mirror of
git://anongit.mindrot.org/openssh.git
synced 2024-12-22 10:00:14 +00:00
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 $ */
|
|
/*
|
|
* Cryptographic attack detector for ssh - source code
|
|
*
|
|
* Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
|
|
*
|
|
* All rights reserved. Redistribution and use in source and binary
|
|
* forms, with or without modification, are permitted provided that
|
|
* this copyright notice is retained.
|
|
*
|
|
* THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
|
|
* WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
|
|
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
|
|
* CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
|
|
* SOFTWARE.
|
|
*
|
|
* Ariel Futoransky <futo@core-sdi.com>
|
|
* <http://www.core-sdi.com>
|
|
*/
|
|
|
|
#include "includes.h"
|
|
|
|
#include <sys/types.h>
|
|
|
|
#include <string.h>
|
|
#include <stdio.h>
|
|
#include <stdarg.h>
|
|
|
|
#include "xmalloc.h"
|
|
#include "deattack.h"
|
|
#include "log.h"
|
|
#include "crc32.h"
|
|
#include "misc.h"
|
|
|
|
/*
|
|
* CRC attack detection has a worst-case behaviour that is O(N^3) over
|
|
* the number of identical blocks in a packet. This behaviour can be
|
|
* exploited to create a limited denial of service attack.
|
|
*
|
|
* However, because we are dealing with encrypted data, identical
|
|
* blocks should only occur every 2^35 maximally-sized packets or so.
|
|
* Consequently, we can detect this DoS by looking for identical blocks
|
|
* in a packet.
|
|
*
|
|
* The parameter below determines how many identical blocks we will
|
|
* accept in a single packet, trading off between attack detection and
|
|
* likelihood of terminating a legitimate connection. A value of 32
|
|
* corresponds to an average of 2^40 messages before an attack is
|
|
* misdetected
|
|
*/
|
|
#define MAX_IDENTICAL 32
|
|
|
|
/* SSH Constants */
|
|
#define SSH_MAXBLOCKS (32 * 1024)
|
|
#define SSH_BLOCKSIZE (8)
|
|
|
|
/* Hashing constants */
|
|
#define HASH_MINSIZE (8 * 1024)
|
|
#define HASH_ENTRYSIZE (2)
|
|
#define HASH_FACTOR(x) ((x)*3/2)
|
|
#define HASH_UNUSEDCHAR (0xff)
|
|
#define HASH_UNUSED (0xffff)
|
|
#define HASH_IV (0xfffe)
|
|
|
|
#define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
|
|
|
|
|
|
/* Hash function (Input keys are cipher results) */
|
|
#define HASH(x) get_u32(x)
|
|
|
|
#define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
|
|
|
|
static void
|
|
crc_update(u_int32_t *a, u_int32_t b)
|
|
{
|
|
b ^= *a;
|
|
*a = ssh_crc32((u_char *)&b, sizeof(b));
|
|
}
|
|
|
|
/* detect if a block is used in a particular pattern */
|
|
static int
|
|
check_crc(u_char *S, u_char *buf, u_int32_t len)
|
|
{
|
|
u_int32_t crc;
|
|
u_char *c;
|
|
|
|
crc = 0;
|
|
for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
|
|
if (!CMP(S, c)) {
|
|
crc_update(&crc, 1);
|
|
crc_update(&crc, 0);
|
|
} else {
|
|
crc_update(&crc, 0);
|
|
crc_update(&crc, 0);
|
|
}
|
|
}
|
|
return (crc == 0);
|
|
}
|
|
|
|
|
|
/* Detect a crc32 compensation attack on a packet */
|
|
int
|
|
detect_attack(u_char *buf, u_int32_t len)
|
|
{
|
|
static u_int16_t *h = (u_int16_t *) NULL;
|
|
static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE;
|
|
u_int32_t i, j;
|
|
u_int32_t l, same;
|
|
u_char *c;
|
|
u_char *d;
|
|
|
|
if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
|
|
len % SSH_BLOCKSIZE != 0) {
|
|
fatal("detect_attack: bad length %d", len);
|
|
}
|
|
for (l = n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
|
|
;
|
|
|
|
if (h == NULL) {
|
|
debug("Installing crc compensation attack detector.");
|
|
h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE);
|
|
n = l;
|
|
} else {
|
|
if (l > n) {
|
|
h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
|
|
n = l;
|
|
}
|
|
}
|
|
|
|
if (len <= HASH_MINBLOCKS) {
|
|
for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
|
|
for (d = buf; d < c; d += SSH_BLOCKSIZE) {
|
|
if (!CMP(c, d)) {
|
|
if ((check_crc(c, buf, len)))
|
|
return (DEATTACK_DETECTED);
|
|
else
|
|
break;
|
|
}
|
|
}
|
|
}
|
|
return (DEATTACK_OK);
|
|
}
|
|
memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
|
|
|
|
for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
|
|
for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED;
|
|
i = (i + 1) & (n - 1)) {
|
|
if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
|
|
if (++same > MAX_IDENTICAL)
|
|
return (DEATTACK_DOS_DETECTED);
|
|
if (check_crc(c, buf, len))
|
|
return (DEATTACK_DETECTED);
|
|
else
|
|
break;
|
|
}
|
|
}
|
|
h[i] = j;
|
|
}
|
|
return (DEATTACK_OK);
|
|
}
|