crypto/external/bsd/openssh/dist/deattack.c

   1 /*      $NetBSD$        */
   2 /* $OpenBSD: deattack.c,v 1.30 2006/09/16 19:53:37 djm Exp $ */
   3 /*
   4  * Cryptographic attack detector for ssh - source code
   5  *
   6  * Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
   7  *
   8  * All rights reserved. Redistribution and use in source and binary
   9  * forms, with or without modification, are permitted provided that
  10  * this copyright notice is retained.
  11  *
  12  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
  13  * WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
  14  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
  15  * CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
  16  * SOFTWARE.
  17  *
  18  * Ariel Futoransky <futo@core-sdi.com>
  19  * <http://www.core-sdi.com>
  20  */
  21
  22 #include "includes.h"
  23 __RCSID("$NetBSD: deattack.c,v 1.17 2009/02/16 20:53:54 christos Exp $");
  24 #include <sys/types.h>
  25
  26 #include <string.h>
  27 #include <stdio.h>
  28 #include <stdarg.h>
  29 #include <time.h>
  30
  31 #include "xmalloc.h"
  32 #include "deattack.h"
  33 #include "log.h"
  34 #include "crc32.h"
  35 #include "misc.h"
  36
  37 /*
  38  * CRC attack detection has a worst-case behaviour that is O(N^3) over
  39  * the number of identical blocks in a packet. This behaviour can be
  40  * exploited to create a limited denial of service attack.
  41  *
  42  * However, because we are dealing with encrypted data, identical
  43  * blocks should only occur every 2^35 maximally-sized packets or so.
  44  * Consequently, we can detect this DoS by looking for identical blocks
  45  * in a packet.
  46  *
  47  * The parameter below determines how many identical blocks we will
  48  * accept in a single packet, trading off between attack detection and
  49  * likelihood of terminating a legitimate connection. A value of 32
  50  * corresponds to an average of 2^40 messages before an attack is
  51  * misdetected
  52  */
  53 #define MAX_IDENTICAL   32
  54
  55 /* SSH Constants */
  56 #define SSH_MAXBLOCKS   (32 * 1024)
  57 #define SSH_BLOCKSIZE   (8)
  58
  59 /* Hashing constants */
  60 #define HASH_MINSIZE    (8 * 1024)
  61 #define HASH_ENTRYSIZE  (2)
  62 #define HASH_FACTOR(x)  ((x)*3/2)
  63 #define HASH_UNUSEDCHAR (0xff)
  64 #define HASH_UNUSED     (0xffff)
  65 #define HASH_IV         (0xfffe)
  66
  67 #define HASH_MINBLOCKS  (7*SSH_BLOCKSIZE)
  68
  69
  70 /* Hash function (Input keys are cipher results) */
  71 #define HASH(x)         get_u32(x)
  72
  73 #define CMP(a, b)       (memcmp(a, b, SSH_BLOCKSIZE))
  74
  75 static void
  76 crc_update(u_int32_t *a, u_int32_t b)
  77 {
  78         b ^= *a;
  79         *a = ssh_crc32((u_char *)&b, sizeof(b));
  80 }
  81
  82 /* detect if a block is used in a particular pattern */
  83 static int
  84 check_crc(u_char *S, u_char *buf, u_int32_t len)
  85 {
  86         u_int32_t crc;
  87         u_char *c;
  88
  89         crc = 0;
  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)
 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                 h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE);
 124                 n = l;
 125         } else {
 126                 if (l > n) {
 127                         h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
 128                         n = l;
 129                 }
 130         }
 131
 132         if (len <= HASH_MINBLOCKS) {
 133                 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
 134                         for (d = buf; d < c; d += SSH_BLOCKSIZE) {
 135                                 if (!CMP(c, d)) {
 136                                         if ((check_crc(c, buf, len)))
 137                                                 return (DEATTACK_DETECTED);
 138                                         else
 139                                                 break;
 140                                 }
 141                         }
 142                 }
 143                 return (DEATTACK_OK);
 144         }
 145         memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
 146
 147         for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
 148                 for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
 149                     i = (i + 1) & (n - 1)) {
 150                         if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
 151                                 if (++same > MAX_IDENTICAL)
 152                                         return (DEATTACK_DOS_DETECTED);
 153                                 if (check_crc(c, buf, len))
 154                                         return (DEATTACK_DETECTED);
 155                                 else
 156                                         break;
 157                         }
 158                 }
 159                 h[i] = j;
 160         }
 161         return (DEATTACK_OK);
 162 }