deattack.c

   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
  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 }