1 /* $OpenBSD: pf_syncookies.c,v 1.7 2018/09/10 15:54:28 henning Exp $ */
3 /* Copyright (c) 2016,2017 Henning Brauer <henning@openbsd.org>
4 * Copyright (c) 2016 Alexandr Nedvedicky <sashan@openbsd.org>
6 * syncookie parts based on FreeBSD sys/netinet/tcp_syncache.c
8 * Copyright (c) 2001 McAfee, Inc.
9 * Copyright (c) 2006,2013 Andre Oppermann, Internet Business Solutions AG
10 * All rights reserved.
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.]
17 * Redistribution and use in source and binary forms, with or without
18 * modification, are permitted provided that the following conditions
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.
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
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
48 * syncookie enabled Initial Sequence Number:
52 * 1 bit SACK permitted
53 * 1 bit odd/even secret
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)
63 #include <sys/param.h>
64 #include <sys/systm.h>
66 #include <sys/filio.h>
67 #include <sys/socket.h>
68 #include <sys/socketvar.h>
69 #include <sys/kernel.h>
72 #include <sys/rwlock.h>
73 #include <sys/syslog.h>
75 #include <crypto/siphash/siphash.h>
78 #include <net/if_var.h>
79 #include <net/if_types.h>
80 #include <net/route.h>
82 #include <netinet/in.h>
83 #include <netinet/ip.h>
84 #include <netinet/tcp.h>
85 #include <netinet/tcp_var.h>
87 #include <net/pfvar.h>
88 #include <netpfil/pf/pf_nv.h>
90 #define DPFPRINTF(n, x) if (V_pf_status.debug >= (n)) printf x
102 #define PF_SYNCOOKIE_SECRET_SIZE SIPHASH_KEY_LENGTH
103 #define PF_SYNCOOKIE_SECRET_LIFETIME 15 /* seconds */
105 /* Protected by PF_RULES_xLOCK. */
106 struct pf_syncookie_status {
107 struct callout keytimeout;
109 uint8_t key[2][SIPHASH_KEY_LENGTH];
110 uint32_t hiwat; /* absolute; # of states */
113 VNET_DEFINE_STATIC(struct pf_syncookie_status, pf_syncookie_status);
114 #define V_pf_syncookie_status VNET(pf_syncookie_status)
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,
121 uint32_t pf_syncookie_generate(struct mbuf *m, int off, struct pf_pdesc *,
125 pf_syncookies_init(void)
127 callout_init(&V_pf_syncookie_status.keytimeout, 1);
129 pf_syncookies_setmode(PF_SYNCOOKIES_NEVER);
134 pf_syncookies_cleanup(void)
136 callout_stop(&V_pf_syncookie_status.keytimeout);
140 pf_get_syncookies(struct pfioc_nv *nv)
142 nvlist_t *nvl = NULL;
143 void *nvlpacked = NULL;
146 #define ERROUT(x) ERROUT_FUNCTION(errout, x)
148 nvl = nvlist_create(0);
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);
159 nvlpacked = nvlist_pack(nvl, &nv->len);
160 if (nvlpacked == NULL)
165 } else if (nv->size < nv->len) {
169 error = copyout(nvlpacked, nv->data, nv->len);
174 free(nvlpacked, M_NVLIST);
180 pf_set_syncookies(struct pfioc_nv *nv)
182 nvlist_t *nvl = NULL;
183 void *nvlpacked = NULL;
185 bool enabled, adaptive;
186 uint32_t hiwat, lowat;
189 #define ERROUT(x) ERROUT_FUNCTION(errout, x)
191 if (nv->len > pf_ioctl_maxcount)
194 nvlpacked = malloc(nv->len, M_NVLIST, M_WAITOK);
195 if (nvlpacked == NULL)
198 error = copyin(nv->data, nvlpacked, nv->len);
202 nvl = nvlist_unpack(nvlpacked, nv->len, 0);
206 if (! nvlist_exists_bool(nvl, "enabled")
207 || ! nvlist_exists_bool(nvl, "adaptive"))
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));
220 newmode = PF_SYNCOOKIES_NEVER;
222 newmode = adaptive ? PF_SYNCOOKIES_ADAPTIVE : PF_SYNCOOKIES_ALWAYS;
225 error = pf_syncookies_setmode(newmode);
227 V_pf_syncookie_status.lowat = lowat;
228 V_pf_syncookie_status.hiwat = hiwat;
235 free(nvlpacked, M_NVLIST);
241 pf_syncookies_setmode(u_int8_t mode)
243 if (mode > PF_SYNCOOKIES_MODE_MAX)
246 if (V_pf_status.syncookies_mode == mode)
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;
258 pf_synflood_check(struct pf_pdesc *pd)
260 MPASS(pd->proto == IPPROTO_TCP);
263 if (pd->pf_mtag && (pd->pf_mtag->tag & PF_TAG_SYNCOOKIE_RECREATED))
266 if (V_pf_status.syncookies_mode != PF_SYNCOOKIES_ADAPTIVE)
267 return (V_pf_status.syncookies_mode);
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
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]++;
283 return (V_pf_status.syncookies_active);
287 pf_syncookie_send(struct mbuf *m, int off, struct pf_pdesc *pd)
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,
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],
304 pf_syncookie_validate(struct pf_pdesc *pd)
306 uint32_t hash, ack, seq;
307 union pf_syncookie cookie;
309 MPASS(pd->proto == IPPROTO_TCP);
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);
316 /* we don't know oddeven before setting the cookie (union) */
317 if (atomic_load_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven])
321 hash = pf_syncookie_mac(pd, cookie, seq);
322 if ((ack & ~0xff) != (hash & ~0xff))
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);
332 * all following functions private
335 pf_syncookie_rotate(void *arg)
337 CURVNET_SET((struct vnet *)arg);
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)
348 V_pf_status.syncookies_active = false;
349 DPFPRINTF(PF_DEBUG_MISC, ("syncookies disabled\n"));
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);
365 /* new key, including timeout */
366 pf_syncookie_newkey();
373 pf_syncookie_newkey(void)
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);
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%
392 static int pf_syncookie_msstab[] =
393 { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
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
402 static int pf_syncookie_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
405 pf_syncookie_mac(struct pf_pdesc *pd, union pf_syncookie cookie, uint32_t seq)
411 MPASS(pd->proto == IPPROTO_TCP);
413 SipHash24_Init(&ctx);
414 SipHash_SetKey(&ctx, V_pf_syncookie_status.key[cookie.flags.oddeven]);
418 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v4));
419 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v4));
422 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v6));
423 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v6));
426 panic("unknown address family");
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);
435 return (siphash[0] ^ siphash[1]);
439 pf_syncookie_generate(struct mbuf *m, int off, struct pf_pdesc *pd,
444 union pf_syncookie cookie;
451 for (i = nitems(pf_syncookie_msstab) - 1;
452 pf_syncookie_msstab[i] > mss && i > 0; i--)
454 cookie.flags.mss_idx = i;
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--)
461 cookie.flags.wscale_idx = i;
462 cookie.flags.sack_ok = 0; /* XXX */
464 cookie.flags.oddeven = V_pf_syncookie_status.oddeven;
465 hash = pf_syncookie_mac(pd, cookie, ntohl(pd->hdr.tcp.th_seq));
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
474 iss |= cookie.cookie ^ (hash >> 24);
480 pf_syncookie_recreate_syn(uint8_t ttl, int off, struct pf_pdesc *pd)
485 union pf_syncookie cookie;
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);
491 if (cookie.flags.mss_idx >= nitems(pf_syncookie_msstab) ||
492 cookie.flags.wscale_idx >= nitems(pf_syncookie_wstab))
495 mss = pf_syncookie_msstab[cookie.flags.mss_idx];
496 wscale = pf_syncookie_wstab[cookie.flags.wscale_idx];
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));