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
131 V_pf_syncookie_status.hiwat = PF_SYNCOOKIES_HIWATPCT *
132 V_pf_limits[PF_LIMIT_STATES].limit / 100;
133 V_pf_syncookie_status.lowat = PF_SYNCOOKIES_LOWATPCT *
134 V_pf_limits[PF_LIMIT_STATES].limit / 100;
135 pf_syncookies_setmode(PF_SYNCOOKIES_ADAPTIVE);
136
137 PF_RULES_WUNLOCK();
138 }
139
140 void
141 pf_syncookies_cleanup(void)
142 {
143 callout_stop(&V_pf_syncookie_status.keytimeout);
144 }
145
146 int
147 pf_get_syncookies(struct pfioc_nv *nv)
148 {
149 nvlist_t *nvl = NULL;
150 void *nvlpacked = NULL;
151 int error;
152
153 #define ERROUT(x) ERROUT_FUNCTION(errout, x)
154
155 nvl = nvlist_create(0);
156 if (nvl == NULL)
157 ERROUT(ENOMEM);
158
159 nvlist_add_bool(nvl, "enabled",
160 V_pf_status.syncookies_mode != PF_SYNCOOKIES_NEVER);
161 nvlist_add_bool(nvl, "adaptive",
162 V_pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE);
163 nvlist_add_number(nvl, "highwater", V_pf_syncookie_status.hiwat);
164 nvlist_add_number(nvl, "lowwater", V_pf_syncookie_status.lowat);
165
166 nvlpacked = nvlist_pack(nvl, &nv->len);
167 if (nvlpacked == NULL)
168 ERROUT(ENOMEM);
169
170 if (nv->size == 0) {
171 ERROUT(0);
172 } else if (nv->size < nv->len) {
173 ERROUT(ENOSPC);
174 }
175
176 error = copyout(nvlpacked, nv->data, nv->len);
177
178 #undef ERROUT
179 errout:
180 nvlist_destroy(nvl);
181 free(nvlpacked, M_NVLIST);
182
183 return (error);
184 }
185
186 int
187 pf_set_syncookies(struct pfioc_nv *nv)
188 {
189 nvlist_t *nvl = NULL;
190 void *nvlpacked = NULL;
191 int error;
192 bool enabled, adaptive;
193 uint32_t hiwat, lowat;
194 uint8_t newmode;
195
196 #define ERROUT(x) ERROUT_FUNCTION(errout, x)
197
198 if (nv->len > pf_ioctl_maxcount)
199 return (ENOMEM);
200
201 nvlpacked = malloc(nv->len, M_NVLIST, M_WAITOK);
202 if (nvlpacked == NULL)
203 return (ENOMEM);
204
205 error = copyin(nv->data, nvlpacked, nv->len);
206 if (error)
207 ERROUT(error);
208
209 nvl = nvlist_unpack(nvlpacked, nv->len, 0);
210 if (nvl == NULL)
211 ERROUT(EBADMSG);
212
213 if (! nvlist_exists_bool(nvl, "enabled")
214 || ! nvlist_exists_bool(nvl, "adaptive"))
215 ERROUT(EBADMSG);
216
217 enabled = nvlist_get_bool(nvl, "enabled");
218 adaptive = nvlist_get_bool(nvl, "adaptive");
219 PFNV_CHK(pf_nvuint32_opt(nvl, "highwater", &hiwat,
220 V_pf_syncookie_status.hiwat));
221 PFNV_CHK(pf_nvuint32_opt(nvl, "lowwater", &lowat,
222 V_pf_syncookie_status.lowat));
223
224 if (lowat >= hiwat)
225 ERROUT(EINVAL);
226
227 newmode = PF_SYNCOOKIES_NEVER;
228 if (enabled)
229 newmode = adaptive ? PF_SYNCOOKIES_ADAPTIVE : PF_SYNCOOKIES_ALWAYS;
230
231 PF_RULES_WLOCK();
232 error = pf_syncookies_setmode(newmode);
233
234 V_pf_syncookie_status.lowat = lowat;
235 V_pf_syncookie_status.hiwat = hiwat;
236
237 PF_RULES_WUNLOCK();
238
239 #undef ERROUT
240 errout:
241 nvlist_destroy(nvl);
242 free(nvlpacked, M_NVLIST);
243
244 return (error);
245 }
246
247 static int
248 pf_syncookies_setmode(u_int8_t mode)
249 {
250 if (mode > PF_SYNCOOKIES_MODE_MAX)
251 return (EINVAL);
252
253 if (V_pf_status.syncookies_mode == mode)
254 return (0);
255
256 V_pf_status.syncookies_mode = mode;
257 if (V_pf_status.syncookies_mode == PF_SYNCOOKIES_ALWAYS) {
258 pf_syncookie_newkey();
259 V_pf_status.syncookies_active = true;
260 }
261 return (0);
262 }
263
264 int
265 pf_synflood_check(struct pf_pdesc *pd)
266 {
267 MPASS(pd->proto == IPPROTO_TCP);
268 PF_RULES_RASSERT();
269
270 if (pd->pf_mtag && (pd->pf_mtag->tag & PF_TAG_SYNCOOKIE_RECREATED))
271 return (0);
272
273 if (V_pf_status.syncookies_mode != PF_SYNCOOKIES_ADAPTIVE)
274 return (V_pf_status.syncookies_mode);
275
276 if (!V_pf_status.syncookies_active &&
277 atomic_load_32(&V_pf_status.states_halfopen) >
278 V_pf_syncookie_status.hiwat) {
279 /* We'd want to 'pf_syncookie_newkey()' here, but that requires
280 * the rules write lock, which we can't get with the read lock
281 * held. */
282 callout_reset(&V_pf_syncookie_status.keytimeout, 0,
283 pf_syncookie_rotate, curvnet);
284 V_pf_status.syncookies_active = true;
285 DPFPRINTF(LOG_WARNING,
286 ("synflood detected, enabling syncookies\n"));
287 // XXXTODO V_pf_status.lcounters[LCNT_SYNFLOODS]++;
288 }
289
290 return (V_pf_status.syncookies_active);
291 }
292
293 void
294 pf_syncookie_send(struct mbuf *m, int off, struct pf_pdesc *pd)
295 {
296 uint16_t mss;
297 uint32_t iss;
298
299 mss = max(V_tcp_mssdflt, pf_get_mss(m, off, pd->hdr.tcp.th_off, pd->af));
300 iss = pf_syncookie_generate(m, off, pd, mss);
301 pf_send_tcp(NULL, pd->af, pd->dst, pd->src, *pd->dport, *pd->sport,
302 iss, ntohl(pd->hdr.tcp.th_seq) + 1, TH_SYN|TH_ACK, 0, mss,
303 0, 1, 0);
304 counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_SENT], 1);
305 /* XXX Maybe only in adaptive mode? */
306 atomic_add_64(&V_pf_status.syncookies_inflight[V_pf_syncookie_status.oddeven],
307 1);
308 }
309
310 bool
311 pf_syncookie_check(struct pf_pdesc *pd)
312 {
313 uint32_t hash, ack, seq;
314 union pf_syncookie cookie;
315
316 MPASS(pd->proto == IPPROTO_TCP);
317 PF_RULES_RASSERT();
318
319 seq = ntohl(pd->hdr.tcp.th_seq) - 1;
320 ack = ntohl(pd->hdr.tcp.th_ack) - 1;
321 cookie.cookie = (ack & 0xff) ^ (ack >> 24);
322
323 /* we don't know oddeven before setting the cookie (union) */
324 if (atomic_load_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven])
325 == 0)
326 return (0);
327
328 hash = pf_syncookie_mac(pd, cookie, seq);
329 if ((ack & ~0xff) != (hash & ~0xff))
330 return (false);
331
332 return (true);
333 }
334
335 uint8_t
336 pf_syncookie_validate(struct pf_pdesc *pd)
337 {
338 uint32_t ack;
339 union pf_syncookie cookie;
340
341 if (! pf_syncookie_check(pd))
342 return (0);
343
344 ack = ntohl(pd->hdr.tcp.th_ack) - 1;
345 cookie.cookie = (ack & 0xff) ^ (ack >> 24);
346
347 counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_VALID], 1);
348 atomic_add_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven], -1);
349
350 return (1);
351 }
352
353 /*
354 * all following functions private
355 */
356 void
357 pf_syncookie_rotate(void *arg)
358 {
359 CURVNET_SET((struct vnet *)arg);
360
361 /* do we want to disable syncookies? */
362 if (V_pf_status.syncookies_active &&
363 ((V_pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE &&
364 (atomic_load_32(&V_pf_status.states_halfopen) +
365 atomic_load_64(&V_pf_status.syncookies_inflight[0]) +
366 atomic_load_64(&V_pf_status.syncookies_inflight[1])) <
367 V_pf_syncookie_status.lowat) ||
368 V_pf_status.syncookies_mode == PF_SYNCOOKIES_NEVER)
369 ) {
370 V_pf_status.syncookies_active = false;
371 DPFPRINTF(PF_DEBUG_MISC, ("syncookies disabled\n"));
372 }
373
374 /* nothing in flight any more? delete keys and return */
375 if (!V_pf_status.syncookies_active &&
376 atomic_load_64(&V_pf_status.syncookies_inflight[0]) == 0 &&
377 atomic_load_64(&V_pf_status.syncookies_inflight[1]) == 0) {
378 memset(V_pf_syncookie_status.key[0], 0,
379 PF_SYNCOOKIE_SECRET_SIZE);
380 memset(V_pf_syncookie_status.key[1], 0,
381 PF_SYNCOOKIE_SECRET_SIZE);
382 CURVNET_RESTORE();
383 return;
384 }
385
386 PF_RULES_WLOCK();
387 /* new key, including timeout */
388 pf_syncookie_newkey();
389 PF_RULES_WUNLOCK();
390
391 CURVNET_RESTORE();
392 }
393
394 void
395 pf_syncookie_newkey(void)
396 {
397 PF_RULES_WASSERT();
398
399 MPASS(V_pf_syncookie_status.oddeven < 2);
400 V_pf_syncookie_status.oddeven = (V_pf_syncookie_status.oddeven + 1) & 0x1;
401 atomic_store_64(&V_pf_status.syncookies_inflight[V_pf_syncookie_status.oddeven], 0);
402 arc4random_buf(V_pf_syncookie_status.key[V_pf_syncookie_status.oddeven],
403 PF_SYNCOOKIE_SECRET_SIZE);
404 callout_reset(&V_pf_syncookie_status.keytimeout,
405 PF_SYNCOOKIE_SECRET_LIFETIME * hz, pf_syncookie_rotate, curvnet);
406 }
407
408 /*
409 * Distribution and probability of certain MSS values. Those in between are
410 * rounded down to the next lower one.
411 * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011]
412 * .2% .3% 5% 7% 7% 20% 15% 45%
413 */
414 static int pf_syncookie_msstab[] =
415 { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
416
417 /*
418 * Distribution and probability of certain WSCALE values.
419 * The absence of the WSCALE option is encoded with index zero.
420 * [WSCALE values histograms, Allman, 2012]
421 * X 10 10 35 5 6 14 10% by host
422 * X 11 4 5 5 18 49 3% by connections
423 */
424 static int pf_syncookie_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
425
426 uint32_t
427 pf_syncookie_mac(struct pf_pdesc *pd, union pf_syncookie cookie, uint32_t seq)
428 {
429 SIPHASH_CTX ctx;
430 uint32_t siphash[2];
431
432 PF_RULES_RASSERT();
433 MPASS(pd->proto == IPPROTO_TCP);
434
435 SipHash24_Init(&ctx);
436 SipHash_SetKey(&ctx, V_pf_syncookie_status.key[cookie.flags.oddeven]);
437
438 switch (pd->af) {
439 case AF_INET:
440 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v4));
441 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v4));
442 break;
443 case AF_INET6:
444 SipHash_Update(&ctx, pd->src, sizeof(pd->src->v6));
445 SipHash_Update(&ctx, pd->dst, sizeof(pd->dst->v6));
446 break;
447 default:
448 panic("unknown address family");
449 }
450
451 SipHash_Update(&ctx, pd->sport, sizeof(*pd->sport));
452 SipHash_Update(&ctx, pd->dport, sizeof(*pd->dport));
453 SipHash_Update(&ctx, &seq, sizeof(seq));
454 SipHash_Update(&ctx, &cookie, sizeof(cookie));
455 SipHash_Final((uint8_t *)&siphash, &ctx);
456
457 return (siphash[0] ^ siphash[1]);
458 }
459
460 uint32_t
461 pf_syncookie_generate(struct mbuf *m, int off, struct pf_pdesc *pd,
462 uint16_t mss)
463 {
464 uint8_t i, wscale;
465 uint32_t iss, hash;
466 union pf_syncookie cookie;
467
468 PF_RULES_RASSERT();
469
470 cookie.cookie = 0;
471
472 /* map MSS */
473 for (i = nitems(pf_syncookie_msstab) - 1;
474 pf_syncookie_msstab[i] > mss && i > 0; i--)
475 /* nada */;
476 cookie.flags.mss_idx = i;
477
478 /* map WSCALE */
479 wscale = pf_get_wscale(m, off, pd->hdr.tcp.th_off, pd->af);
480 for (i = nitems(pf_syncookie_wstab) - 1;
481 pf_syncookie_wstab[i] > wscale && i > 0; i--)
482 /* nada */;
483 cookie.flags.wscale_idx = i;
484 cookie.flags.sack_ok = 0; /* XXX */
485
486 cookie.flags.oddeven = V_pf_syncookie_status.oddeven;
487 hash = pf_syncookie_mac(pd, cookie, ntohl(pd->hdr.tcp.th_seq));
488
489 /*
490 * Put the flags into the hash and XOR them to get better ISS number
491 * variance. This doesn't enhance the cryptographic strength and is
492 * done to prevent the 8 cookie bits from showing up directly on the
493 * wire.
494 */
495 iss = hash & ~0xff;
496 iss |= cookie.cookie ^ (hash >> 24);
497
498 return (iss);
499 }
500
501 struct mbuf *
502 pf_syncookie_recreate_syn(uint8_t ttl, int off, struct pf_pdesc *pd)
503 {
504 uint8_t wscale;
505 uint16_t mss;
506 uint32_t ack, seq;
507 union pf_syncookie cookie;
508
509 seq = ntohl(pd->hdr.tcp.th_seq) - 1;
510 ack = ntohl(pd->hdr.tcp.th_ack) - 1;
511 cookie.cookie = (ack & 0xff) ^ (ack >> 24);
512
513 if (cookie.flags.mss_idx >= nitems(pf_syncookie_msstab) ||
514 cookie.flags.wscale_idx >= nitems(pf_syncookie_wstab))
515 return (NULL);
516
517 mss = pf_syncookie_msstab[cookie.flags.mss_idx];
518 wscale = pf_syncookie_wstab[cookie.flags.wscale_idx];
519
520 return (pf_build_tcp(NULL, pd->af, pd->src, pd->dst, *pd->sport,
521 *pd->dport, seq, 0, TH_SYN, wscale, mss, ttl, 0,
522 PF_TAG_SYNCOOKIE_RECREATED));
523 }
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