- djm@cvs.openbsd.org 2010/01/30 02:54:53

[openssh.git] / deattack.c

diff --git a/deattack.c b/deattack.c
index ff9ca4dd542f2ba6e87af210442a8dca6b1fb244..1b37e4dabe59e4e89736b5148f85781d6ae697b6 100644 (file)
--- a/deattack.c
+++ b/deattack.c
@@ -1,3 +1,4 @@
+/* $OpenBSD: deattack.c,v 1.30 2006/09/16 19:53:37 djm Exp $ */
 /*
  * Cryptographic attack detector for ssh - source code
  *
@@ -19,11 +20,35 @@
 
 #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 "getput.h"
-#include "xmalloc.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)
@@ -41,7 +66,7 @@
 
 
 /* Hash function (Input keys are cipher results) */
-#define HASH(x)                GET_32BIT(x)
+#define HASH(x)                get_u32(x)
 
 #define CMP(a, b)      (memcmp(a, b, SSH_BLOCKSIZE))
 
@@ -80,7 +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;
 
@@ -117,10 +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