deattack.c revision 295367 
1/* $OpenBSD: deattack.c,v 1.32 2015/01/20 23:14:00 deraadt Exp $ */
2/*
3 * Cryptographic attack detector for ssh - source code
4 *
5 * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
6 *
7 * All rights reserved. Redistribution and use in source and binary
8 * forms, with or without modification, are permitted provided that
9 * this copyright notice is retained.
10 *
11 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
12 * WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
13 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
14 * CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
15 * SOFTWARE.
16 *
17 * Ariel Futoransky <futo@core-sdi.com>
18 * <http://www.core-sdi.com>
19 */
20
21#include "includes.h"
22
23#include <string.h>
24#include <stdio.h>
25#include <stdlib.h>
26
27#include "deattack.h"
28#include "crc32.h"
29#include "sshbuf.h"
30#include "misc.h"
31
32/*
33 * CRC attack detection has a worst-case behaviour that is O(N^3) over
34 * the number of identical blocks in a packet. This behaviour can be
35 * exploited to create a limited denial of service attack.
36 *
37 * However, because we are dealing with encrypted data, identical
38 * blocks should only occur every 2^35 maximally-sized packets or so.
39 * Consequently, we can detect this DoS by looking for identical blocks
40 * in a packet.
41 *
42 * The parameter below determines how many identical blocks we will
43 * accept in a single packet, trading off between attack detection and
44 * likelihood of terminating a legitimate connection. A value of 32
45 * corresponds to an average of 2^40 messages before an attack is
46 * misdetected
47 */
48#define MAX_IDENTICAL	32
49
50/* SSH Constants */
51#define SSH_MAXBLOCKS	(32 * 1024)
52#define SSH_BLOCKSIZE	(8)
53
54/* Hashing constants */
55#define HASH_MINSIZE	(8 * 1024)
56#define HASH_ENTRYSIZE	(2)
57#define HASH_FACTOR(x)	((x)*3/2)
58#define HASH_UNUSEDCHAR	(0xff)
59#define HASH_UNUSED	(0xffff)
60#define HASH_IV		(0xfffe)
61
62#define HASH_MINBLOCKS	(7*SSH_BLOCKSIZE)
63
64
65/* Hash function (Input keys are cipher results) */
66#define HASH(x)		PEEK_U32(x)
67
68#define CMP(a, b)	(memcmp(a, b, SSH_BLOCKSIZE))
69
70static void
71crc_update(u_int32_t *a, u_int32_t b)
72{
73	b ^= *a;
74	*a = ssh_crc32((u_char *)&b, sizeof(b));
75}
76
77/* detect if a block is used in a particular pattern */
78static int
79check_crc(const u_char *S, const u_char *buf, u_int32_t len)
80{
81	u_int32_t crc;
82	const u_char *c;
83
84	crc = 0;
85	for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
86		if (!CMP(S, c)) {
87			crc_update(&crc, 1);
88			crc_update(&crc, 0);
89		} else {
90			crc_update(&crc, 0);
91			crc_update(&crc, 0);
92		}
93	}
94	return crc == 0;
95}
96
97void
98deattack_init(struct deattack_ctx *dctx)
99{
100	bzero(dctx, sizeof(*dctx));
101	dctx->n = HASH_MINSIZE / HASH_ENTRYSIZE;
102}
103
104/* Detect a crc32 compensation attack on a packet */
105int
106detect_attack(struct deattack_ctx *dctx, const u_char *buf, u_int32_t len)
107{
108	u_int32_t i, j, l, same;
109	u_int16_t *tmp;
110	const u_char *c, *d;
111
112	if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
113	    len % SSH_BLOCKSIZE != 0)
114		return DEATTACK_ERROR;
115	for (l = dctx->n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
116		;
117
118	if (dctx->h == NULL) {
119		if ((dctx->h = calloc(l, HASH_ENTRYSIZE)) == NULL)
120			return DEATTACK_ERROR;
121		dctx->n = l;
122	} else {
123		if (l > dctx->n) {
124			if ((tmp = reallocarray(dctx->h, l, HASH_ENTRYSIZE))
125			    == NULL) {
126				free(dctx->h);
127				dctx->h = NULL;
128				return DEATTACK_ERROR;
129			}
130			dctx->h = tmp;
131			dctx->n = l;
132		}
133	}
134
135	if (len <= HASH_MINBLOCKS) {
136		for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
137			for (d = buf; d < c; d += SSH_BLOCKSIZE) {
138				if (!CMP(c, d)) {
139					if ((check_crc(c, buf, len)))
140						return DEATTACK_DETECTED;
141					else
142						break;
143				}
144			}
145		}
146		return DEATTACK_OK;
147	}
148	memset(dctx->h, HASH_UNUSEDCHAR, dctx->n * HASH_ENTRYSIZE);
149
150	for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
151		for (i = HASH(c) & (dctx->n - 1); dctx->h[i] != HASH_UNUSED;
152		    i = (i + 1) & (dctx->n - 1)) {
153			if (!CMP(c, buf + dctx->h[i] * SSH_BLOCKSIZE)) {
154				if (++same > MAX_IDENTICAL)
155					return DEATTACK_DOS_DETECTED;
156				if (check_crc(c, buf, len))
157					return DEATTACK_DETECTED;
158				else
159					break;
160			}
161		}
162		dctx->h[i] = j;
163	}
164	return DEATTACK_OK;
165}
166