Fix BER decoder integer overflow

doc/security.rst

@@ -21,7 +21,16 @@ Advisories
 2016
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-* 2016-10-8871 (CVE-2016-8871) OAEP side channel
+* 2016-11-27 (CVE-2016-xxxx) Integer overflow in BER decoder
+
+  While decoding BER length fields, an integer overflow could occur. This could
+  occur while parsing untrusted inputs such as X.509 certificates. The overflow
+  does not seem to lead to any obviously exploitable condition, but exploitation
+  cannot be positively ruled out. Only 32-bit platforms are likely affected; to
+  cause an overflow on 64-bit the parsed data would have to be many gigabytes.
+  Bug found by Falko Strenzke, cryptosource GmbH.
+
+* 2016-10-26 (CVE-2016-8871) OAEP side channel
 
   A side channel in OAEP decoding could be used to distinguish RSA ciphertexts
   that did or did not have a leading 0 byte. For an attacker capable of

src/lib/asn1/ber_dec.cpp

@@ -9,6 +9,7 @@
 #include <botan/ber_dec.h>
 #include <botan/bigint.h>
 #include <botan/loadstor.h>
+#include <botan/internal/safeint.h>
 
 namespace Botan {
 
@@ -126,7 +127,9 @@ size_t find_eoc(DataSource* ber)
       size_t item_size = decode_length(&source, length_size);
       source.discard_next(item_size);
 
-      length += item_size + length_size + tag_size;
+      length = BOTAN_CHECKED_ADD(length, item_size);
+      length = BOTAN_CHECKED_ADD(length, tag_size);
+      length = BOTAN_CHECKED_ADD(length, length_size);
 
       if(type_tag == EOC && class_tag == UNIVERSAL)
          break;

src/lib/utils/info.txt

@@ -1,4 +1,4 @@
-define UTIL_FUNCTIONS 20150919
+define UTIL_FUNCTIONS 20161127
 
 load_on always
 
@@ -31,6 +31,7 @@ filesystem.h
 os_utils.h
 prefetch.h
 rounding.h
+safeint.h
 semaphore.h
 stl_util.h
 </header:internal>

src/lib/utils/safeint.h

@@ -0,0 +1,39 @@
+/*
+* Safe(r) Integer Handling
+* (C) 2016 Jack Lloyd
+*
+* Botan is released under the Simplified BSD License (see license.txt)
+*/
+
+#ifndef BOTAN_UTILS_SAFE_INT_H__
+#define BOTAN_UTILS_SAFE_INT_H__
+
+#include <botan/exceptn.h>
+#include <string>
+
+namespace Botan {
+
+class Integer_Overflow_Detected : public Exception
+   {
+   public:
+      Integer_Overflow_Detected(const std::string& file, int line) :
+         Exception("Integer overflow detected at " + file + ":" + std::to_string(line))
+         {}
+   };
+
+inline size_t checked_add(size_t x, size_t y, const char* file, int line)
+   {
+   // TODO: use __builtin_x_overflow on GCC and Clang
+   size_t z = x + y;
+   if(z < x)
+      {
+      throw Integer_Overflow_Detected(file, line);
+      }
+   return z;
+   }
+
+#define BOTAN_CHECKED_ADD(x,y) checked_add(x,y,__FILE__,__LINE__)
+
+}
+
+#endif