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3c9c1fbd21
[deattack.c deattack.h packet.c]
limit maximum work performed by the CRC compensation attack detector,
problem reported by Tavis Ormandy, Google Security Team;
ok markus@ deraadt@
161 lines
3.9 KiB
C
161 lines
3.9 KiB
C
/* $OpenBSD: deattack.c,v 1.30 2006/09/16 19:53:37 djm 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 <sys/types.h>
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#include <string.h>
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#include <stdio.h>
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#include <stdarg.h>
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#include "xmalloc.h"
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#include "deattack.h"
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#include "log.h"
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#include "crc32.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) get_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(u_char *S, u_char *buf, u_int32_t len)
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{
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u_int32_t crc;
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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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/* Detect a crc32 compensation attack on a packet */
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int
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detect_attack(u_char *buf, u_int32_t len)
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{
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static u_int16_t *h = (u_int16_t *) NULL;
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static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
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u_int32_t i, j;
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u_int32_t l, same;
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u_char *c;
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u_char *d;
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if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
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len % SSH_BLOCKSIZE != 0) {
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fatal("detect_attack: bad length %d", len);
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}
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for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
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;
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if (h == NULL) {
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debug("Installing crc compensation attack detector.");
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h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE);
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n = l;
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} else {
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if (l > n) {
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h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
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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(h, HASH_UNUSEDCHAR, 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) & (n - 1); h[i] != HASH_UNUSED;
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i = (i + 1) & (n - 1)) {
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if (!CMP(c, buf + 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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h[i] = j;
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}
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return (DEATTACK_OK);
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}
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