[patch] openssh-4.3_p2-identical-simple-dos.patch

openssh-4.3_p2-identical-simple-dos.patch (text/plain), 3.44 KB, created by SpanKY on 2006-09-19 21:23:35 UTC

http://bugs.gentoo.org/148228

taken from upstream cvs and munged a little to apply against 4.3p2

===================================================================
RCS file: /usr/OpenBSD/cvs/src/usr.bin/ssh/deattack.c,v
retrieving revision 1.29
retrieving revision 1.30
diff -u -r1.29 -r1.30
--- src/usr.bin/ssh/deattack.c	2006/08/03 03:34:42	1.29
+++ src/usr.bin/ssh/deattack.c	2006/09/16 19:53:37	1.30
@@ -30,6 +30,24 @@
 #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)
@@ -85,7 +103,7 @@
 	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;
 
@@ -122,11 +140,13 @@
 	if (IV)
 		h[HASH(IV) & (n - 1)] = HASH_IV;
 
-	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 (h[i] == HASH_IV) {
 				if (!CMP(c, IV)) {
+					if (++same > MAX_IDENTICAL)
+						return (DEATTACK_DOS_DETECTED);
 					if (check_crc(c, buf, len, IV))
 						return (DEATTACK_DETECTED);
 					else
===================================================================
RCS file: /usr/OpenBSD/cvs/src/usr.bin/ssh/packet.c,v
retrieving revision 1.143
retrieving revision 1.144
diff -u -r1.143 -r1.144
--- src/usr.bin/ssh/packet.c	2006/08/05 08:34:04	1.143
+++ src/usr.bin/ssh/packet.c	2006/09/16 19:53:37	1.144
@@ -991,9 +991,16 @@
 	 * (C)1998 CORE-SDI, Buenos Aires Argentina
 	 * Ariel Futoransky([e-mail])
 	 */
-	if (!receive_context.plaintext &&
-	    detect_attack(buffer_ptr(&input), padded_len, NULL) == DEATTACK_DETECTED)
-		packet_disconnect("crc32 compensation attack: network attack detected");
+	if (!receive_context.plaintext) {
+		switch (detect_attack(buffer_ptr(&input), padded_len, NULL)) {
+		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);
===================================================================
RCS file: /usr/OpenBSD/cvs/src/usr.bin/ssh/deattack.h,v
retrieving revision 1.9
retrieving revision 1.10
diff -u -r1.9 -r1.10
--- src/usr.bin/ssh/deattack.h	2006/03/25 22:22:43	1.9
+++ src/usr.bin/ssh/deattack.h	2006/09/16 19:53:37	1.10
@@ -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