166 lines
4.1 KiB
C
166 lines
4.1 KiB
C
/* $OpenBSD: deattack.c,v 1.32 2015/01/20 23:14:00 deraadt Exp $ */
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/*
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* Cryptographic attack detector for ssh - source code
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*
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* Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
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*
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* All rights reserved. Redistribution and use in source and binary
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* forms, with or without modification, are permitted provided that
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* this copyright notice is retained.
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*
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* THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
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* WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
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* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
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* CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
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* SOFTWARE.
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*
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* Ariel Futoransky <futo@core-sdi.com>
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* <http://www.core-sdi.com>
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*/
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#include "includes.h"
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#include <string.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include "deattack.h"
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#include "crc32.h"
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#include "sshbuf.h"
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#include "misc.h"
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/*
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* CRC attack detection has a worst-case behaviour that is O(N^3) over
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* the number of identical blocks in a packet. This behaviour can be
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* exploited to create a limited denial of service attack.
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*
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* However, because we are dealing with encrypted data, identical
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* blocks should only occur every 2^35 maximally-sized packets or so.
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* Consequently, we can detect this DoS by looking for identical blocks
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* in a packet.
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*
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* The parameter below determines how many identical blocks we will
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* accept in a single packet, trading off between attack detection and
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* likelihood of terminating a legitimate connection. A value of 32
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* corresponds to an average of 2^40 messages before an attack is
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* misdetected
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*/
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#define MAX_IDENTICAL 32
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/* SSH Constants */
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#define SSH_MAXBLOCKS (32 * 1024)
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#define SSH_BLOCKSIZE (8)
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/* Hashing constants */
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#define HASH_MINSIZE (8 * 1024)
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#define HASH_ENTRYSIZE (2)
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#define HASH_FACTOR(x) ((x)*3/2)
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#define HASH_UNUSEDCHAR (0xff)
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#define HASH_UNUSED (0xffff)
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#define HASH_IV (0xfffe)
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#define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
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/* Hash function (Input keys are cipher results) */
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#define HASH(x) PEEK_U32(x)
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#define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
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static void
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crc_update(u_int32_t *a, u_int32_t b)
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{
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b ^= *a;
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*a = ssh_crc32((u_char *)&b, sizeof(b));
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}
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/* detect if a block is used in a particular pattern */
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static int
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check_crc(const u_char *S, const u_char *buf, u_int32_t len)
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{
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u_int32_t crc;
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const u_char *c;
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crc = 0;
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for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
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if (!CMP(S, c)) {
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crc_update(&crc, 1);
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crc_update(&crc, 0);
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} else {
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crc_update(&crc, 0);
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crc_update(&crc, 0);
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}
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}
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return crc == 0;
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}
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void
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deattack_init(struct deattack_ctx *dctx)
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{
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bzero(dctx, sizeof(*dctx));
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dctx->n = HASH_MINSIZE / HASH_ENTRYSIZE;
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}
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/* Detect a crc32 compensation attack on a packet */
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int
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detect_attack(struct deattack_ctx *dctx, const u_char *buf, u_int32_t len)
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{
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u_int32_t i, j, l, same;
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u_int16_t *tmp;
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const u_char *c, *d;
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if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
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len % SSH_BLOCKSIZE != 0)
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return DEATTACK_ERROR;
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for (l = dctx->n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
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;
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if (dctx->h == NULL) {
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if ((dctx->h = calloc(l, HASH_ENTRYSIZE)) == NULL)
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return DEATTACK_ERROR;
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dctx->n = l;
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} else {
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if (l > dctx->n) {
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if ((tmp = reallocarray(dctx->h, l, HASH_ENTRYSIZE))
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== NULL) {
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free(dctx->h);
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dctx->h = NULL;
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return DEATTACK_ERROR;
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}
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dctx->h = tmp;
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dctx->n = l;
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}
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}
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if (len <= HASH_MINBLOCKS) {
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for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
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for (d = buf; d < c; d += SSH_BLOCKSIZE) {
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if (!CMP(c, d)) {
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if ((check_crc(c, buf, len)))
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return DEATTACK_DETECTED;
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else
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break;
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}
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}
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}
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return DEATTACK_OK;
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}
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memset(dctx->h, HASH_UNUSEDCHAR, dctx->n * HASH_ENTRYSIZE);
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for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
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for (i = HASH(c) & (dctx->n - 1); dctx->h[i] != HASH_UNUSED;
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i = (i + 1) & (dctx->n - 1)) {
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if (!CMP(c, buf + dctx->h[i] * SSH_BLOCKSIZE)) {
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if (++same > MAX_IDENTICAL)
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return DEATTACK_DOS_DETECTED;
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if (check_crc(c, buf, len))
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return DEATTACK_DETECTED;
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else
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break;
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}
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}
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dctx->h[i] = j;
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}
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return DEATTACK_OK;
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}
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