1 /* $OpenBSD: pf_syncookies.c,v 1.7 2018/09/10 15:54:28 henning Exp $ */ 2 3 /* Copyright (c) 2016,2017 Henning Brauer <henning@openbsd.org> 4 * Copyright (c) 2016 Alexandr Nedvedicky <sashan@openbsd.org> 5 * 6 * syncookie parts based on FreeBSD sys/netinet/tcp_syncache.c 7 * 8 * Copyright (c) 2001 McAfee, Inc. 9 * Copyright (c) 2006,2013 Andre Oppermann, Internet Business Solutions AG 10 * All rights reserved. 11 * 12 * This software was developed for the FreeBSD Project by Jonathan Lemon 13 * and McAfee Research, the Security Research Division of McAfee, Inc. under 14 * DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the 15 * DARPA CHATS research program. [2001 McAfee, Inc.] 16 * 17 * Redistribution and use in source and binary forms, with or without 18 * modification, are permitted provided that the following conditions 19 * are met: 20 * 1. Redistributions of source code must retain the above copyright 21 * notice, this list of conditions and the following disclaimer. 22 * 2. Redistributions in binary form must reproduce the above copyright 23 * notice, this list of conditions and the following disclaimer in the 24 * documentation and/or other materials provided with the distribution. 25 * 26 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 27 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 28 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 29 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 30 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 31 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 32 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 33 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 34 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 35 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 36 * SUCH DAMAGE. 37 */ 38 39 /* 40 * when we're under synflood, we use syncookies to prevent state table 41 * exhaustion. Trigger for the synflood mode is the number of half-open 42 * connections in the state table. 43 * We leave synflood mode when the number of half-open states - including 44 * in-flight syncookies - drops far enough again 45 */ 46 47 /* 48 * syncookie enabled Initial Sequence Number: 49 * 24 bit MAC 50 * 3 bit WSCALE index 51 * 3 bit MSS index 52 * 1 bit SACK permitted 53 * 1 bit odd/even secret 54 * 55 * References: 56 * RFC4987 TCP SYN Flooding Attacks and Common Mitigations 57 * http://cr.yp.to/syncookies.html (overview) 58 * http://cr.yp.to/syncookies/archive (details) 59 */ 60 61 //#include "pflog.h" 62 63 #include <sys/param.h> 64 #include <sys/systm.h> 65 #include <sys/mbuf.h> 66 #include <sys/filio.h> 67 #include <sys/socket.h> 68 #include <sys/socketvar.h> 69 #include <sys/kernel.h> 70 #include <sys/time.h> 71 #include <sys/proc.h> 72 #include <sys/rwlock.h> 73 #include <sys/syslog.h> 74 75 #include <crypto/siphash/siphash.h> 76 77 #include <net/if.h> 78 #include <net/if_var.h> 79 #include <net/if_types.h> 80 #include <net/route.h> 81 82 #include <netinet/in.h> 83 #include <netinet/ip.h> 84 #include <netinet/tcp.h> 85 #include <netinet/tcp_var.h> 86 87 #include <net/pfvar.h> 88 #include <netpfil/pf/pf_nv.h> 89 90 #define DPFPRINTF(n, x) if (V_pf_status.debug >= (n)) printf x 91 92 union pf_syncookie { 93 uint8_t cookie; 94 struct { 95 uint8_t oddeven:1, 96 sack_ok:1, 97 wscale_idx:3, 98 mss_idx:3; 99 } flags; 100 }; 101 102 #define PF_SYNCOOKIE_SECRET_SIZE SIPHASH_KEY_LENGTH 103 #define PF_SYNCOOKIE_SECRET_LIFETIME 15 /* seconds */ 104 105 /* Protected by PF_RULES_xLOCK. */ 106 struct pf_syncookie_status { 107 struct callout keytimeout; 108 uint8_t oddeven; 109 uint8_t key[2][SIPHASH_KEY_LENGTH]; 110 uint32_t hiwat; /* absolute; # of states */ 111 uint32_t lowat; 112 }; 113 VNET_DEFINE_STATIC(struct pf_syncookie_status, pf_syncookie_status); 114 #define V_pf_syncookie_status VNET(pf_syncookie_status) 115 116 static int pf_syncookies_setmode(u_int8_t); 117 void pf_syncookie_rotate(void *); 118 void pf_syncookie_newkey(void); 119 uint32_t pf_syncookie_mac(struct pf_pdesc *, union pf_syncookie, 120 uint32_t); 121 uint32_t pf_syncookie_generate(struct mbuf *m, int off, struct pf_pdesc *, 122 uint16_t); 123 124 void 125 pf_syncookies_init(void) 126 { 127 callout_init(&V_pf_syncookie_status.keytimeout, 1); 128 PF_RULES_WLOCK(); 129 pf_syncookies_setmode(PF_SYNCOOKIES_NEVER); 130 PF_RULES_WUNLOCK(); 131 } 132 133 void 134 pf_syncookies_cleanup(void) 135 { 136 callout_stop(&V_pf_syncookie_status.keytimeout); 137 } 138 139 int 140 pf_get_syncookies(struct pfioc_nv *nv) 141 { 142 nvlist_t *nvl = NULL; 143 void *nvlpacked = NULL; 144 145 nvl = nvlist_create(0); 146 if (nvl == NULL) 147 return (ENOMEM); 148 149 nvlist_add_bool(nvl, "enabled", 150 V_pf_status.syncookies_mode != PF_SYNCOOKIES_NEVER); 151 nvlist_add_bool(nvl, "adaptive", 152 V_pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE); 153 nvlist_add_number(nvl, "highwater", V_pf_syncookie_status.hiwat); 154 nvlist_add_number(nvl, "lowwater", V_pf_syncookie_status.lowat); 155 156 nvlpacked = nvlist_pack(nvl, &nv->len); 157 if (nvlpacked == NULL) { 158 nvlist_destroy(nvl); 159 return (ENOMEM); 160 } 161 if (nv->size == 0) { 162 nvlist_destroy(nvl); 163 free(nvlpacked, M_TEMP); 164 return (0); 165 } else if (nv->size < nv->len) { 166 nvlist_destroy(nvl); 167 free(nvlpacked, M_TEMP); 168 return (ENOSPC); 169 } 170 171 return (copyout(nvlpacked, nv->data, nv->len)); 172 } 173 174 int 175 pf_set_syncookies(struct pfioc_nv *nv) 176 { 177 nvlist_t *nvl = NULL; 178 void *nvlpacked = NULL; 179 int error; 180 bool enabled, adaptive; 181 uint32_t hiwat, lowat; 182 uint8_t newmode; 183 184 #define ERROUT(x) ERROUT_FUNCTION(errout, x) 185 186 if (nv->len > pf_ioctl_maxcount) 187 return (ENOMEM); 188 189 nvlpacked = malloc(nv->len, M_TEMP, M_WAITOK); 190 if (nvlpacked == NULL) 191 return (ENOMEM); 192 193 error = copyin(nv->data, nvlpacked, nv->len); 194 if (error) 195 ERROUT(error); 196 197 nvl = nvlist_unpack(nvlpacked, nv->len, 0); 198 if (nvl == NULL) 199 ERROUT(EBADMSG); 200 201 if (! nvlist_exists_bool(nvl, "enabled") 202 || ! nvlist_exists_bool(nvl, "adaptive")) 203 ERROUT(EBADMSG); 204 205 enabled = nvlist_get_bool(nvl, "enabled"); 206 adaptive = nvlist_get_bool(nvl, "adaptive"); 207 PFNV_CHK(pf_nvuint32_opt(nvl, "highwater", &hiwat, 208 V_pf_syncookie_status.hiwat)); 209 PFNV_CHK(pf_nvuint32_opt(nvl, "lowwater", &lowat, 210 V_pf_syncookie_status.lowat)); 211 212 if (lowat >= hiwat) 213 ERROUT(EINVAL); 214 215 newmode = PF_SYNCOOKIES_NEVER; 216 if (enabled) 217 newmode = adaptive ? PF_SYNCOOKIES_ADAPTIVE : PF_SYNCOOKIES_ALWAYS; 218 219 PF_RULES_WLOCK(); 220 error = pf_syncookies_setmode(newmode); 221 222 V_pf_syncookie_status.lowat = lowat; 223 V_pf_syncookie_status.hiwat = hiwat; 224 225 PF_RULES_WUNLOCK(); 226 227 #undef ERROUT 228 errout: 229 nvlist_destroy(nvl); 230 free(nvlpacked, M_TEMP); 231 232 return (error); 233 } 234 235 static int 236 pf_syncookies_setmode(u_int8_t mode) 237 { 238 if (mode > PF_SYNCOOKIES_MODE_MAX) 239 return (EINVAL); 240 241 if (V_pf_status.syncookies_mode == mode) 242 return (0); 243 244 V_pf_status.syncookies_mode = mode; 245 if (V_pf_status.syncookies_mode == PF_SYNCOOKIES_ALWAYS) { 246 pf_syncookie_newkey(); 247 V_pf_status.syncookies_active = true; 248 } 249 return (0); 250 } 251 252 int 253 pf_synflood_check(struct pf_pdesc *pd) 254 { 255 MPASS(pd->proto == IPPROTO_TCP); 256 PF_RULES_RASSERT(); 257 258 if (pd->pf_mtag && (pd->pf_mtag->tag & PF_TAG_SYNCOOKIE_RECREATED)) 259 return (0); 260 261 if (V_pf_status.syncookies_mode != PF_SYNCOOKIES_ADAPTIVE) 262 return (V_pf_status.syncookies_mode); 263 264 if (!V_pf_status.syncookies_active && 265 atomic_load_32(&V_pf_status.states_halfopen) > 266 V_pf_syncookie_status.hiwat) { 267 /* We'd want to 'pf_syncookie_newkey()' here, but that requires 268 * the rules write lock, which we can't get with the read lock 269 * held. */ 270 callout_reset(&V_pf_syncookie_status.keytimeout, 0, 271 pf_syncookie_rotate, curvnet); 272 V_pf_status.syncookies_active = true; 273 DPFPRINTF(LOG_WARNING, 274 ("synflood detected, enabling syncookies\n")); 275 // XXXTODO V_pf_status.lcounters[LCNT_SYNFLOODS]++; 276 } 277 278 return (V_pf_status.syncookies_active); 279 } 280 281 void 282 pf_syncookie_send(struct mbuf *m, int off, struct pf_pdesc *pd) 283 { 284 uint16_t mss; 285 uint32_t iss; 286 287 mss = max(V_tcp_mssdflt, pf_get_mss(m, off, pd->hdr.tcp.th_off, pd->af)); 288 iss = pf_syncookie_generate(m, off, pd, mss); 289 pf_send_tcp(NULL, pd->af, pd->dst, pd->src, *pd->dport, *pd->sport, 290 iss, ntohl(pd->hdr.tcp.th_seq) + 1, TH_SYN|TH_ACK, 0, mss, 291 0, 1, 0); 292 counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_SENT], 1); 293 /* XXX Maybe only in adaptive mode? */ 294 atomic_add_64(&V_pf_status.syncookies_inflight[V_pf_syncookie_status.oddeven], 295 1); 296 } 297 298 uint8_t 299 pf_syncookie_validate(struct pf_pdesc *pd) 300 { 301 uint32_t hash, ack, seq; 302 union pf_syncookie cookie; 303 304 MPASS(pd->proto == IPPROTO_TCP); 305 PF_RULES_RASSERT(); 306 307 seq = ntohl(pd->hdr.tcp.th_seq) - 1; 308 ack = ntohl(pd->hdr.tcp.th_ack) - 1; 309 cookie.cookie = (ack & 0xff) ^ (ack >> 24); 310 311 /* we don't know oddeven before setting the cookie (union) */ 312 if (atomic_load_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven]) 313 == 0) 314 return (0); 315 316 hash = pf_syncookie_mac(pd, cookie, seq); 317 if ((ack & ~0xff) != (hash & ~0xff)) 318 return (0); 319 320 counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_VALID], 1); 321 atomic_add_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven], -1); 322 323 return (1); 324 } 325 326 /* 327 * all following functions private 328 */ 329 void 330 pf_syncookie_rotate(void *arg) 331 { 332 CURVNET_SET((struct vnet *)arg); 333 334 /* do we want to disable syncookies? */ 335 if (V_pf_status.syncookies_active && 336 ((V_pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE && 337 (atomic_load_32(&V_pf_status.states_halfopen) + 338 atomic_load_64(&V_pf_status.syncookies_inflight[0]) + 339 atomic_load_64(&V_pf_status.syncookies_inflight[1])) < 340 V_pf_syncookie_status.lowat) || 341 V_pf_status.syncookies_mode == PF_SYNCOOKIES_NEVER) 342 ) { 343 V_pf_status.syncookies_active = false; 344 DPFPRINTF(PF_DEBUG_MISC, ("syncookies disabled\n")); 345 } 346 347 /* nothing in flight any more? delete keys and return */ 348 if (!V_pf_status.syncookies_active && 349 atomic_load_64(&V_pf_status.syncookies_inflight[0]) == 0 && 350 atomic_load_64(&V_pf_status.syncookies_inflight[1]) == 0) { 351 memset(V_pf_syncookie_status.key[0], 0, 352 PF_SYNCOOKIE_SECRET_SIZE); 353 memset(V_pf_syncookie_status.key[1], 0, 354 PF_SYNCOOKIE_SECRET_SIZE); 355 CURVNET_RESTORE(); 356 return; 357 } 358 359 PF_RULES_WLOCK(); 360 /* new key, including timeout */ 361 pf_syncookie_newkey(); 362 PF_RULES_WUNLOCK(); 363 364 CURVNET_RESTORE(); 365 } 366 367 void 368 pf_syncookie_newkey(void) 369 { 370 PF_RULES_WASSERT(); 371 372 MPASS(V_pf_syncookie_status.oddeven < 2); 373 V_pf_syncookie_status.oddeven = (V_pf_syncookie_status.oddeven + 1) & 0x1; 374 atomic_store_64(&V_pf_status.syncookies_inflight[V_pf_syncookie_status.oddeven], 0); 375 arc4random_buf(V_pf_syncookie_status.key[V_pf_syncookie_status.oddeven], 376 PF_SYNCOOKIE_SECRET_SIZE); 377 callout_reset(&V_pf_syncookie_status.keytimeout, 378 PF_SYNCOOKIE_SECRET_LIFETIME * hz, pf_syncookie_rotate, curvnet); 379 } 380 381 /* 382 * Distribution and probability of certain MSS values. Those in between are 383 * rounded down to the next lower one. 384 * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011] 385 * .2% .3% 5% 7% 7% 20% 15% 45% 386 */ 387 static int pf_syncookie_msstab[] = 388 { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 }; 389 390 /* 391 * Distribution and probability of certain WSCALE values. 392 * The absence of the WSCALE option is encoded with index zero. 393 * [WSCALE values histograms, Allman, 2012] 394 * X 10 10 35 5 6 14 10% by host 395 * X 11 4 5 5 18 49 3% by connections 396 */ 397 static int pf_syncookie_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 }; 398 399 uint32_t 400 pf_syncookie_mac(struct pf_pdesc *pd, union pf_syncookie cookie, uint32_t seq) 401 { 402 SIPHASH_CTX ctx; 403 uint32_t siphash[2]; 404 405 PF_RULES_RASSERT(); 406 MPASS(pd->proto == IPPROTO_TCP); 407 408 SipHash24_Init(&ctx); 409 SipHash_SetKey(&ctx, V_pf_syncookie_status.key[cookie.flags.oddeven]); 410 411 switch (pd->af) { 412 case AF_INET: 413 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v4)); 414 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v4)); 415 break; 416 case AF_INET6: 417 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v6)); 418 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v6)); 419 break; 420 default: 421 panic("unknown address family"); 422 } 423 424 SipHash_Update(&ctx, pd->sport, sizeof(*pd->sport)); 425 SipHash_Update(&ctx, pd->dport, sizeof(*pd->dport)); 426 SipHash_Update(&ctx, &seq, sizeof(seq)); 427 SipHash_Update(&ctx, &cookie, sizeof(cookie)); 428 SipHash_Final((uint8_t *)&siphash, &ctx); 429 430 return (siphash[0] ^ siphash[1]); 431 } 432 433 uint32_t 434 pf_syncookie_generate(struct mbuf *m, int off, struct pf_pdesc *pd, 435 uint16_t mss) 436 { 437 uint8_t i, wscale; 438 uint32_t iss, hash; 439 union pf_syncookie cookie; 440 441 PF_RULES_RASSERT(); 442 443 cookie.cookie = 0; 444 445 /* map MSS */ 446 for (i = nitems(pf_syncookie_msstab) - 1; 447 pf_syncookie_msstab[i] > mss && i > 0; i--) 448 /* nada */; 449 cookie.flags.mss_idx = i; 450 451 /* map WSCALE */ 452 wscale = pf_get_wscale(m, off, pd->hdr.tcp.th_off, pd->af); 453 for (i = nitems(pf_syncookie_wstab) - 1; 454 pf_syncookie_wstab[i] > wscale && i > 0; i--) 455 /* nada */; 456 cookie.flags.wscale_idx = i; 457 cookie.flags.sack_ok = 0; /* XXX */ 458 459 cookie.flags.oddeven = V_pf_syncookie_status.oddeven; 460 hash = pf_syncookie_mac(pd, cookie, ntohl(pd->hdr.tcp.th_seq)); 461 462 /* 463 * Put the flags into the hash and XOR them to get better ISS number 464 * variance. This doesn't enhance the cryptographic strength and is 465 * done to prevent the 8 cookie bits from showing up directly on the 466 * wire. 467 */ 468 iss = hash & ~0xff; 469 iss |= cookie.cookie ^ (hash >> 24); 470 471 return (iss); 472 } 473 474 struct mbuf * 475 pf_syncookie_recreate_syn(uint8_t ttl, int off, struct pf_pdesc *pd) 476 { 477 uint8_t wscale; 478 uint16_t mss; 479 uint32_t ack, seq; 480 union pf_syncookie cookie; 481 482 seq = ntohl(pd->hdr.tcp.th_seq) - 1; 483 ack = ntohl(pd->hdr.tcp.th_ack) - 1; 484 cookie.cookie = (ack & 0xff) ^ (ack >> 24); 485 486 if (cookie.flags.mss_idx >= nitems(pf_syncookie_msstab) || 487 cookie.flags.wscale_idx >= nitems(pf_syncookie_wstab)) 488 return (NULL); 489 490 mss = pf_syncookie_msstab[cookie.flags.mss_idx]; 491 wscale = pf_syncookie_wstab[cookie.flags.wscale_idx]; 492 493 return (pf_build_tcp(NULL, pd->af, pd->src, pd->dst, *pd->sport, 494 *pd->dport, seq, 0, TH_SYN, wscale, mss, ttl, 0, 495 PF_TAG_SYNCOOKIE_RECREATED)); 496 } 497