deattack.c revision 1.2 
1/*	$NetBSD: deattack.c,v 1.2 2009/06/07 22:38:46 christos Exp $	*/
2/* $OpenBSD: deattack.c,v 1.30 2006/09/16 19:53:37 djm Exp $ */
3/*
4 * Cryptographic attack detector for ssh - source code
5 *
6 * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
7 *
8 * All rights reserved. Redistribution and use in source and binary
9 * forms, with or without modification, are permitted provided that
10 * this copyright notice is retained.
11 *
12 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
13 * WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
14 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
15 * CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
16 * SOFTWARE.
17 *
18 * Ariel Futoransky <futo@core-sdi.com>
19 * <http://www.core-sdi.com>
20 */
21
22#include "includes.h"
23__RCSID("$NetBSD: deattack.c,v 1.2 2009/06/07 22:38:46 christos Exp $");
24#include <sys/types.h>
25
26#include <string.h>
27#include <stdio.h>
28#include <stdarg.h>
29#include <time.h>
30
31#include "xmalloc.h"
32#include "deattack.h"
33#include "log.h"
34#include "crc32.h"
35#include "misc.h"
36
37/*
38 * CRC attack detection has a worst-case behaviour that is O(N^3) over
39 * the number of identical blocks in a packet. This behaviour can be
40 * exploited to create a limited denial of service attack.
41 *
42 * However, because we are dealing with encrypted data, identical
43 * blocks should only occur every 2^35 maximally-sized packets or so.
44 * Consequently, we can detect this DoS by looking for identical blocks
45 * in a packet.
46 *
47 * The parameter below determines how many identical blocks we will
48 * accept in a single packet, trading off between attack detection and
49 * likelihood of terminating a legitimate connection. A value of 32
50 * corresponds to an average of 2^40 messages before an attack is
51 * misdetected
52 */
53#define MAX_IDENTICAL	32
54
55/* SSH Constants */
56#define SSH_MAXBLOCKS	(32 * 1024)
57#define SSH_BLOCKSIZE	(8)
58
59/* Hashing constants */
60#define HASH_MINSIZE	(8 * 1024)
61#define HASH_ENTRYSIZE	(2)
62#define HASH_FACTOR(x)	((x)*3/2)
63#define HASH_UNUSEDCHAR	(0xff)
64#define HASH_UNUSED	(0xffff)
65#define HASH_IV		(0xfffe)
66
67#define HASH_MINBLOCKS	(7*SSH_BLOCKSIZE)
68
69
70/* Hash function (Input keys are cipher results) */
71#define HASH(x)		get_u32(x)
72
73#define CMP(a, b)	(memcmp(a, b, SSH_BLOCKSIZE))
74
75static void
76crc_update(u_int32_t *a, u_int32_t b)
77{
78	b ^= *a;
79	*a = ssh_crc32((u_char *)&b, sizeof(b));
80}
81
82/* detect if a block is used in a particular pattern */
83static int
84check_crc(u_char *S, u_char *buf, u_int32_t len)
85{
86	u_int32_t crc;
87	u_char *c;
88
89	crc = 0;
90	for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
91		if (!CMP(S, c)) {
92			crc_update(&crc, 1);
93			crc_update(&crc, 0);
94		} else {
95			crc_update(&crc, 0);
96			crc_update(&crc, 0);
97		}
98	}
99	return (crc == 0);
100}
101
102
103/* Detect a crc32 compensation attack on a packet */
104int
105detect_attack(u_char *buf, u_int32_t len)
106{
107	static u_int16_t *h = (u_int16_t *) NULL;
108	static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
109	u_int32_t i, j;
110	u_int32_t l, same;
111	u_char *c;
112	u_char *d;
113
114	if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
115	    len % SSH_BLOCKSIZE != 0) {
116		fatal("detect_attack: bad length %d", len);
117	}
118	for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
119		;
120
121	if (h == NULL) {
122		debug("Installing crc compensation attack detector.");
123		h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE);
124		n = l;
125	} else {
126		if (l > n) {
127			h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
128			n = l;
129		}
130	}
131
132	if (len <= HASH_MINBLOCKS) {
133		for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
134			for (d = buf; d < c; d += SSH_BLOCKSIZE) {
135				if (!CMP(c, d)) {
136					if ((check_crc(c, buf, len)))
137						return (DEATTACK_DETECTED);
138					else
139						break;
140				}
141			}
142		}
143		return (DEATTACK_OK);
144	}
145	memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
146
147	for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
148		for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
149		    i = (i + 1) & (n - 1)) {
150			if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
151				if (++same > MAX_IDENTICAL)
152					return (DEATTACK_DOS_DETECTED);
153				if (check_crc(c, buf, len))
154					return (DEATTACK_DETECTED);
155				else
156					break;
157			}
158		}
159		h[i] = j;
160	}
161	return (DEATTACK_OK);
162}
163