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 union pf_syncookie {
92 uint8_t cookie;
93 struct {
94 uint8_t oddeven:1,
95 sack_ok:1,
96 wscale_idx:3,
97 mss_idx:3;
98 } flags;
99 };
100
101 #define PF_SYNCOOKIE_SECRET_SIZE SIPHASH_KEY_LENGTH
102 #define PF_SYNCOOKIE_SECRET_LIFETIME 15 /* seconds */
103
104 /* Protected by PF_RULES_xLOCK. */
105 struct pf_syncookie_status {
106 struct callout keytimeout;
107 uint8_t oddeven;
108 uint8_t key[2][SIPHASH_KEY_LENGTH];
109 uint32_t hiwat; /* absolute; # of states */
110 uint32_t lowat;
111 };
112 VNET_DEFINE_STATIC(struct pf_syncookie_status, pf_syncookie_status);
113 #define V_pf_syncookie_status VNET(pf_syncookie_status)
114
115 static int pf_syncookies_setmode(u_int8_t);
116 void pf_syncookie_rotate(void *);
117 void pf_syncookie_newkey(void);
118 uint32_t pf_syncookie_mac(struct pf_pdesc *, union pf_syncookie,
119 uint32_t);
120 uint32_t pf_syncookie_generate(struct pf_pdesc *, uint16_t);
121
122 void
pf_syncookies_init(void)123 pf_syncookies_init(void)
124 {
125 callout_init(&V_pf_syncookie_status.keytimeout, 1);
126 PF_RULES_WLOCK();
127
128 V_pf_syncookie_status.hiwat = PF_SYNCOOKIES_HIWATPCT *
129 V_pf_limits[PF_LIMIT_STATES].limit / 100;
130 V_pf_syncookie_status.lowat = PF_SYNCOOKIES_LOWATPCT *
131 V_pf_limits[PF_LIMIT_STATES].limit / 100;
132 pf_syncookies_setmode(PF_SYNCOOKIES_ADAPTIVE);
133
134 PF_RULES_WUNLOCK();
135 }
136
137 void
pf_syncookies_cleanup(void)138 pf_syncookies_cleanup(void)
139 {
140 callout_stop(&V_pf_syncookie_status.keytimeout);
141 }
142
143 int
pf_get_syncookies(struct pfioc_nv * nv)144 pf_get_syncookies(struct pfioc_nv *nv)
145 {
146 nvlist_t *nvl = NULL;
147 void *nvlpacked = NULL;
148 int error;
149
150 #define ERROUT(x) ERROUT_FUNCTION(errout, x)
151
152 nvl = nvlist_create(0);
153 if (nvl == NULL)
154 ERROUT(ENOMEM);
155
156 nvlist_add_bool(nvl, "enabled",
157 V_pf_status.syncookies_mode != PF_SYNCOOKIES_NEVER);
158 nvlist_add_bool(nvl, "adaptive",
159 V_pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE);
160 nvlist_add_number(nvl, "highwater", V_pf_syncookie_status.hiwat);
161 nvlist_add_number(nvl, "lowwater", V_pf_syncookie_status.lowat);
162 nvlist_add_number(nvl, "halfopen_states",
163 atomic_load_32(&V_pf_status.states_halfopen));
164
165 nvlpacked = nvlist_pack(nvl, &nv->len);
166 if (nvlpacked == NULL)
167 ERROUT(ENOMEM);
168
169 if (nv->size == 0) {
170 ERROUT(0);
171 } else if (nv->size < nv->len) {
172 ERROUT(ENOSPC);
173 }
174
175 error = copyout(nvlpacked, nv->data, nv->len);
176
177 #undef ERROUT
178 errout:
179 nvlist_destroy(nvl);
180 free(nvlpacked, M_NVLIST);
181
182 return (error);
183 }
184
185 int
pf_set_syncookies(struct pfioc_nv * nv)186 pf_set_syncookies(struct pfioc_nv *nv)
187 {
188 nvlist_t *nvl = NULL;
189 void *nvlpacked = NULL;
190 int error;
191 bool enabled, adaptive;
192 uint32_t hiwat, lowat;
193 uint8_t newmode;
194
195 #define ERROUT(x) ERROUT_FUNCTION(errout, x)
196
197 if (nv->len > pf_ioctl_maxcount)
198 return (ENOMEM);
199
200 nvlpacked = malloc(nv->len, M_NVLIST, M_WAITOK);
201 error = copyin(nv->data, nvlpacked, nv->len);
202 if (error)
203 ERROUT(error);
204
205 nvl = nvlist_unpack(nvlpacked, nv->len, 0);
206 if (nvl == NULL)
207 ERROUT(EBADMSG);
208
209 if (! nvlist_exists_bool(nvl, "enabled")
210 || ! nvlist_exists_bool(nvl, "adaptive"))
211 ERROUT(EBADMSG);
212
213 enabled = nvlist_get_bool(nvl, "enabled");
214 adaptive = nvlist_get_bool(nvl, "adaptive");
215 PFNV_CHK(pf_nvuint32_opt(nvl, "highwater", &hiwat,
216 V_pf_syncookie_status.hiwat));
217 PFNV_CHK(pf_nvuint32_opt(nvl, "lowwater", &lowat,
218 V_pf_syncookie_status.lowat));
219
220 if (lowat >= hiwat)
221 ERROUT(EINVAL);
222
223 newmode = PF_SYNCOOKIES_NEVER;
224 if (enabled)
225 newmode = adaptive ? PF_SYNCOOKIES_ADAPTIVE : PF_SYNCOOKIES_ALWAYS;
226
227 PF_RULES_WLOCK();
228 error = pf_syncookies_setmode(newmode);
229
230 V_pf_syncookie_status.lowat = lowat;
231 V_pf_syncookie_status.hiwat = hiwat;
232
233 PF_RULES_WUNLOCK();
234
235 #undef ERROUT
236 errout:
237 nvlist_destroy(nvl);
238 free(nvlpacked, M_NVLIST);
239
240 return (error);
241 }
242
243 static int
pf_syncookies_setmode(u_int8_t mode)244 pf_syncookies_setmode(u_int8_t mode)
245 {
246 if (mode > PF_SYNCOOKIES_MODE_MAX)
247 return (EINVAL);
248
249 if (V_pf_status.syncookies_mode == mode)
250 return (0);
251
252 V_pf_status.syncookies_mode = mode;
253 if (V_pf_status.syncookies_mode == PF_SYNCOOKIES_ALWAYS) {
254 pf_syncookie_newkey();
255 V_pf_status.syncookies_active = true;
256 }
257 return (0);
258 }
259
260 int
pf_synflood_check(struct pf_pdesc * pd)261 pf_synflood_check(struct pf_pdesc *pd)
262 {
263 MPASS(pd->proto == IPPROTO_TCP);
264 PF_RULES_RASSERT();
265
266 if (pd->pf_mtag && (pd->pf_mtag->flags & PF_MTAG_FLAG_SYNCOOKIE_RECREATED))
267 return (0);
268
269 if (V_pf_status.syncookies_mode != PF_SYNCOOKIES_ADAPTIVE)
270 return (V_pf_status.syncookies_mode);
271
272 if (!V_pf_status.syncookies_active &&
273 atomic_load_32(&V_pf_status.states_halfopen) >
274 V_pf_syncookie_status.hiwat) {
275 /* We'd want to 'pf_syncookie_newkey()' here, but that requires
276 * the rules write lock, which we can't get with the read lock
277 * held. */
278 callout_reset(&V_pf_syncookie_status.keytimeout, 0,
279 pf_syncookie_rotate, curvnet);
280 V_pf_status.syncookies_active = true;
281 DPFPRINTF(LOG_WARNING,
282 "synflood detected, enabling syncookies");
283 // XXXTODO V_pf_status.lcounters[LCNT_SYNFLOODS]++;
284 }
285
286 return (V_pf_status.syncookies_active);
287 }
288
289 void
pf_syncookie_send(struct pf_pdesc * pd)290 pf_syncookie_send(struct pf_pdesc *pd)
291 {
292 uint16_t mss;
293 uint32_t iss;
294
295 mss = max(V_tcp_mssdflt, pf_get_mss(pd));
296 iss = pf_syncookie_generate(pd, mss);
297 pf_send_tcp(NULL, pd->af, pd->dst, pd->src, *pd->dport, *pd->sport,
298 iss, ntohl(pd->hdr.tcp.th_seq) + 1, TH_SYN|TH_ACK, 0, mss,
299 0, M_SKIP_FIREWALL | (pd->m->m_flags & M_LOOP), 0, 0,
300 pd->act.rtableid);
301 counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_SENT], 1);
302 /* XXX Maybe only in adaptive mode? */
303 atomic_add_64(&V_pf_status.syncookies_inflight[V_pf_syncookie_status.oddeven],
304 1);
305 }
306
307 bool
pf_syncookie_check(struct pf_pdesc * pd)308 pf_syncookie_check(struct pf_pdesc *pd)
309 {
310 uint32_t hash, ack, seq;
311 union pf_syncookie cookie;
312
313 MPASS(pd->proto == IPPROTO_TCP);
314 PF_RULES_RASSERT();
315
316 seq = ntohl(pd->hdr.tcp.th_seq) - 1;
317 ack = ntohl(pd->hdr.tcp.th_ack) - 1;
318 cookie.cookie = (ack & 0xff) ^ (ack >> 24);
319
320 /* we don't know oddeven before setting the cookie (union) */
321 if (atomic_load_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven])
322 == 0)
323 return (0);
324
325 hash = pf_syncookie_mac(pd, cookie, seq);
326 if ((ack & ~0xff) != (hash & ~0xff))
327 return (false);
328
329 return (true);
330 }
331
332 uint8_t
pf_syncookie_validate(struct pf_pdesc * pd)333 pf_syncookie_validate(struct pf_pdesc *pd)
334 {
335 uint32_t ack;
336 union pf_syncookie cookie;
337
338 if (! pf_syncookie_check(pd))
339 return (0);
340
341 ack = ntohl(pd->hdr.tcp.th_ack) - 1;
342 cookie.cookie = (ack & 0xff) ^ (ack >> 24);
343
344 counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_VALID], 1);
345 atomic_add_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven], -1);
346
347 return (1);
348 }
349
350 /*
351 * all following functions private
352 */
353 void
pf_syncookie_rotate(void * arg)354 pf_syncookie_rotate(void *arg)
355 {
356 CURVNET_SET((struct vnet *)arg);
357
358 /* do we want to disable syncookies? */
359 if (V_pf_status.syncookies_active &&
360 ((V_pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE &&
361 (atomic_load_32(&V_pf_status.states_halfopen) +
362 atomic_load_64(&V_pf_status.syncookies_inflight[0]) +
363 atomic_load_64(&V_pf_status.syncookies_inflight[1])) <
364 V_pf_syncookie_status.lowat) ||
365 V_pf_status.syncookies_mode == PF_SYNCOOKIES_NEVER)
366 ) {
367 V_pf_status.syncookies_active = false;
368 DPFPRINTF(PF_DEBUG_MISC, "syncookies disabled");
369 }
370
371 /* nothing in flight any more? delete keys and return */
372 if (!V_pf_status.syncookies_active &&
373 atomic_load_64(&V_pf_status.syncookies_inflight[0]) == 0 &&
374 atomic_load_64(&V_pf_status.syncookies_inflight[1]) == 0) {
375 memset(V_pf_syncookie_status.key[0], 0,
376 PF_SYNCOOKIE_SECRET_SIZE);
377 memset(V_pf_syncookie_status.key[1], 0,
378 PF_SYNCOOKIE_SECRET_SIZE);
379 CURVNET_RESTORE();
380 return;
381 }
382
383 PF_RULES_WLOCK();
384 /* new key, including timeout */
385 pf_syncookie_newkey();
386 PF_RULES_WUNLOCK();
387
388 CURVNET_RESTORE();
389 }
390
391 void
pf_syncookie_newkey(void)392 pf_syncookie_newkey(void)
393 {
394 PF_RULES_WASSERT();
395
396 MPASS(V_pf_syncookie_status.oddeven < 2);
397 V_pf_syncookie_status.oddeven = (V_pf_syncookie_status.oddeven + 1) & 0x1;
398 atomic_store_64(&V_pf_status.syncookies_inflight[V_pf_syncookie_status.oddeven], 0);
399 arc4random_buf(V_pf_syncookie_status.key[V_pf_syncookie_status.oddeven],
400 PF_SYNCOOKIE_SECRET_SIZE);
401 callout_reset(&V_pf_syncookie_status.keytimeout,
402 PF_SYNCOOKIE_SECRET_LIFETIME * hz, pf_syncookie_rotate, curvnet);
403 }
404
405 /*
406 * Distribution and probability of certain MSS values. Those in between are
407 * rounded down to the next lower one.
408 * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011]
409 * .2% .3% 5% 7% 7% 20% 15% 45%
410 */
411 static int pf_syncookie_msstab[] =
412 { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
413
414 /*
415 * Distribution and probability of certain WSCALE values.
416 * The absence of the WSCALE option is encoded with index zero.
417 * [WSCALE values histograms, Allman, 2012]
418 * X 10 10 35 5 6 14 10% by host
419 * X 11 4 5 5 18 49 3% by connections
420 */
421 static int pf_syncookie_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
422
423 uint32_t
pf_syncookie_mac(struct pf_pdesc * pd,union pf_syncookie cookie,uint32_t seq)424 pf_syncookie_mac(struct pf_pdesc *pd, union pf_syncookie cookie, uint32_t seq)
425 {
426 SIPHASH_CTX ctx;
427 uint32_t siphash[2];
428
429 PF_RULES_RASSERT();
430 MPASS(pd->proto == IPPROTO_TCP);
431
432 SipHash24_Init(&ctx);
433 SipHash_SetKey(&ctx, V_pf_syncookie_status.key[cookie.flags.oddeven]);
434
435 switch (pd->af) {
436 case AF_INET:
437 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v4));
438 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v4));
439 break;
440 case AF_INET6:
441 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v6));
442 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v6));
443 break;
444 default:
445 panic("unknown address family");
446 }
447
448 SipHash_Update(&ctx, pd->sport, sizeof(*pd->sport));
449 SipHash_Update(&ctx, pd->dport, sizeof(*pd->dport));
450 SipHash_Update(&ctx, &seq, sizeof(seq));
451 SipHash_Update(&ctx, &cookie, sizeof(cookie));
452 SipHash_Final((uint8_t *)&siphash, &ctx);
453
454 return (siphash[0] ^ siphash[1]);
455 }
456
457 uint32_t
pf_syncookie_generate(struct pf_pdesc * pd,uint16_t mss)458 pf_syncookie_generate(struct pf_pdesc *pd, uint16_t mss)
459 {
460 uint8_t i, wscale;
461 uint32_t iss, hash;
462 union pf_syncookie cookie;
463
464 PF_RULES_RASSERT();
465
466 cookie.cookie = 0;
467
468 /* map MSS */
469 for (i = nitems(pf_syncookie_msstab) - 1;
470 pf_syncookie_msstab[i] > mss && i > 0; i--)
471 /* nada */;
472 cookie.flags.mss_idx = i;
473
474 /* map WSCALE */
475 wscale = pf_get_wscale(pd);
476 for (i = nitems(pf_syncookie_wstab) - 1;
477 pf_syncookie_wstab[i] > wscale && i > 0; i--)
478 /* nada */;
479 cookie.flags.wscale_idx = i;
480 cookie.flags.sack_ok = 0; /* XXX */
481
482 cookie.flags.oddeven = V_pf_syncookie_status.oddeven;
483 hash = pf_syncookie_mac(pd, cookie, ntohl(pd->hdr.tcp.th_seq));
484
485 /*
486 * Put the flags into the hash and XOR them to get better ISS number
487 * variance. This doesn't enhance the cryptographic strength and is
488 * done to prevent the 8 cookie bits from showing up directly on the
489 * wire.
490 */
491 iss = hash & ~0xff;
492 iss |= cookie.cookie ^ (hash >> 24);
493
494 return (iss);
495 }
496
497 struct mbuf *
pf_syncookie_recreate_syn(struct pf_pdesc * pd)498 pf_syncookie_recreate_syn(struct pf_pdesc *pd)
499 {
500 uint8_t wscale;
501 uint16_t mss;
502 uint32_t ack, seq;
503 union pf_syncookie cookie;
504
505 seq = ntohl(pd->hdr.tcp.th_seq) - 1;
506 ack = ntohl(pd->hdr.tcp.th_ack) - 1;
507 cookie.cookie = (ack & 0xff) ^ (ack >> 24);
508
509 if (cookie.flags.mss_idx >= nitems(pf_syncookie_msstab) ||
510 cookie.flags.wscale_idx >= nitems(pf_syncookie_wstab))
511 return (NULL);
512
513 mss = pf_syncookie_msstab[cookie.flags.mss_idx];
514 wscale = pf_syncookie_wstab[cookie.flags.wscale_idx];
515
516 return (pf_build_tcp(NULL, pd->af, pd->src, pd->dst, *pd->sport,
517 *pd->dport, seq, 0, TH_SYN, wscale, mss, pd->ttl,
518 (pd->m->m_flags & M_LOOP), 0, PF_MTAG_FLAG_SYNCOOKIE_RECREATED,
519 cookie.flags.sack_ok, pd->act.rtableid));
520 }
521