1 files changed, 40 insertions, 36 deletions

diff --git a/deattack.c b/deattack.c
index 8b55d6686..1b37e4dab 100644
--- 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
  *
@@ -18,14 +19,36 @@
  */
 
 #include "includes.h"
-RCSID("$OpenBSD: deattack.c,v 1.19 2003/09/18 08:49:45 markus Exp $");
 
+#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 "deattack.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)
@@ -43,7 +66,7 @@ RCSID("$OpenBSD: deattack.c,v 1.19 2003/09/18 08:49:45 markus Exp $");
 
 
 /* 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))
 
@@ -51,22 +74,17 @@ static void
 crc_update(u_int32_t *a, u_int32_t b)
 {
         b ^= *a;
-        *a = ssh_crc32((u_char *) &b, sizeof(b));
+        *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_char *IV)
+check_crc(u_char *S, u_char *buf, u_int32_t len)
 {
         u_int32_t crc;
         u_char *c;
 
         crc = 0;
-        if (IV && !CMP(S, IV)) {
-                crc_update(&crc, 1);
-                crc_update(&crc, 0);
-        }
         for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
                 if (!CMP(S, c)) {
                         crc_update(&crc, 1);
@@ -82,12 +100,12 @@ check_crc(u_char *S, u_char *buf, u_int32_t len,
 
 /* Detect a crc32 compensation attack on a packet */
 int
-detect_attack(u_char *buf, u_int32_t len, u_char *IV)
+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;
 
@@ -100,26 +118,20 @@ detect_attack(u_char *buf, u_int32_t len, u_char *IV)
 
         if (h == NULL) {
                 debug("Installing crc compensation attack detector.");
-                h = (u_int16_t *) xmalloc(l * HASH_ENTRYSIZE);
+                h = (u_int16_t *) xcalloc(l, HASH_ENTRYSIZE);
                 n = l;
         } else {
                 if (l > n) {
-                        h = (u_int16_t *) xrealloc(h, l * HASH_ENTRYSIZE);
+                        h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
                         n = l;
                 }
         }
 
         if (len <= HASH_MINBLOCKS) {
                 for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
-                        if (IV && (!CMP(c, IV))) {
-                                if ((check_crc(c, buf, len, IV)))
-                                        return (DEATTACK_DETECTED);
-                                else
-                                        break;
-                        }
                         for (d = buf; d < c; d += SSH_BLOCKSIZE) {
                                 if (!CMP(c, d)) {
-                                        if ((check_crc(c, buf, len, IV)))
+                                        if ((check_crc(c, buf, len)))
                                                 return (DEATTACK_DETECTED);
                                         else
                                                 break;
@@ -130,21 +142,13 @@ detect_attack(u_char *buf, u_int32_t len, u_char *IV)
         }
         memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE);
 
-        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 (check_crc(c, buf, len, IV))
-                                                return (DEATTACK_DETECTED);
-                                        else
-                                                break;
-                                }
-                        } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
-                                if (check_crc(c, buf, len, IV))
+                        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;