Fix silent truncations when extracting values from CheckedNumeric

components/webcrypto/algorithms/aes_cbc.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/numerics/safe_math.h"
@@ skipping 37 matching lines @@
     return Status::ErrorIncorrectSizeAesCbcIv();
 
   // According to the openssl docs, the amount of data written may be as large
   // as (data_size + cipher_block_size - 1), constrained to a multiple of
   // cipher_block_size.
   base::CheckedNumeric<int> output_max_len = data.byte_length();
   output_max_len += AES_BLOCK_SIZE - 1;
   if (!output_max_len.IsValid())
     return Status::ErrorDataTooLarge();
 
-  const unsigned remainder = output_max_len.ValueOrDie() % AES_BLOCK_SIZE;
+  const unsigned remainder =
+      (output_max_len % AES_BLOCK_SIZE).template ValueOrDie<unsigned>();
   if (remainder != 0)

[Tom Sepez, 2016/11/29 20:06:26]

I'm not grokking this.  Can you explain?  May be too complicate for mere mortals
to use.

[jschuh, 2016/11/29 21:04:47]

It's the template disambiguator for dependent names. Without the template
keyword the compiler may assume it's getting a less-than token rather than a
dependent template parameter. MSVS doesn't need it, but clang and GCC both do.
However, I assume at some point in the future this will be fixed (like how
nested template brackets no longer get treated as right shift operations).

It's described in the last section of this page:
http://en.cppreference.com/w/cpp/language/dependent_name

[jschuh, 2016/11/30 23:38:52]

I added some helpers and am using those at the call sites.

     output_max_len += AES_BLOCK_SIZE - remainder;
   if (!output_max_len.IsValid())
     return Status::ErrorDataTooLarge();
 
   // Note: PKCS padding is enabled by default
   const EVP_CIPHER* const cipher = GetAESCipherByKeyLength(raw_key.size());
   DCHECK(cipher);
 
   bssl::ScopedEVP_CIPHER_CTX context;
   if (!EVP_CipherInit_ex(context.get(), cipher, NULL, &raw_key[0],
                          params->iv().data(), cipher_operation)) {
     return Status::OperationError();
   }
 
-  buffer->resize(output_max_len.ValueOrDie());
+  buffer->resize(output_max_len.template ValueOrDie<size_t>());
 
   int output_len = 0;
   if (!EVP_CipherUpdate(context.get(), buffer->data(), &output_len,
                         data.bytes(), data.byte_length())) {
     return Status::OperationError();
   }
   int final_output_chunk_len = 0;
   if (!EVP_CipherFinal_ex(context.get(), buffer->data() + output_len,
                           &final_output_chunk_len)) {
     return Status::OperationError();
@@ skipping 27 matching lines @@
   }
 };
 
 }  // namespace
 
 std::unique_ptr<AlgorithmImplementation> CreateAesCbcImplementation() {
   return base::WrapUnique(new AesCbcImplementation);
 }
 
 }  // namespace webcrypto