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/in_pcb.h> 84 #include <netinet/ip.h> 85 #include <netinet/tcp.h> 86 #include <netinet/tcp_var.h> 87 88 #include <net/pfvar.h> 89 #include <netpfil/pf/pf_nv.h> 90 91 #define DPFPRINTF(n, x) if (V_pf_status.debug >= (n)) printf x 92 93 union pf_syncookie { 94 uint8_t cookie; 95 struct { 96 uint8_t oddeven:1, 97 sack_ok:1, 98 wscale_idx:3, 99 mss_idx:3; 100 } flags; 101 }; 102 103 #define PF_SYNCOOKIE_SECRET_SIZE SIPHASH_KEY_LENGTH 104 #define PF_SYNCOOKIE_SECRET_LIFETIME 15 /* seconds */ 105 106 /* Protected by PF_RULES_xLOCK. */ 107 struct pf_syncookie_status { 108 struct callout keytimeout; 109 uint8_t oddeven; 110 uint8_t key[2][SIPHASH_KEY_LENGTH]; 111 uint32_t hiwat; /* absolute; # of states */ 112 uint32_t lowat; 113 }; 114 VNET_DEFINE_STATIC(struct pf_syncookie_status, pf_syncookie_status); 115 #define V_pf_syncookie_status VNET(pf_syncookie_status) 116 117 static int pf_syncookies_setmode(u_int8_t); 118 void pf_syncookie_rotate(void *); 119 void pf_syncookie_newkey(void); 120 uint32_t pf_syncookie_mac(struct pf_pdesc *, union pf_syncookie, 121 uint32_t); 122 uint32_t pf_syncookie_generate(struct mbuf *m, int off, struct pf_pdesc *, 123 uint16_t); 124 125 void 126 pf_syncookies_init(void) 127 { 128 callout_init(&V_pf_syncookie_status.keytimeout, 1); 129 PF_RULES_WLOCK(); 130 pf_syncookies_setmode(PF_SYNCOOKIES_NEVER); 131 PF_RULES_WUNLOCK(); 132 } 133 134 void 135 pf_syncookies_cleanup(void) 136 { 137 callout_stop(&V_pf_syncookie_status.keytimeout); 138 } 139 140 int 141 pf_get_syncookies(struct pfioc_nv *nv) 142 { 143 nvlist_t *nvl = NULL; 144 void *nvlpacked = NULL; 145 int error; 146 147 #define ERROUT(x) ERROUT_FUNCTION(errout, x) 148 149 nvl = nvlist_create(0); 150 if (nvl == NULL) 151 ERROUT(ENOMEM); 152 153 nvlist_add_bool(nvl, "enabled", 154 V_pf_status.syncookies_mode != PF_SYNCOOKIES_NEVER); 155 nvlist_add_bool(nvl, "adaptive", 156 V_pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE); 157 nvlist_add_number(nvl, "highwater", V_pf_syncookie_status.hiwat); 158 nvlist_add_number(nvl, "lowwater", V_pf_syncookie_status.lowat); 159 160 nvlpacked = nvlist_pack(nvl, &nv->len); 161 if (nvlpacked == NULL) 162 ERROUT(ENOMEM); 163 164 if (nv->size == 0) { 165 ERROUT(0); 166 } else if (nv->size < nv->len) { 167 ERROUT(ENOSPC); 168 } 169 170 error = copyout(nvlpacked, nv->data, nv->len); 171 172 #undef ERROUT 173 errout: 174 nvlist_destroy(nvl); 175 free(nvlpacked, M_NVLIST); 176 177 return (error); 178 } 179 180 int 181 pf_set_syncookies(struct pfioc_nv *nv) 182 { 183 nvlist_t *nvl = NULL; 184 void *nvlpacked = NULL; 185 int error; 186 bool enabled, adaptive; 187 uint32_t hiwat, lowat; 188 uint8_t newmode; 189 190 #define ERROUT(x) ERROUT_FUNCTION(errout, x) 191 192 if (nv->len > pf_ioctl_maxcount) 193 return (ENOMEM); 194 195 nvlpacked = malloc(nv->len, M_NVLIST, M_WAITOK); 196 if (nvlpacked == NULL) 197 return (ENOMEM); 198 199 error = copyin(nv->data, nvlpacked, nv->len); 200 if (error) 201 ERROUT(error); 202 203 nvl = nvlist_unpack(nvlpacked, nv->len, 0); 204 if (nvl == NULL) 205 ERROUT(EBADMSG); 206 207 if (! nvlist_exists_bool(nvl, "enabled") 208 || ! nvlist_exists_bool(nvl, "adaptive")) 209 ERROUT(EBADMSG); 210 211 enabled = nvlist_get_bool(nvl, "enabled"); 212 adaptive = nvlist_get_bool(nvl, "adaptive"); 213 PFNV_CHK(pf_nvuint32_opt(nvl, "highwater", &hiwat, 214 V_pf_syncookie_status.hiwat)); 215 PFNV_CHK(pf_nvuint32_opt(nvl, "lowwater", &lowat, 216 V_pf_syncookie_status.lowat)); 217 218 if (lowat >= hiwat) 219 ERROUT(EINVAL); 220 221 newmode = PF_SYNCOOKIES_NEVER; 222 if (enabled) 223 newmode = adaptive ? PF_SYNCOOKIES_ADAPTIVE : PF_SYNCOOKIES_ALWAYS; 224 225 PF_RULES_WLOCK(); 226 error = pf_syncookies_setmode(newmode); 227 228 V_pf_syncookie_status.lowat = lowat; 229 V_pf_syncookie_status.hiwat = hiwat; 230 231 PF_RULES_WUNLOCK(); 232 233 #undef ERROUT 234 errout: 235 nvlist_destroy(nvl); 236 free(nvlpacked, M_NVLIST); 237 238 return (error); 239 } 240 241 static int 242 pf_syncookies_setmode(u_int8_t mode) 243 { 244 if (mode > PF_SYNCOOKIES_MODE_MAX) 245 return (EINVAL); 246 247 if (V_pf_status.syncookies_mode == mode) 248 return (0); 249 250 V_pf_status.syncookies_mode = mode; 251 if (V_pf_status.syncookies_mode == PF_SYNCOOKIES_ALWAYS) { 252 pf_syncookie_newkey(); 253 V_pf_status.syncookies_active = true; 254 } 255 return (0); 256 } 257 258 int 259 pf_synflood_check(struct pf_pdesc *pd) 260 { 261 MPASS(pd->proto == IPPROTO_TCP); 262 PF_RULES_RASSERT(); 263 264 if (pd->pf_mtag && (pd->pf_mtag->tag & PF_TAG_SYNCOOKIE_RECREATED)) 265 return (0); 266 267 if (V_pf_status.syncookies_mode != PF_SYNCOOKIES_ADAPTIVE) 268 return (V_pf_status.syncookies_mode); 269 270 if (!V_pf_status.syncookies_active && 271 atomic_load_32(&V_pf_status.states_halfopen) > 272 V_pf_syncookie_status.hiwat) { 273 /* We'd want to 'pf_syncookie_newkey()' here, but that requires 274 * the rules write lock, which we can't get with the read lock 275 * held. */ 276 callout_reset(&V_pf_syncookie_status.keytimeout, 0, 277 pf_syncookie_rotate, curvnet); 278 V_pf_status.syncookies_active = true; 279 DPFPRINTF(LOG_WARNING, 280 ("synflood detected, enabling syncookies\n")); 281 // XXXTODO V_pf_status.lcounters[LCNT_SYNFLOODS]++; 282 } 283 284 return (V_pf_status.syncookies_active); 285 } 286 287 void 288 pf_syncookie_send(struct mbuf *m, int off, struct pf_pdesc *pd) 289 { 290 uint16_t mss; 291 uint32_t iss; 292 293 mss = max(V_tcp_mssdflt, pf_get_mss(m, off, pd->hdr.tcp.th_off, pd->af)); 294 iss = pf_syncookie_generate(m, off, pd, mss); 295 pf_send_tcp(NULL, pd->af, pd->dst, pd->src, *pd->dport, *pd->sport, 296 iss, ntohl(pd->hdr.tcp.th_seq) + 1, TH_SYN|TH_ACK, 0, mss, 297 0, 1, 0); 298 counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_SENT], 1); 299 /* XXX Maybe only in adaptive mode? */ 300 atomic_add_64(&V_pf_status.syncookies_inflight[V_pf_syncookie_status.oddeven], 301 1); 302 } 303 304 uint8_t 305 pf_syncookie_validate(struct pf_pdesc *pd) 306 { 307 uint32_t hash, ack, seq; 308 union pf_syncookie cookie; 309 310 MPASS(pd->proto == IPPROTO_TCP); 311 PF_RULES_RASSERT(); 312 313 seq = ntohl(pd->hdr.tcp.th_seq) - 1; 314 ack = ntohl(pd->hdr.tcp.th_ack) - 1; 315 cookie.cookie = (ack & 0xff) ^ (ack >> 24); 316 317 /* we don't know oddeven before setting the cookie (union) */ 318 if (atomic_load_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven]) 319 == 0) 320 return (0); 321 322 hash = pf_syncookie_mac(pd, cookie, seq); 323 if ((ack & ~0xff) != (hash & ~0xff)) 324 return (0); 325 326 counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_VALID], 1); 327 atomic_add_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven], -1); 328 329 return (1); 330 } 331 332 /* 333 * all following functions private 334 */ 335 void 336 pf_syncookie_rotate(void *arg) 337 { 338 CURVNET_SET((struct vnet *)arg); 339 340 /* do we want to disable syncookies? */ 341 if (V_pf_status.syncookies_active && 342 ((V_pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE && 343 (atomic_load_32(&V_pf_status.states_halfopen) + 344 atomic_load_64(&V_pf_status.syncookies_inflight[0]) + 345 atomic_load_64(&V_pf_status.syncookies_inflight[1])) < 346 V_pf_syncookie_status.lowat) || 347 V_pf_status.syncookies_mode == PF_SYNCOOKIES_NEVER) 348 ) { 349 V_pf_status.syncookies_active = false; 350 DPFPRINTF(PF_DEBUG_MISC, ("syncookies disabled\n")); 351 } 352 353 /* nothing in flight any more? delete keys and return */ 354 if (!V_pf_status.syncookies_active && 355 atomic_load_64(&V_pf_status.syncookies_inflight[0]) == 0 && 356 atomic_load_64(&V_pf_status.syncookies_inflight[1]) == 0) { 357 memset(V_pf_syncookie_status.key[0], 0, 358 PF_SYNCOOKIE_SECRET_SIZE); 359 memset(V_pf_syncookie_status.key[1], 0, 360 PF_SYNCOOKIE_SECRET_SIZE); 361 CURVNET_RESTORE(); 362 return; 363 } 364 365 PF_RULES_WLOCK(); 366 /* new key, including timeout */ 367 pf_syncookie_newkey(); 368 PF_RULES_WUNLOCK(); 369 370 CURVNET_RESTORE(); 371 } 372 373 void 374 pf_syncookie_newkey(void) 375 { 376 PF_RULES_WASSERT(); 377 378 MPASS(V_pf_syncookie_status.oddeven < 2); 379 V_pf_syncookie_status.oddeven = (V_pf_syncookie_status.oddeven + 1) & 0x1; 380 atomic_store_64(&V_pf_status.syncookies_inflight[V_pf_syncookie_status.oddeven], 0); 381 arc4random_buf(V_pf_syncookie_status.key[V_pf_syncookie_status.oddeven], 382 PF_SYNCOOKIE_SECRET_SIZE); 383 callout_reset(&V_pf_syncookie_status.keytimeout, 384 PF_SYNCOOKIE_SECRET_LIFETIME * hz, pf_syncookie_rotate, curvnet); 385 } 386 387 /* 388 * Distribution and probability of certain MSS values. Those in between are 389 * rounded down to the next lower one. 390 * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011] 391 * .2% .3% 5% 7% 7% 20% 15% 45% 392 */ 393 static int pf_syncookie_msstab[] = 394 { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 }; 395 396 /* 397 * Distribution and probability of certain WSCALE values. 398 * The absence of the WSCALE option is encoded with index zero. 399 * [WSCALE values histograms, Allman, 2012] 400 * X 10 10 35 5 6 14 10% by host 401 * X 11 4 5 5 18 49 3% by connections 402 */ 403 static int pf_syncookie_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 }; 404 405 uint32_t 406 pf_syncookie_mac(struct pf_pdesc *pd, union pf_syncookie cookie, uint32_t seq) 407 { 408 SIPHASH_CTX ctx; 409 uint32_t siphash[2]; 410 411 PF_RULES_RASSERT(); 412 MPASS(pd->proto == IPPROTO_TCP); 413 414 SipHash24_Init(&ctx); 415 SipHash_SetKey(&ctx, V_pf_syncookie_status.key[cookie.flags.oddeven]); 416 417 switch (pd->af) { 418 case AF_INET: 419 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v4)); 420 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v4)); 421 break; 422 case AF_INET6: 423 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v6)); 424 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v6)); 425 break; 426 default: 427 panic("unknown address family"); 428 } 429 430 SipHash_Update(&ctx, pd->sport, sizeof(*pd->sport)); 431 SipHash_Update(&ctx, pd->dport, sizeof(*pd->dport)); 432 SipHash_Update(&ctx, &seq, sizeof(seq)); 433 SipHash_Update(&ctx, &cookie, sizeof(cookie)); 434 SipHash_Final((uint8_t *)&siphash, &ctx); 435 436 return (siphash[0] ^ siphash[1]); 437 } 438 439 uint32_t 440 pf_syncookie_generate(struct mbuf *m, int off, struct pf_pdesc *pd, 441 uint16_t mss) 442 { 443 uint8_t i, wscale; 444 uint32_t iss, hash; 445 union pf_syncookie cookie; 446 447 PF_RULES_RASSERT(); 448 449 cookie.cookie = 0; 450 451 /* map MSS */ 452 for (i = nitems(pf_syncookie_msstab) - 1; 453 pf_syncookie_msstab[i] > mss && i > 0; i--) 454 /* nada */; 455 cookie.flags.mss_idx = i; 456 457 /* map WSCALE */ 458 wscale = pf_get_wscale(m, off, pd->hdr.tcp.th_off, pd->af); 459 for (i = nitems(pf_syncookie_wstab) - 1; 460 pf_syncookie_wstab[i] > wscale && i > 0; i--) 461 /* nada */; 462 cookie.flags.wscale_idx = i; 463 cookie.flags.sack_ok = 0; /* XXX */ 464 465 cookie.flags.oddeven = V_pf_syncookie_status.oddeven; 466 hash = pf_syncookie_mac(pd, cookie, ntohl(pd->hdr.tcp.th_seq)); 467 468 /* 469 * Put the flags into the hash and XOR them to get better ISS number 470 * variance. This doesn't enhance the cryptographic strength and is 471 * done to prevent the 8 cookie bits from showing up directly on the 472 * wire. 473 */ 474 iss = hash & ~0xff; 475 iss |= cookie.cookie ^ (hash >> 24); 476 477 return (iss); 478 } 479 480 struct mbuf * 481 pf_syncookie_recreate_syn(uint8_t ttl, int off, struct pf_pdesc *pd) 482 { 483 uint8_t wscale; 484 uint16_t mss; 485 uint32_t ack, seq; 486 union pf_syncookie cookie; 487 488 seq = ntohl(pd->hdr.tcp.th_seq) - 1; 489 ack = ntohl(pd->hdr.tcp.th_ack) - 1; 490 cookie.cookie = (ack & 0xff) ^ (ack >> 24); 491 492 if (cookie.flags.mss_idx >= nitems(pf_syncookie_msstab) || 493 cookie.flags.wscale_idx >= nitems(pf_syncookie_wstab)) 494 return (NULL); 495 496 mss = pf_syncookie_msstab[cookie.flags.mss_idx]; 497 wscale = pf_syncookie_wstab[cookie.flags.wscale_idx]; 498 499 return (pf_build_tcp(NULL, pd->af, pd->src, pd->dst, *pd->sport, 500 *pd->dport, seq, 0, TH_SYN, wscale, mss, ttl, 0, 501 PF_TAG_SYNCOOKIE_RECREATED)); 502 } 503