Home | History | Annotate | Download | only in openssh 
      1 /* $OpenBSD: deattack.c,v 1.32 2015/01/20 23:14:00 deraadt 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 <string.h>
     24 #include <stdio.h>
     25 #include <stdlib.h>
     26 
     27 #include "deattack.h"
     28 #include "crc32.h"
     29 #include "sshbuf.h"
     30 #include "misc.h"
     31 
     32 /*
     33  * CRC attack detection has a worst-case behaviour that is O(N^3) 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)		PEEK_U32(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(const u_char *S, const u_char *buf, u_int32_t len)
     80 {
     81 	u_int32_t crc;
     82 	const u_char *c;
     83 
     84 	crc = 0;
     85 	for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
     86 		if (!CMP(S, c)) {
     87 			crc_update(&crc, 1);
     88 			crc_update(&crc, 0);
     89 		} else {
     90 			crc_update(&crc, 0);
     91 			crc_update(&crc, 0);
     92 		}
     93 	}
     94 	return crc == 0;
     95 }
     96 
     97 void
     98 deattack_init(struct deattack_ctx *dctx)
     99 {
    100 	bzero(dctx, sizeof(*dctx));
    101 	dctx->n = HASH_MINSIZE / HASH_ENTRYSIZE;
    102 }
    103 
    104 /* Detect a crc32 compensation attack on a packet */
    105 int
    106 detect_attack(struct deattack_ctx *dctx, const u_char *buf, u_int32_t len)
    107 {
    108 	u_int32_t i, j, l, same;
    109 	u_int16_t *tmp;
    110 	const u_char *c, *d;
    111 
    112 	if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
    113 	    len % SSH_BLOCKSIZE != 0)
    114 		return DEATTACK_ERROR;
    115 	for (l = dctx->n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
    116 		;
    117 
    118 	if (dctx->h == NULL) {
    119 		if ((dctx->h = calloc(l, HASH_ENTRYSIZE)) == NULL)
    120 			return DEATTACK_ERROR;
    121 		dctx->n = l;
    122 	} else {
    123 		if (l > dctx->n) {
    124 			if ((tmp = reallocarray(dctx->h, l, HASH_ENTRYSIZE))
    125 			    == NULL) {
    126 				free(dctx->h);
    127 				dctx->h = NULL;
    128 				return DEATTACK_ERROR;
    129 			}
    130 			dctx->h = tmp;
    131 			dctx->n = l;
    132 		}
    133 	}
    134 
    135 	if (len <= HASH_MINBLOCKS) {
    136 		for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
    137 			for (d = buf; d < c; d += SSH_BLOCKSIZE) {
    138 				if (!CMP(c, d)) {
    139 					if ((check_crc(c, buf, len)))
    140 						return DEATTACK_DETECTED;
    141 					else
    142 						break;
    143 				}
    144 			}
    145 		}
    146 		return DEATTACK_OK;
    147 	}
    148 	memset(dctx->h, HASH_UNUSEDCHAR, dctx->n * HASH_ENTRYSIZE);
    149 
    150 	for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
    151 		for (i = HASH(c) & (dctx->n - 1); dctx->h[i] != HASH_UNUSED;
    152 		    i = (i + 1) & (dctx->n - 1)) {
    153 			if (!CMP(c, buf + dctx->h[i] * SSH_BLOCKSIZE)) {
    154 				if (++same > MAX_IDENTICAL)
    155 					return DEATTACK_DOS_DETECTED;
    156 				if (check_crc(c, buf, len))
    157 					return DEATTACK_DETECTED;
    158 				else
    159 					break;
    160 			}
    161 		}
    162 		dctx->h[i] = j;
    163 	}
    164 	return DEATTACK_OK;
    165 }
    166