1 /* $OpenBSD: deattack.c,v 1.30 2006/09/16 19:53:37 djm 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 <sys/types.h>
24
25 #include <string.h>
26 #include <stdio.h>
27 #include <stdarg.h>
28
29 #include "xmalloc.h"
30 #include "deattack.h"
31 #include "log.h"
32 #include "crc32.h"
33 #include "misc.h"
34
35 /*
36 * CRC attack detection has a worst-case behaviour that is O(N^3) over
37 * the number of identical blocks in a packet. This behaviour can be
38 * exploited to create a limited denial of service attack.
39 *
40 * However, because we are dealing with encrypted data, identical
41 * blocks should only occur every 2^35 maximally-sized packets or so.
42 * Consequently, we can detect this DoS by looking for identical blocks
43 * in a packet.
44 *
45 * The parameter below determines how many identical blocks we will
46 * accept in a single packet, trading off between attack detection and
47 * likelihood of terminating a legitimate connection. A value of 32
48 * corresponds to an average of 2^40 messages before an attack is
49 * misdetected
50 */
51 #define MAX_IDENTICAL 32
52
53 /* SSH Constants */
54 #define SSH_MAXBLOCKS (32 * 1024)
55 #define SSH_BLOCKSIZE (8)
56
57 /* Hashing constants */
58 #define HASH_MINSIZE (8 * 1024)
59 #define HASH_ENTRYSIZE (2)
60 #define HASH_FACTOR(x) ((x)*3/2)
61 #define HASH_UNUSEDCHAR (0xff)
62 #define HASH_UNUSED (0xffff)
63 #define HASH_IV (0xfffe)
64
65 #define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
66
67
68 /* Hash function (Input keys are cipher results) */
69 #define HASH(x) get_u32(x)
70
71 #define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
72
73 static void
crc_update(u_int32_t * a,u_int32_t b)74 crc_update(u_int32_t *a, u_int32_t b)
75 {
76 b ^= *a;
77 *a = ssh_crc32((u_char *)&b, sizeof(b));
78 }
79
80 /* detect if a block is used in a particular pattern */
81 static int
check_crc(u_char * S,u_char * buf,u_int32_t len)82 check_crc(u_char *S, u_char *buf, u_int32_t len)
83 {
84 u_int32_t crc;
85 u_char *c;
86
87 crc = 0;
88 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
89 if (!CMP(S, c)) {
90 crc_update(&crc, 1);
91 crc_update(&crc, 0);
92 } else {
93 crc_update(&crc, 0);
94 crc_update(&crc, 0);
95 }
96 }
97 return (crc == 0);
98 }
99
100
101 /* Detect a crc32 compensation attack on a packet */
102 int
detect_attack(u_char * buf,u_int32_t len)103 detect_attack(u_char *buf, u_int32_t len)
104 {
105 static u_int16_t *h = (u_int16_t *) NULL;
106 static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
107 u_int32_t i, j;
108 u_int32_t l, same;
109 u_char *c;
110 u_char *d;
111
112 if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
113 len % SSH_BLOCKSIZE != 0) {
114 fatal("detect_attack: bad length %d", len);
115 }
116 for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
117 ;
118
119 if (h == NULL) {
120 debug("Installing crc compensation attack detector.");
121 h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE);
122 n = l;
123 } else {
124 if (l > n) {
125 h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
126 n = l;
127 }
128 }
129
130 if (len <= HASH_MINBLOCKS) {
131 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
132 for (d = buf; d < c; d += SSH_BLOCKSIZE) {
133 if (!CMP(c, d)) {
134 if ((check_crc(c, buf, len)))
135 return (DEATTACK_DETECTED);
136 else
137 break;
138 }
139 }
140 }
141 return (DEATTACK_OK);
142 }
143 memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
144
145 for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
146 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
147 i = (i + 1) & (n - 1)) {
148 if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
149 if (++same > MAX_IDENTICAL)
150 return (DEATTACK_DOS_DETECTED);
151 if (check_crc(c, buf, len))
152 return (DEATTACK_DETECTED);
153 else
154 break;
155 }
156 }
157 h[i] = j;
158 }
159 return (DEATTACK_OK);
160 }
161