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 (at) 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 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 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 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