Pass a minimally sized buffer to BoringSSL's HMAC().

components/webcrypto/algorithms/hmac.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 <openssl/hmac.h>
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
@@ skipping 78 matching lines @@
                 const CryptoData& data,
                 std::vector<uint8_t>* buffer) {
   crypto::OpenSSLErrStackTracer err_tracer(FROM_HERE);
 
   const EVP_MD* digest_algorithm = GetDigest(hash);
   if (!digest_algorithm)
     return Status::ErrorUnsupported();
   size_t hmac_expected_length = EVP_MD_size(digest_algorithm);
 
   buffer->resize(hmac_expected_length);
-  crypto::ScopedOpenSSLSafeSizeBuffer<EVP_MAX_MD_SIZE> hmac_result(
-      buffer->data(), hmac_expected_length);
 
   unsigned int hmac_actual_length;
   unsigned char* const success =
       HMAC(digest_algorithm, raw_key.data(), raw_key.size(), data.bytes(),
-           data.byte_length(), hmac_result.safe_buffer(), &hmac_actual_length);
-  if (!success || hmac_actual_length != hmac_expected_length)
+           data.byte_length(), buffer->data(), &hmac_actual_length);
+  if (!success)

[davidben, 2016/08/02 18:17:54]

Optional: You can also just write:

if (!HMAC(....)) {
  return Status::OperationError();
}

[eroman, 2016/08/02 18:28:21]

Done.

     return Status::OperationError();
 
+  if (hmac_actual_length != hmac_expected_length) {
+    // HMAC() promises to use at most EVP_MD_CTX_size(). If this was not the
+    // case then memory corruption may have just occurred.
+    CHECK(false);
+  }

[davidben, 2016/08/02 18:17:54]

CHECK_EQ(hmac_expected_length, hmac_actual_length);

[eroman, 2016/08/02 18:28:21]

Done.

+
   return Status::Success();
 }
 
 class HmacImplementation : public AlgorithmImplementation {
  public:
   HmacImplementation() {}
 
   Status GenerateKey(const blink::WebCryptoAlgorithm& algorithm,
                      bool extractable,
                      blink::WebCryptoKeyUsageMask usages,
@@ skipping 168 matching lines @@
   }
 };
 
 }  // namespace
 
 std::unique_ptr<AlgorithmImplementation> CreateHmacImplementation() {
   return base::WrapUnique(new HmacImplementation);
 }
 
 }  // namespace webcrypto