sys/netpfil/pf/pf_syncookies.c

   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 }