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
70 static void
crc_update(u_int32_t * a,u_int32_t b)71 crc_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 */
78 static int
check_crc(const u_char * S,const u_char * buf,u_int32_t len)79 check_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
97 void
deattack_init(struct deattack_ctx * dctx)98 deattack_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 */
105 int
detect_attack(struct deattack_ctx * dctx,const u_char * buf,u_int32_t len)106 detect_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