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

[openssh.git] / deattack.c

diff --git a/deattack.c b/deattack.c
index 44220491d8c33e2ddb2f29638748dc43cccb64f0..1b37e4dabe59e4e89736b5148f85781d6ae697b6 100644 (file)
--- a/deattack.c
+++ b/deattack.c
@@ -1,5 +1,4 @@
-/*     $OpenBSD: deattack.c,v 1.15 2001/11/19 19:02:16 mpech Exp $     */
-
+/* $OpenBSD: deattack.c,v 1.30 2006/09/16 19:53:37 djm Exp $ */
 /*
  * Cryptographic attack detector for ssh - source code
  *
@@ -20,11 +19,36 @@
  */
 
 #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)
@@ -36,13 +60,13 @@
 #define HASH_FACTOR(x) ((x)*3/2)
 #define HASH_UNUSEDCHAR        (0xff)
 #define HASH_UNUSED    (0xffff)
-#define HASH_IV        (0xfffe)
+#define HASH_IV                (0xfffe)
 
 #define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
 
 
 /* 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))
 
@@ -50,22 +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);
@@ -81,12 +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;
 
@@ -99,26 +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 *) xcalloc(l, HASH_ENTRYSIZE);
                n = l;
-               h = (u_int16_t *) xmalloc(n * HASH_ENTRYSIZE);
        } else {
                if (l > n) {
+                       h = (u_int16_t *)xrealloc(h, l, HASH_ENTRYSIZE);
                        n = l;
-                       h = (u_int16_t *) xrealloc(h, n * HASH_ENTRYSIZE);
                }
        }
 
        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;
@@ -129,21 +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))
+                   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
                                        break;