usr/src/cmd/ssh/libssh/common/deattack.c

   1 /*
   2  * Cryptographic attack detector for ssh - source code
   3  *
   4  * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
   5  *
   6  * All rights reserved. Redistribution and use in source and binary
   7  * forms, with or without modification, are permitted provided that
   8  * this copyright notice is retained.
   9  *
  10  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
  11  * WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
  12  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
  13  * CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
  14  * SOFTWARE.
  15  *
  16  * Ariel Futoransky <futo@core-sdi.com>
  17  * <http://www.core-sdi.com>
  18  */
  19
  20 #include "includes.h"
  21 RCSID("$OpenBSD: deattack.c,v 1.18 2002/03/04 17:27:39 stevesk Exp $");
  22
  23 #pragma ident   "%Z%%M% %I%     %E% SMI"
  24
  25 #include "deattack.h"
  26 #include "log.h"
  27 #include "crc32.h"
  28 #include "getput.h"
  29 #include "xmalloc.h"
  30 #include "deattack.h"
  31
  32 /*
  33  * CRC attack detection has a worst-case behaviour that is O(N^2) 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)         GET_32BIT(x)
  67
  68 #define CMP(a, b)       (memcmp(a, b, SSH_BLOCKSIZE))
  69
  70 static void
  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
  79 check_crc(u_char *S, u_char *buf, u_int32_t len,
  80           u_char *IV)
  81 {
  82         u_int32_t crc;
  83         u_char *c;
  84
  85         crc = 0;
  86         if (IV && !CMP(S, IV)) {
  87                 crc_update(&crc, 1);
  88                 crc_update(&crc, 0);
  89         }
  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 */
 104 int
 105 detect_attack(u_char *buf, u_int32_t len, u_char *IV)
 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                 n = l;
 124                 h = (u_int16_t *) xmalloc(n * HASH_ENTRYSIZE);
 125         } else {
 126                 if (l > n) {
 127                         n = l;
 128                         h = (u_int16_t *) xrealloc(h, n * HASH_ENTRYSIZE);
 129                 }
 130         }
 131
 132         if (len <= HASH_MINBLOCKS) {
 133                 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
 134                         if (IV && (!CMP(c, IV))) {
 135                                 if ((check_crc(c, buf, len, IV)))
 136                                         return (DEATTACK_DETECTED);
 137                                 else
 138                                         break;
 139                         }
 140                         for (d = buf; d < c; d += SSH_BLOCKSIZE) {
 141                                 if (!CMP(c, d)) {
 142                                         if ((check_crc(c, buf, len, IV)))
 143                                                 return (DEATTACK_DETECTED);
 144                                         else
 145                                                 break;
 146                                 }
 147                         }
 148                 }
 149                 return (DEATTACK_OK);
 150         }
 151         memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
 152
 153         if (IV)
 154                 h[HASH(IV) & (n - 1)] = HASH_IV;
 155
 156         for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
 157                 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
 158                     i = (i + 1) & (n - 1)) {
 159                         if (h[i] == HASH_IV) {
 160                                 if (!CMP(c, IV)) {
 161                                         if (check_crc(c, buf, len, IV))
 162                                                 return (DEATTACK_DETECTED);
 163                                         else
 164                                                 break;
 165                                 }
 166                         } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
 167                                 if (++same > MAX_IDENTICAL)
 168                                         return (DEATTACK_DOS_DETECTED);
 169                                 if (check_crc(c, buf, len, IV))
 170                                         return (DEATTACK_DETECTED);
 171                                 else
 172                                         break;
 173                         }
 174                 }
 175                 h[i] = j;
 176         }
 177         return (DEATTACK_OK);
 178 }