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 int error; 145 146 #define ERROUT(x) ERROUT_FUNCTION(errout, x) 147 148 nvl = nvlist_create(0); 149 if (nvl == NULL) 150 ERROUT(ENOMEM); 151 152 nvlist_add_bool(nvl, "enabled", 153 V_pf_status.syncookies_mode != PF_SYNCOOKIES_NEVER); 154 nvlist_add_bool(nvl, "adaptive", 155 V_pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE); 156 nvlist_add_number(nvl, "highwater", V_pf_syncookie_status.hiwat); 157 nvlist_add_number(nvl, "lowwater", V_pf_syncookie_status.lowat); 158 159 nvlpacked = nvlist_pack(nvl, &nv->len); 160 if (nvlpacked == NULL) 161 ERROUT(ENOMEM); 162 163 if (nv->size == 0) { 164 ERROUT(0); 165 } else if (nv->size < nv->len) { 166 ERROUT(ENOSPC); 167 } 168 169 error = copyout(nvlpacked, nv->data, nv->len); 170 171 #undef ERROUT 172 errout: 173 nvlist_destroy(nvl); 174 free(nvlpacked, M_TEMP); 175 176 return (error); 177 } 178 179 int 180 pf_set_syncookies(struct pfioc_nv *nv) 181 { 182 nvlist_t *nvl = NULL; 183 void *nvlpacked = NULL; 184 int error; 185 bool enabled, adaptive; 186 uint32_t hiwat, lowat; 187 uint8_t newmode; 188 189 #define ERROUT(x) ERROUT_FUNCTION(errout, x) 190 191 if (nv->len > pf_ioctl_maxcount) 192 return (ENOMEM); 193 194 nvlpacked = malloc(nv->len, M_TEMP, M_WAITOK); 195 if (nvlpacked == NULL) 196 return (ENOMEM); 197 198 error = copyin(nv->data, nvlpacked, nv->len); 199 if (error) 200 ERROUT(error); 201 202 nvl = nvlist_unpack(nvlpacked, nv->len, 0); 203 if (nvl == NULL) 204 ERROUT(EBADMSG); 205 206 if (! nvlist_exists_bool(nvl, "enabled") 207 || ! nvlist_exists_bool(nvl, "adaptive")) 208 ERROUT(EBADMSG); 209 210 enabled = nvlist_get_bool(nvl, "enabled"); 211 adaptive = nvlist_get_bool(nvl, "adaptive"); 212 PFNV_CHK(pf_nvuint32_opt(nvl, "highwater", &hiwat, 213 V_pf_syncookie_status.hiwat)); 214 PFNV_CHK(pf_nvuint32_opt(nvl, "lowwater", &lowat, 215 V_pf_syncookie_status.lowat)); 216 217 if (lowat >= hiwat) 218 ERROUT(EINVAL); 219 220 newmode = PF_SYNCOOKIES_NEVER; 221 if (enabled) 222 newmode = adaptive ? PF_SYNCOOKIES_ADAPTIVE : PF_SYNCOOKIES_ALWAYS; 223 224 PF_RULES_WLOCK(); 225 error = pf_syncookies_setmode(newmode); 226 227 V_pf_syncookie_status.lowat = lowat; 228 V_pf_syncookie_status.hiwat = hiwat; 229 230 PF_RULES_WUNLOCK(); 231 232 #undef ERROUT 233 errout: 234 nvlist_destroy(nvl); 235 free(nvlpacked, M_TEMP); 236 237 return (error); 238 } 239 240 static int 241 pf_syncookies_setmode(u_int8_t mode) 242 { 243 if (mode > PF_SYNCOOKIES_MODE_MAX) 244 return (EINVAL); 245 246 if (V_pf_status.syncookies_mode == mode) 247 return (0); 248 249 V_pf_status.syncookies_mode = mode; 250 if (V_pf_status.syncookies_mode == PF_SYNCOOKIES_ALWAYS) { 251 pf_syncookie_newkey(); 252 V_pf_status.syncookies_active = true; 253 } 254 return (0); 255 } 256 257 int 258 pf_synflood_check(struct pf_pdesc *pd) 259 { 260 MPASS(pd->proto == IPPROTO_TCP); 261 PF_RULES_RASSERT(); 262 263 if (pd->pf_mtag && (pd->pf_mtag->tag & PF_TAG_SYNCOOKIE_RECREATED)) 264 return (0); 265 266 if (V_pf_status.syncookies_mode != PF_SYNCOOKIES_ADAPTIVE) 267 return (V_pf_status.syncookies_mode); 268 269 if (!V_pf_status.syncookies_active && 270 atomic_load_32(&V_pf_status.states_halfopen) > 271 V_pf_syncookie_status.hiwat) { 272 /* We'd want to 'pf_syncookie_newkey()' here, but that requires 273 * the rules write lock, which we can't get with the read lock 274 * held. */ 275 callout_reset(&V_pf_syncookie_status.keytimeout, 0, 276 pf_syncookie_rotate, curvnet); 277 V_pf_status.syncookies_active = true; 278 DPFPRINTF(LOG_WARNING, 279 ("synflood detected, enabling syncookies\n")); 280 // XXXTODO V_pf_status.lcounters[LCNT_SYNFLOODS]++; 281 } 282 283 return (V_pf_status.syncookies_active); 284 } 285 286 void 287 pf_syncookie_send(struct mbuf *m, int off, struct pf_pdesc *pd) 288 { 289 uint16_t mss; 290 uint32_t iss; 291 292 mss = max(V_tcp_mssdflt, pf_get_mss(m, off, pd->hdr.tcp.th_off, pd->af)); 293 iss = pf_syncookie_generate(m, off, pd, mss); 294 pf_send_tcp(NULL, pd->af, pd->dst, pd->src, *pd->dport, *pd->sport, 295 iss, ntohl(pd->hdr.tcp.th_seq) + 1, TH_SYN|TH_ACK, 0, mss, 296 0, 1, 0); 297 counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_SENT], 1); 298 /* XXX Maybe only in adaptive mode? */ 299 atomic_add_64(&V_pf_status.syncookies_inflight[V_pf_syncookie_status.oddeven], 300 1); 301 } 302 303 uint8_t 304 pf_syncookie_validate(struct pf_pdesc *pd) 305 { 306 uint32_t hash, ack, seq; 307 union pf_syncookie cookie; 308 309 MPASS(pd->proto == IPPROTO_TCP); 310 PF_RULES_RASSERT(); 311 312 seq = ntohl(pd->hdr.tcp.th_seq) - 1; 313 ack = ntohl(pd->hdr.tcp.th_ack) - 1; 314 cookie.cookie = (ack & 0xff) ^ (ack >> 24); 315 316 /* we don't know oddeven before setting the cookie (union) */ 317 if (atomic_load_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven]) 318 == 0) 319 return (0); 320 321 hash = pf_syncookie_mac(pd, cookie, seq); 322 if ((ack & ~0xff) != (hash & ~0xff)) 323 return (0); 324 325 counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_VALID], 1); 326 atomic_add_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven], -1); 327 328 return (1); 329 } 330 331 /* 332 * all following functions private 333 */ 334 void 335 pf_syncookie_rotate(void *arg) 336 { 337 CURVNET_SET((struct vnet *)arg); 338 339 /* do we want to disable syncookies? */ 340 if (V_pf_status.syncookies_active && 341 ((V_pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE && 342 (atomic_load_32(&V_pf_status.states_halfopen) + 343 atomic_load_64(&V_pf_status.syncookies_inflight[0]) + 344 atomic_load_64(&V_pf_status.syncookies_inflight[1])) < 345 V_pf_syncookie_status.lowat) || 346 V_pf_status.syncookies_mode == PF_SYNCOOKIES_NEVER) 347 ) { 348 V_pf_status.syncookies_active = false; 349 DPFPRINTF(PF_DEBUG_MISC, ("syncookies disabled\n")); 350 } 351 352 /* nothing in flight any more? delete keys and return */ 353 if (!V_pf_status.syncookies_active && 354 atomic_load_64(&V_pf_status.syncookies_inflight[0]) == 0 && 355 atomic_load_64(&V_pf_status.syncookies_inflight[1]) == 0) { 356 memset(V_pf_syncookie_status.key[0], 0, 357 PF_SYNCOOKIE_SECRET_SIZE); 358 memset(V_pf_syncookie_status.key[1], 0, 359 PF_SYNCOOKIE_SECRET_SIZE); 360 CURVNET_RESTORE(); 361 return; 362 } 363 364 PF_RULES_WLOCK(); 365 /* new key, including timeout */ 366 pf_syncookie_newkey(); 367 PF_RULES_WUNLOCK(); 368 369 CURVNET_RESTORE(); 370 } 371 372 void 373 pf_syncookie_newkey(void) 374 { 375 PF_RULES_WASSERT(); 376 377 MPASS(V_pf_syncookie_status.oddeven < 2); 378 V_pf_syncookie_status.oddeven = (V_pf_syncookie_status.oddeven + 1) & 0x1; 379 atomic_store_64(&V_pf_status.syncookies_inflight[V_pf_syncookie_status.oddeven], 0); 380 arc4random_buf(V_pf_syncookie_status.key[V_pf_syncookie_status.oddeven], 381 PF_SYNCOOKIE_SECRET_SIZE); 382 callout_reset(&V_pf_syncookie_status.keytimeout, 383 PF_SYNCOOKIE_SECRET_LIFETIME * hz, pf_syncookie_rotate, curvnet); 384 } 385 386 /* 387 * Distribution and probability of certain MSS values. Those in between are 388 * rounded down to the next lower one. 389 * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011] 390 * .2% .3% 5% 7% 7% 20% 15% 45% 391 */ 392 static int pf_syncookie_msstab[] = 393 { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 }; 394 395 /* 396 * Distribution and probability of certain WSCALE values. 397 * The absence of the WSCALE option is encoded with index zero. 398 * [WSCALE values histograms, Allman, 2012] 399 * X 10 10 35 5 6 14 10% by host 400 * X 11 4 5 5 18 49 3% by connections 401 */ 402 static int pf_syncookie_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 }; 403 404 uint32_t 405 pf_syncookie_mac(struct pf_pdesc *pd, union pf_syncookie cookie, uint32_t seq) 406 { 407 SIPHASH_CTX ctx; 408 uint32_t siphash[2]; 409 410 PF_RULES_RASSERT(); 411 MPASS(pd->proto == IPPROTO_TCP); 412 413 SipHash24_Init(&ctx); 414 SipHash_SetKey(&ctx, V_pf_syncookie_status.key[cookie.flags.oddeven]); 415 416 switch (pd->af) { 417 case AF_INET: 418 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v4)); 419 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v4)); 420 break; 421 case AF_INET6: 422 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v6)); 423 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v6)); 424 break; 425 default: 426 panic("unknown address family"); 427 } 428 429 SipHash_Update(&ctx, pd->sport, sizeof(*pd->sport)); 430 SipHash_Update(&ctx, pd->dport, sizeof(*pd->dport)); 431 SipHash_Update(&ctx, &seq, sizeof(seq)); 432 SipHash_Update(&ctx, &cookie, sizeof(cookie)); 433 SipHash_Final((uint8_t *)&siphash, &ctx); 434 435 return (siphash[0] ^ siphash[1]); 436 } 437 438 uint32_t 439 pf_syncookie_generate(struct mbuf *m, int off, struct pf_pdesc *pd, 440 uint16_t mss) 441 { 442 uint8_t i, wscale; 443 uint32_t iss, hash; 444 union pf_syncookie cookie; 445 446 PF_RULES_RASSERT(); 447 448 cookie.cookie = 0; 449 450 /* map MSS */ 451 for (i = nitems(pf_syncookie_msstab) - 1; 452 pf_syncookie_msstab[i] > mss && i > 0; i--) 453 /* nada */; 454 cookie.flags.mss_idx = i; 455 456 /* map WSCALE */ 457 wscale = pf_get_wscale(m, off, pd->hdr.tcp.th_off, pd->af); 458 for (i = nitems(pf_syncookie_wstab) - 1; 459 pf_syncookie_wstab[i] > wscale && i > 0; i--) 460 /* nada */; 461 cookie.flags.wscale_idx = i; 462 cookie.flags.sack_ok = 0; /* XXX */ 463 464 cookie.flags.oddeven = V_pf_syncookie_status.oddeven; 465 hash = pf_syncookie_mac(pd, cookie, ntohl(pd->hdr.tcp.th_seq)); 466 467 /* 468 * Put the flags into the hash and XOR them to get better ISS number 469 * variance. This doesn't enhance the cryptographic strength and is 470 * done to prevent the 8 cookie bits from showing up directly on the 471 * wire. 472 */ 473 iss = hash & ~0xff; 474 iss |= cookie.cookie ^ (hash >> 24); 475 476 return (iss); 477 } 478 479 struct mbuf * 480 pf_syncookie_recreate_syn(uint8_t ttl, int off, struct pf_pdesc *pd) 481 { 482 uint8_t wscale; 483 uint16_t mss; 484 uint32_t ack, seq; 485 union pf_syncookie cookie; 486 487 seq = ntohl(pd->hdr.tcp.th_seq) - 1; 488 ack = ntohl(pd->hdr.tcp.th_ack) - 1; 489 cookie.cookie = (ack & 0xff) ^ (ack >> 24); 490 491 if (cookie.flags.mss_idx >= nitems(pf_syncookie_msstab) || 492 cookie.flags.wscale_idx >= nitems(pf_syncookie_wstab)) 493 return (NULL); 494 495 mss = pf_syncookie_msstab[cookie.flags.mss_idx]; 496 wscale = pf_syncookie_wstab[cookie.flags.wscale_idx]; 497 498 return (pf_build_tcp(NULL, pd->af, pd->src, pd->dst, *pd->sport, 499 *pd->dport, seq, 0, TH_SYN, wscale, mss, ttl, 0, 500 PF_TAG_SYNCOOKIE_RECREATED)); 501 } 502