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/pool.h>
72 #include <sys/proc.h>
73 #include <sys/rwlock.h>
74 #include <sys/syslog.h>
75
76 #include <net/if.h>
77 #include <net/if_var.h>
78 #include <net/if_types.h>
79 #include <net/route.h>
80
81 #include <netinet/in.h>
82 #include <netinet/ip.h>
83 #include <netinet/ip_var.h>
84 #include <netinet/tcp.h>
85 #include <netinet/tcp_seq.h>
86 #include <netinet/udp.h>
87 #include <netinet/ip_icmp.h>
88 #include <netinet/in_pcb.h>
89 #include <netinet/tcp_timer.h>
90 #include <netinet/tcp_var.h>
91 #include <netinet/tcp_fsm.h>
92 #include <netinet/udp_var.h>
93 #include <netinet/icmp_var.h>
94 #include <netinet/ip_divert.h>
95
96 #include <net/pfvar.h>
97 #include <net/pfvar_priv.h>
98
99 #if NPFLOG > 0
100 #include <net/if_pflog.h>
101 #endif /* NPFLOG > 0 */
102
103 union pf_syncookie {
104 uint8_t cookie;
105 struct {
106 uint8_t oddeven:1,
107 sack_ok:1,
108 wscale_idx:3,
109 mss_idx:3;
110 } flags;
111 };
112
113 #define PF_SYNCOOKIE_SECRET_SIZE SIPHASH_KEY_LENGTH
114 #define PF_SYNCOOKIE_SECRET_LIFETIME 15 /* seconds */
115
116 static struct {
117 struct timeout keytimeout;
118 volatile uint oddeven;
119 SIPHASH_KEY key[2];
120 uint32_t hiwat; /* absolute; # of states */
121 uint32_t lowat;
122 } pf_syncookie_status;
123
124 void pf_syncookie_rotate(void *);
125 void pf_syncookie_newkey(void);
126 uint32_t pf_syncookie_mac(struct pf_pdesc *, union pf_syncookie,
127 uint32_t);
128 uint32_t pf_syncookie_generate(struct pf_pdesc *, uint16_t);
129
130 void
pf_syncookies_init(void)131 pf_syncookies_init(void)
132 {
133 timeout_set(&pf_syncookie_status.keytimeout,
134 pf_syncookie_rotate, NULL);
135 pf_syncookie_status.hiwat = PFSTATE_HIWAT * PF_SYNCOOKIES_HIWATPCT/100;
136 pf_syncookie_status.lowat = PFSTATE_HIWAT * PF_SYNCOOKIES_LOWATPCT/100;
137 pf_syncookies_setmode(PF_SYNCOOKIES_NEVER);
138 }
139
140 int
pf_syncookies_setmode(u_int8_t mode)141 pf_syncookies_setmode(u_int8_t mode)
142 {
143 if (mode > PF_SYNCOOKIES_MODE_MAX)
144 return (EINVAL);
145
146 if (pf_status.syncookies_mode == mode)
147 return (0);
148
149 pf_status.syncookies_mode = mode;
150 if (pf_status.syncookies_mode == PF_SYNCOOKIES_ALWAYS) {
151 pf_syncookie_newkey();
152 pf_status.syncookies_active = 1;
153 }
154 return (0);
155 }
156
157 int
pf_syncookies_setwats(u_int32_t hiwat,u_int32_t lowat)158 pf_syncookies_setwats(u_int32_t hiwat, u_int32_t lowat)
159 {
160 if (lowat > hiwat)
161 return (EINVAL);
162
163 pf_syncookie_status.hiwat = hiwat;
164 pf_syncookie_status.lowat = lowat;
165 return (0);
166 }
167
168 int
pf_syncookies_getwats(struct pfioc_synflwats * wats)169 pf_syncookies_getwats(struct pfioc_synflwats *wats)
170 {
171 wats->hiwat = pf_syncookie_status.hiwat;
172 wats->lowat = pf_syncookie_status.lowat;
173 return (0);
174 }
175
176 int
pf_synflood_check(struct pf_pdesc * pd)177 pf_synflood_check(struct pf_pdesc *pd)
178 {
179 KASSERT (pd->proto == IPPROTO_TCP);
180
181 if (pd->m && (pd->m->m_pkthdr.pf.tag & PF_TAG_SYNCOOKIE_RECREATED))
182 return (0);
183
184 if (pf_status.syncookies_mode != PF_SYNCOOKIES_ADAPTIVE)
185 return (pf_status.syncookies_mode);
186
187 if (!pf_status.syncookies_active &&
188 pf_status.states_halfopen > pf_syncookie_status.hiwat) {
189 pf_syncookie_newkey();
190 pf_status.syncookies_active = 1;
191 DPFPRINTF(LOG_WARNING,
192 "synflood detected, enabling syncookies");
193 pf_status.lcounters[LCNT_SYNFLOODS]++;
194 }
195
196 return (pf_status.syncookies_active);
197 }
198
199 void
pf_syncookie_send(struct pf_pdesc * pd)200 pf_syncookie_send(struct pf_pdesc *pd)
201 {
202 uint16_t mss;
203 uint32_t iss;
204
205 mss = max(tcp_mssdflt, pf_get_mss(pd));
206 iss = pf_syncookie_generate(pd, mss);
207 pf_send_tcp(NULL, pd->af, pd->dst, pd->src, *pd->dport, *pd->sport,
208 iss, ntohl(pd->hdr.tcp.th_seq) + 1, TH_SYN|TH_ACK, 0, mss,
209 0, 1, 0, pd->rdomain);
210 pf_status.syncookies_inflight[pf_syncookie_status.oddeven]++;
211 pf_status.lcounters[LCNT_SYNCOOKIES_SENT]++;
212 }
213
214 uint8_t
pf_syncookie_validate(struct pf_pdesc * pd)215 pf_syncookie_validate(struct pf_pdesc *pd)
216 {
217 uint32_t hash, ack, seq;
218 union pf_syncookie cookie;
219
220 KASSERT(pd->proto == IPPROTO_TCP);
221
222 seq = ntohl(pd->hdr.tcp.th_seq) - 1;
223 ack = ntohl(pd->hdr.tcp.th_ack) - 1;
224 cookie.cookie = (ack & 0xff) ^ (ack >> 24);
225
226 /* we don't know oddeven before setting the cookie (union) */
227 if (pf_status.syncookies_inflight[cookie.flags.oddeven] == 0)
228 return (0);
229
230 hash = pf_syncookie_mac(pd, cookie, seq);
231 if ((ack & ~0xff) != (hash & ~0xff))
232 return (0);
233
234 pf_status.syncookies_inflight[cookie.flags.oddeven]--;
235 pf_status.lcounters[LCNT_SYNCOOKIES_VALID]++;
236 return (1);
237 }
238
239 /*
240 * all following functions private
241 */
242 void
pf_syncookie_rotate(void * arg)243 pf_syncookie_rotate(void *arg)
244 {
245 /* do we want to disable syncookies? */
246 if (pf_status.syncookies_active &&
247 ((pf_status.syncookies_mode == PF_SYNCOOKIES_ADAPTIVE &&
248 pf_status.states_halfopen + pf_status.syncookies_inflight[0] +
249 pf_status.syncookies_inflight[1] < pf_syncookie_status.lowat) ||
250 pf_status.syncookies_mode == PF_SYNCOOKIES_NEVER)) {
251 pf_status.syncookies_active = 0;
252 DPFPRINTF(LOG_WARNING, "syncookies disabled");
253 }
254
255 /* nothing in flight any more? delete keys and return */
256 if (!pf_status.syncookies_active &&
257 pf_status.syncookies_inflight[0] == 0 &&
258 pf_status.syncookies_inflight[1] == 0) {
259 memset(&pf_syncookie_status.key[0], 0,
260 PF_SYNCOOKIE_SECRET_SIZE);
261 memset(&pf_syncookie_status.key[1], 0,
262 PF_SYNCOOKIE_SECRET_SIZE);
263 return;
264 }
265
266 /* new key, including timeout */
267 pf_syncookie_newkey();
268 }
269
270 void
pf_syncookie_newkey(void)271 pf_syncookie_newkey(void)
272 {
273 pf_syncookie_status.oddeven = (pf_syncookie_status.oddeven + 1) & 0x1;
274 pf_status.syncookies_inflight[pf_syncookie_status.oddeven] = 0;
275 arc4random_buf(&pf_syncookie_status.key[pf_syncookie_status.oddeven],
276 PF_SYNCOOKIE_SECRET_SIZE);
277 timeout_add_sec(&pf_syncookie_status.keytimeout,
278 PF_SYNCOOKIE_SECRET_LIFETIME);
279 }
280
281 /*
282 * Distribution and probability of certain MSS values. Those in between are
283 * rounded down to the next lower one.
284 * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011]
285 * .2% .3% 5% 7% 7% 20% 15% 45%
286 */
287 static int pf_syncookie_msstab[] =
288 { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
289
290 /*
291 * Distribution and probability of certain WSCALE values.
292 * The absence of the WSCALE option is encoded with index zero.
293 * [WSCALE values histograms, Allman, 2012]
294 * X 10 10 35 5 6 14 10% by host
295 * X 11 4 5 5 18 49 3% by connections
296 */
297 static int pf_syncookie_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
298
299 uint32_t
pf_syncookie_mac(struct pf_pdesc * pd,union pf_syncookie cookie,uint32_t seq)300 pf_syncookie_mac(struct pf_pdesc *pd, union pf_syncookie cookie, uint32_t seq)
301 {
302 SIPHASH_CTX ctx;
303 uint32_t siphash[2];
304
305 KASSERT(pd->proto == IPPROTO_TCP);
306
307 SipHash24_Init(&ctx, &pf_syncookie_status.key[cookie.flags.oddeven]);
308
309 switch (pd->af) {
310 case AF_INET:
311 SipHash24_Update(&ctx, pd->src, sizeof(pd->src->v4));
312 SipHash24_Update(&ctx, pd->dst, sizeof(pd->dst->v4));
313 break;
314 case AF_INET6:
315 SipHash24_Update(&ctx, pd->src, sizeof(pd->src->v6));
316 SipHash24_Update(&ctx, pd->dst, sizeof(pd->dst->v6));
317 break;
318 default:
319 panic("unknown address family");
320 }
321
322 SipHash24_Update(&ctx, pd->sport, sizeof(*pd->sport));
323 SipHash24_Update(&ctx, pd->dport, sizeof(*pd->dport));
324 SipHash24_Update(&ctx, &seq, sizeof(seq));
325 SipHash24_Update(&ctx, &cookie, sizeof(cookie));
326 SipHash24_Final((uint8_t *)&siphash, &ctx);
327
328 return (siphash[0] ^ siphash[1]);
329 }
330
331 uint32_t
pf_syncookie_generate(struct pf_pdesc * pd,uint16_t mss)332 pf_syncookie_generate(struct pf_pdesc *pd, uint16_t mss)
333 {
334 uint8_t i, wscale;
335 uint32_t iss, hash;
336 union pf_syncookie cookie;
337
338 cookie.cookie = 0;
339
340 /* map MSS */
341 for (i = nitems(pf_syncookie_msstab) - 1;
342 pf_syncookie_msstab[i] > mss && i > 0; i--)
343 /* nada */;
344 cookie.flags.mss_idx = i;
345
346 /* map WSCALE */
347 wscale = pf_get_wscale(pd);
348 for (i = nitems(pf_syncookie_wstab) - 1;
349 pf_syncookie_wstab[i] > wscale && i > 0; i--)
350 /* nada */;
351 cookie.flags.wscale_idx = i;
352 cookie.flags.sack_ok = 0; /* XXX */
353
354 cookie.flags.oddeven = pf_syncookie_status.oddeven;
355 hash = pf_syncookie_mac(pd, cookie, ntohl(pd->hdr.tcp.th_seq));
356
357 /*
358 * Put the flags into the hash and XOR them to get better ISS number
359 * variance. This doesn't enhance the cryptographic strength and is
360 * done to prevent the 8 cookie bits from showing up directly on the
361 * wire.
362 */
363 iss = hash & ~0xff;
364 iss |= cookie.cookie ^ (hash >> 24);
365
366 return (iss);
367 }
368
369 struct mbuf *
pf_syncookie_recreate_syn(struct pf_pdesc * pd)370 pf_syncookie_recreate_syn(struct pf_pdesc *pd)
371 {
372 uint8_t wscale;
373 uint16_t mss;
374 uint32_t ack, seq;
375 union pf_syncookie cookie;
376
377 seq = ntohl(pd->hdr.tcp.th_seq) - 1;
378 ack = ntohl(pd->hdr.tcp.th_ack) - 1;
379 cookie.cookie = (ack & 0xff) ^ (ack >> 24);
380
381 if (cookie.flags.mss_idx >= nitems(pf_syncookie_msstab) ||
382 cookie.flags.wscale_idx >= nitems(pf_syncookie_wstab))
383 return (NULL);
384
385 mss = pf_syncookie_msstab[cookie.flags.mss_idx];
386 wscale = pf_syncookie_wstab[cookie.flags.wscale_idx];
387
388 return (pf_build_tcp(NULL, pd->af, pd->src, pd->dst, *pd->sport,
389 *pd->dport, seq, 0, TH_SYN, wscale, mss, pd->ttl, 0,
390 PF_TAG_SYNCOOKIE_RECREATED, cookie.flags.sack_ok, pd->rdomain));
391 }
392