Fix BN_hex2bn/BN_dec2bn NULL ptr/heap corruption

In the BN_hex2bn function the number of hex digits is calculated using
an int value |i|. Later |bn_expand| is called with a value of |i * 4|.
For large values of |i| this can result in |bn_expand| not allocating any
memory because |i * 4| is negative. This leaves ret->d as NULL leading
to a subsequent NULL ptr deref. For very large values of |i|, the
calculation |i * 4| could be a positive value smaller than |i|. In this
case memory is allocated to ret->d, but it is insufficiently sized
leading to heap corruption. A similar issue exists in BN_dec2bn.

This could have security consequences if BN_hex2bn/BN_dec2bn is ever
called by user applications with very large untrusted hex/dec data. This is
anticipated to be a rare occurrence.

All OpenSSL internal usage of this function uses data that is not expected
to be untrusted, e.g. config file data or application command line
arguments. If user developed applications generate config file data based
on untrusted data then it is possible that this could also lead to security
consequences. This is also anticipated to be a rare.

Issue reported by Guido Vranken.

CVE-2016-0797

Reviewed-by: Andy Polyakov <appro@openssl.org>
(cherry picked from commit c175308407)

crypto/bn/bn.h

@@ -125,6 +125,7 @@
 #ifndef HEADER_BN_H
 # define HEADER_BN_H
 
+# include <limits.h>
 # include <openssl/e_os2.h>
 # ifndef OPENSSL_NO_FP_API
 #  include <stdio.h>            /* FILE */
@@ -739,8 +740,17 @@ const BIGNUM *BN_get0_nist_prime_521(void);
 
 /* library internal functions */
 
-# define bn_expand(a,bits) ((((((bits+BN_BITS2-1))/BN_BITS2)) <= (a)->dmax)?\
+# define bn_expand(a,bits) \
-        (a):bn_expand2((a),(bits+BN_BITS2-1)/BN_BITS2))
+    ( \
+        bits > (INT_MAX - BN_BITS2 + 1) ? \
+            NULL \
+        : \
+            (((bits+BN_BITS2-1)/BN_BITS2) <= (a)->dmax) ? \
+                (a) \
+            : \
+                bn_expand2((a),(bits+BN_BITS2-1)/BN_BITS2) \
+    )
+
 # define bn_wexpand(a,words) (((words) <= (a)->dmax)?(a):bn_expand2((a),(words)))
 BIGNUM *bn_expand2(BIGNUM *a, int words);
 # ifndef OPENSSL_NO_DEPRECATED

crypto/bn/bn_print.c

@@ -58,6 +58,7 @@
 
 #include <stdio.h>
 #include <ctype.h>
+#include <limits.h>
 #include "cryptlib.h"
 #include <openssl/buffer.h>
 #include "bn_lcl.h"
@@ -189,7 +190,11 @@ int BN_hex2bn(BIGNUM **bn, const char *a)
         a++;
     }
 
-    for (i = 0; isxdigit((unsigned char)a[i]); i++) ;
+    for (i = 0; i <= (INT_MAX/4) && isxdigit((unsigned char)a[i]); i++)
+        continue;
+
+    if (i > INT_MAX/4)
+        goto err;
 
     num = i + neg;
     if (bn == NULL)
@@ -204,7 +209,7 @@ int BN_hex2bn(BIGNUM **bn, const char *a)
         BN_zero(ret);
     }
 
-    /* i is the number of hex digests; */
+    /* i is the number of hex digits */
     if (bn_expand(ret, i * 4) == NULL)
         goto err;
 
@@ -260,7 +265,11 @@ int BN_dec2bn(BIGNUM **bn, const char *a)
         a++;
     }
 
-    for (i = 0; isdigit((unsigned char)a[i]); i++) ;
+    for (i = 0; i <= (INT_MAX/4) && isdigit((unsigned char)a[i]); i++)
+        continue;
+
+    if (i > INT_MAX/4)
+        goto err;
 
     num = i + neg;
     if (bn == NULL)
@@ -278,7 +287,7 @@ int BN_dec2bn(BIGNUM **bn, const char *a)
         BN_zero(ret);
     }
 
-    /* i is the number of digests, a bit of an over expand; */
+    /* i is the number of digits, a bit of an over expand */
     if (bn_expand(ret, i * 4) == NULL)
         goto err;