sysctl_handle_int(oidp, &new, 0, req); 179 if (error == 0 && req->newptr) { 180 if (new >= 512 && new <= 32768) 181 ip_initid(new); 182 else 183 error = EINVAL; 184 } 185 return (error); 186 } 187 188 static void 189 ip_initid(int new_size) 190 { 191 uint16_t *new_array; 192 bitstr_t *new_bits; 193 194 new_array = malloc(new_size * sizeof(uint16_t), M_IPID, 195 M_WAITOK | M_ZERO); 196 new_bits = malloc(bitstr_size(65536), M_IPID, M_WAITOK | M_ZERO); 197 198 mtx_lock(&V_ip_id_mtx); 199 if (V_id_array != NULL) { 200 free(V_id_array, M_IPID); 201 free(V_id_bits, M_IPID); 202 } 203 V_id_array = new_array; 204 V_id_bits = new_bits; 205 V_array_size = new_size; 206 V_array_ptr = 0; 207 V_random_id_collisions = 0; 208 V_random_id_total = 0; 209 mtx_unlock(&V_ip_id_mtx); 210 } 211 212 static uint16_t 213 ip_randomid(void) 214 { 215 uint16_t new_id; 216 217 mtx_lock(&V_ip_id_mtx); 218 /* 219 * To avoid a conflict with the zeros that the array is initially 220 * filled with, we never hand out an id of zero. 221 */ 222 new_id = 0; 223 do { 224 if (new_id != 0) 225 V_random_id_collisions++; 226 arc4rand(&new_id, sizeof(new_id), 0); 227 } while (bit_test(V_id_bits, new_id) || new_id == 0); 228 bit_clear(V_id_bits, V_id_array[V_array_ptr]); 229 bit_set(V_id_bits, new_id); 230 V_id_array[V_array_ptr] = new_id; 231 V_array_ptr++; 232 if (V_array_ptr == V_array_size) 233 V_array_ptr = 0; 234 V_random_id_total++; 235 mtx_unlock(&V_ip_id_mtx); 236 return (new_id); 237 } 238 239 void 240 ip_fillid(struct ip *ip, bool do_randomid) 241 { 242 243 /* 244 * Per RFC6864 Section 4 245 * 246 * o Atomic datagrams: (DF==1) && (MF==0) && (frag_offset==0) 247 * o Non-atomic datagrams: (DF==0) || (MF==1) || (frag_offset>0) 248 */ 249 if (V_ip_rfc6864 && (ip->ip_off & htons(IP_DF)) == htons(IP_DF)) 250 ip->ip_id = 0; 251 else if (do_randomid) 252 ip->ip_id = ip_randomid(); 253 else { 254 counter_u64_add(V_ip_id, 1); 255 /* 256 * There are two issues about this trick, to be kept in mind. 257 * 1) We can migrate between counter_u64_add() and next 258 * line, and grab counter from other CPU, resulting in too 259 * quick ID reuse. This is tolerable in our particular case, 260 * since probability of such event is much lower then reuse 261 * of ID due to legitimate overflow, that at modern Internet 262 * speeds happens all the time. 263 * 2) We are relying on the fact that counter(9) is based on 264 * UMA_ZONE_PCPU uma(9) zone. We also take only last 265 * sixteen bits of a counter, so we don't care about the 266 * fact that machines with 32-bit word update their counters 267 * not atomically. 268 */ 269 ip->ip_id = htons((*(uint64_t *)zpcpu_get(V_ip_id)) & 0xffff); 270 } 271 } 272 273 static void 274 ipid_sysinit(void) 275 { 276 int i; 277 278 mtx_init(&V_ip_id_mtx, "ip_id_mtx", NULL, MTX_DEF); 279 V_ip_id = counter_u64_alloc(M_WAITOK); 280 281 CPU_FOREACH(i) 282 arc4rand(zpcpu_get_cpu(V_ip_id, i), sizeof(uint64_t), 0); 283 } 284 VNET_SYSINIT(ip_id, SI_SUB_PROTO_DOMAIN, SI_ORDER_ANY, ipid_sysinit, NULL); 285 286 static void 287 ipid_sysuninit(void) 288 { 289 290 if (V_id_array != NULL) { 291 free(V_id_array, M_IPID); 292 free(V_id_bits, M_IPID); 293 } 294 counter_u64_free(V_ip_id); 295 mtx_destroy(&V_ip_id_mtx); 296 } 297 VNET_SYSUNINIT(ip_id, SI_SUB_PROTO_DOMAIN, SI_ORDER_THIRD, ipid_sysuninit, NULL); 298