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;
145 nvl = nvlist_create(0);
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);
156 nvlpacked = nvlist_pack(nvl, &nv->len);
157 if (nvlpacked == NULL) {
163 free(nvlpacked, M_TEMP);
165 } else if (nv->size < nv->len) {
167 free(nvlpacked, M_TEMP);
171 return (copyout(nvlpacked, nv->data, nv->len));
175 pf_set_syncookies(struct pfioc_nv *nv)
177 nvlist_t *nvl = NULL;
178 void *nvlpacked = NULL;
180 bool enabled, adaptive;
181 uint32_t hiwat, lowat;
184 #define ERROUT(x) ERROUT_FUNCTION(errout, x)
186 if (nv->len > pf_ioctl_maxcount)
189 nvlpacked = malloc(nv->len, M_TEMP, M_WAITOK);
190 if (nvlpacked == NULL)
193 error = copyin(nv->data, nvlpacked, nv->len);
197 nvl = nvlist_unpack(nvlpacked, nv->len, 0);
201 if (! nvlist_exists_bool(nvl, "enabled")
202 || ! nvlist_exists_bool(nvl, "adaptive"))
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));
215 newmode = PF_SYNCOOKIES_NEVER;
217 newmode = adaptive ? PF_SYNCOOKIES_ADAPTIVE : PF_SYNCOOKIES_ALWAYS;
220 error = pf_syncookies_setmode(newmode);
222 V_pf_syncookie_status.lowat = lowat;
223 V_pf_syncookie_status.hiwat = hiwat;
230 free(nvlpacked, M_TEMP);
236 pf_syncookies_setmode(u_int8_t mode)
238 if (mode > PF_SYNCOOKIES_MODE_MAX)
241 if (V_pf_status.syncookies_mode == mode)
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;
253 pf_synflood_check(struct pf_pdesc *pd)
255 MPASS(pd->proto == IPPROTO_TCP);
258 if (pd->pf_mtag && (pd->pf_mtag->tag & PF_TAG_SYNCOOKIE_RECREATED))
261 if (V_pf_status.syncookies_mode != PF_SYNCOOKIES_ADAPTIVE)
262 return (V_pf_status.syncookies_mode);
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
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]++;
278 return (V_pf_status.syncookies_active);
282 pf_syncookie_send(struct mbuf *m, int off, struct pf_pdesc *pd)
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,
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],
299 pf_syncookie_validate(struct pf_pdesc *pd)
301 uint32_t hash, ack, seq;
302 union pf_syncookie cookie;
304 MPASS(pd->proto == IPPROTO_TCP);
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);
311 /* we don't know oddeven before setting the cookie (union) */
312 if (atomic_load_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven])
316 hash = pf_syncookie_mac(pd, cookie, seq);
317 if ((ack & ~0xff) != (hash & ~0xff))
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);
327 * all following functions private
330 pf_syncookie_rotate(void *arg)
332 CURVNET_SET((struct vnet *)arg);
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)
343 V_pf_status.syncookies_active = false;
344 DPFPRINTF(PF_DEBUG_MISC, ("syncookies disabled\n"));
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);
360 /* new key, including timeout */
361 pf_syncookie_newkey();
368 pf_syncookie_newkey(void)
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);
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%
387 static int pf_syncookie_msstab[] =
388 { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
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
397 static int pf_syncookie_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
400 pf_syncookie_mac(struct pf_pdesc *pd, union pf_syncookie cookie, uint32_t seq)
406 MPASS(pd->proto == IPPROTO_TCP);
408 SipHash24_Init(&ctx);
409 SipHash_SetKey(&ctx, V_pf_syncookie_status.key[cookie.flags.oddeven]);
413 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v4));
414 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v4));
417 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v6));
418 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v6));
421 panic("unknown address family");
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);
430 return (siphash[0] ^ siphash[1]);
434 pf_syncookie_generate(struct mbuf *m, int off, struct pf_pdesc *pd,
439 union pf_syncookie cookie;
446 for (i = nitems(pf_syncookie_msstab) - 1;
447 pf_syncookie_msstab[i] > mss && i > 0; i--)
449 cookie.flags.mss_idx = i;
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--)
456 cookie.flags.wscale_idx = i;
457 cookie.flags.sack_ok = 0; /* XXX */
459 cookie.flags.oddeven = V_pf_syncookie_status.oddeven;
460 hash = pf_syncookie_mac(pd, cookie, ntohl(pd->hdr.tcp.th_seq));
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
469 iss |= cookie.cookie ^ (hash >> 24);
475 pf_syncookie_recreate_syn(uint8_t ttl, int off, struct pf_pdesc *pd)
480 union pf_syncookie cookie;
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);
486 if (cookie.flags.mss_idx >= nitems(pf_syncookie_msstab) ||
487 cookie.flags.wscale_idx >= nitems(pf_syncookie_wstab))
490 mss = pf_syncookie_msstab[cookie.flags.mss_idx];
491 wscale = pf_syncookie_wstab[cookie.flags.wscale_idx];
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));