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      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