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- djm@cvs.openbsd.org 2006/09/16 19:53:37 

[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@
This commit is contained in:
Damien Miller 2006-09-17 06:08:53 +10:00
parent 223897a01a
commit 3c9c1fbd21
4 changed files with 45 additions and 9 deletions
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10
ChangeLog
View File

@ -1,3 +1,11 @@
20060916
- OpenBSD CVS Sync
- djm@cvs.openbsd.org 2006/09/16 19:53:37
[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@
20060912
- (djm) [Makefile.in buildpkg.sh.in configure.ac openssh.xml.in]
Support SMF in Solaris Packages if enabled by configure. Patch from
@ -5433,4 +5441,4 @@
- (djm) Trim deprecated options from INSTALL. Mention UsePAM
- (djm) Fix quote handling in sftp; Patch from admorten AT umich.edu
$Id: ChangeLog,v 1.4544 2006/09/12 11:54:10 djm Exp $
$Id: ChangeLog,v 1.4545 2006/09/16 20:08:53 djm Exp $

26
deattack.c
View File

@ -1,4 +1,4 @@
/* $OpenBSD: deattack.c,v 1.29 2006/08/03 03:34:42 deraadt Exp $ */
/* $OpenBSD: deattack.c,v 1.30 2006/09/16 19:53:37 djm Exp $ */
/*
* Cryptographic attack detector for ssh - source code
*
@ -32,6 +32,24 @@
#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)
@ -87,7 +105,7 @@ 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;
u_int32_t l, same;
u_char *c;
u_char *d;
@ -124,10 +142,12 @@ detect_attack(u_char *buf, u_int32_t len)
}
memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
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

3
deattack.h
View File

@ -1,4 +1,4 @@
/* $OpenBSD: deattack.h,v 1.9 2006/03/25 22:22:43 djm Exp $ */
/* $OpenBSD: deattack.h,v 1.10 2006/09/16 19:53:37 djm Exp $ */
/*
* Cryptographic attack detector for ssh - Header file
@ -25,6 +25,7 @@
/* Return codes */
#define DEATTACK_OK 0
#define DEATTACK_DETECTED 1
#define DEATTACK_DOS_DETECTED 2
int detect_attack(u_char *, u_int32_t);
#endif

15
packet.c
View File

@ -1,4 +1,4 @@
/* $OpenBSD: packet.c,v 1.143 2006/08/05 08:34:04 dtucker Exp $ */
/* $OpenBSD: packet.c,v 1.144 2006/09/16 19:53:37 djm Exp $ */
/*
* Author: Tatu Ylonen <ylo@cs.hut.fi>
* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@ -1000,9 +1000,16 @@ packet_read_poll1(void)
* (C)1998 CORE-SDI, Buenos Aires Argentina
* Ariel Futoransky(futo@core-sdi.com)
*/
if (!receive_context.plaintext &&
detect_attack(buffer_ptr(&input), padded_len) == DEATTACK_DETECTED)
packet_disconnect("crc32 compensation attack: network attack detected");
if (!receive_context.plaintext) {
switch (detect_attack(buffer_ptr(&input), padded_len)) {
case DEATTACK_DETECTED:
packet_disconnect("crc32 compensation attack: "
"network attack detected");
case DEATTACK_DOS_DETECTED:
packet_disconnect("deattack denial of "
"service detected");
}
}
/* Decrypt data to incoming_packet. */
buffer_clear(&incoming_packet);