- 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@

4 files changed, 45 insertions, 9 deletions

diff --git a/ChangeLog b/ChangeLog
index c61fd934..3c00fec4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -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 $
diff --git a/deattack.c b/deattack.c
index b4fed7f8..1b37e4da 100644
--- a/deattack.c
+++ b/deattack.c
@@ -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
diff --git a/deattack.h b/deattack.h
index 62759810..0316fb28 100644
--- a/deattack.h
+++ b/deattack.h
@@ -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
diff --git a/packet.c b/packet.c
index a4cb3324..da843b2c 100644
--- a/packet.c
+++ b/packet.c
@@ -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);