Disable the TLS version fallback.

components/ssl_config/ssl_config_service_manager_pref.cc

 // Copyright (c) 2012 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 "components/ssl_config/ssl_config_service_manager.h"
 
 #include <stdint.h>
 
 #include <algorithm>
 #include <string>
 #include <vector>
 
 #include "base/bind.h"
+#include "base/feature_list.h"
 #include "base/macros.h"
 #include "base/metrics/field_trial.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_util.h"
 #include "base/values.h"
 #include "components/content_settings/core/browser/content_settings_utils.h"
 #include "components/content_settings/core/common/content_settings.h"
 #include "components/prefs/pref_change_registrar.h"
 #include "components/prefs/pref_member.h"
 #include "components/prefs/pref_registry_simple.h"
@@ skipping 58 matching lines @@
   }
   return version;
 }
 
 bool IsRC4EnabledByDefault() {
   const std::string group_name =
       base::FieldTrialList::FindFullName("RC4Ciphers");
   return base::StartsWith(group_name, "Enabled", base::CompareCase::SENSITIVE);
 }
 
+const base::Feature kSSLVersionFallbackTLSv11 = {

[Alexei Svitkine (slow), 2016/02/16 16:25:04]

Nit: No =

[davidben, 2016/02/16 17:01:08]

Done.

+    "SSLVersionFallbackTLSv1.1", base::FEATURE_DISABLED_BY_DEFAULT,
+};
+
 }  // namespace
 
 ////////////////////////////////////////////////////////////////////////////////
 //  SSLConfigServicePref
 
 // An SSLConfigService which stores a cached version of the current SSLConfig
 // prefs, which are updated by SSLConfigServiceManagerPref when the prefs
 // change.
 class SSLConfigServicePref : public net::SSLConfigService {
  public:
@@ skipping 89 matching lines @@
     PrefService* local_state,
     const scoped_refptr<base::SingleThreadTaskRunner>& io_task_runner)
     : ssl_config_service_(new SSLConfigServicePref(io_task_runner)),
       io_task_runner_(io_task_runner) {
   DCHECK(local_state);
 
   local_state->SetDefaultPrefValue(
       ssl_config::prefs::kRC4Enabled,
       new base::FundamentalValue(IsRC4EnabledByDefault()));
 
+  // Restore the TLS 1.1 fallback leg if enabled via features.
+  // TODO(davidben): Remove this when the fallback removal has succeeded.
+  // https://crbug.com/536200.
+  if (base::FeatureList::IsEnabled(kSSLVersionFallbackTLSv11)) {
+    local_state->SetDefaultPrefValue(
+        ssl_config::prefs::kSSLVersionFallbackMin,
+        new base::StringValue(switches::kSSLVersionTLSv11));
+  }
+
   PrefChangeRegistrar::NamedChangeCallback local_state_callback =
       base::Bind(&SSLConfigServiceManagerPref::OnPreferenceChanged,
                  base::Unretained(this), local_state);
 
   rev_checking_enabled_.Init(ssl_config::prefs::kCertRevocationCheckingEnabled,
                              local_state, local_state_callback);
   rev_checking_required_local_anchors_.Init(
       ssl_config::prefs::kCertRevocationCheckingRequiredLocalAnchors,
       local_state, local_state_callback);
   ssl_version_min_.Init(ssl_config::prefs::kSSLVersionMin, local_state,
@@ skipping 77 matching lines @@
   uint16_t version_max = SSLProtocolVersionFromString(version_max_str);
   uint16_t version_fallback_min =
       SSLProtocolVersionFromString(version_fallback_min_str);
   if (version_min) {
     config->version_min = version_min;
   }
   if (version_max) {
     uint16_t supported_version_max = config->version_max;
     config->version_max = std::min(supported_version_max, version_max);
   }
-  if (version_fallback_min) {
+  // Values below TLS 1.1 are invalid.
+  if (version_fallback_min &&
+      version_fallback_min >= net::SSL_PROTOCOL_VERSION_TLS1_1) {
     config->version_fallback_min = version_fallback_min;
   }
   config->disabled_cipher_suites = disabled_cipher_suites_;
   config->rc4_enabled = rc4_enabled_.GetValue();
 }
 
 void SSLConfigServiceManagerPref::OnDisabledCipherSuitesChange(
     PrefService* local_state) {
   const base::ListValue* value =
       local_state->GetList(ssl_config::prefs::kCipherSuiteBlacklist);
   disabled_cipher_suites_ = ParseCipherSuites(ListValueToStringVector(value));
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 //  SSLConfigServiceManager
 
 namespace ssl_config {
 // static
 SSLConfigServiceManager* SSLConfigServiceManager::CreateDefaultManager(
     PrefService* local_state,
     const scoped_refptr<base::SingleThreadTaskRunner>& io_task_runner) {
   return new SSLConfigServiceManagerPref(local_state, io_task_runner);
 }
 
 // static
 void SSLConfigServiceManager::RegisterPrefs(PrefRegistrySimple* registry) {
   SSLConfigServiceManagerPref::RegisterPrefs(registry);
 }
 }  // namespace ssl_config