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("$FreeBSD$"); 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); 300