Add an escape hatch for the ECDSA CBC cipher removal.

net/socket/ssl_client_socket_impl.cc

Index: net/socket/ssl_client_socket_impl.cc
diff --git a/net/socket/ssl_client_socket_impl.cc b/net/socket/ssl_client_socket_impl.cc
index a5d2afd7d8483a4996e04f9b68e3756f55af5d1a..9c5de623961687bc39a9f17cf5caeb45e69649a4 100644
--- a/net/socket/ssl_client_socket_impl.cc
+++ b/net/socket/ssl_client_socket_impl.cc
@@ -11,6 +11,7 @@
 
 #include "base/bind.h"
 #include "base/callback_helpers.h"
+#include "base/feature_list.h"
 #include "base/lazy_instance.h"
 #include "base/macros.h"
 #include "base/memory/singleton.h"
@@ -216,6 +217,21 @@ int GetBufferSize(const char* field_trial) {
   return buffer_size;
 }
 
+#if defined(OS_NACL)
+bool AreLegacyECDSACiphersEnabled() {
+  return false;
+}
+#else
+// TODO(davidben): Remove this after the ECDSA CBC removal sticks.
+// https:/crbug.com/666191.
+const base::Feature kLegacyECDSACiphersFeature{
+    "SSLLegacyECDSACiphers", base::FEATURE_DISABLED_BY_DEFAULT};
+
+bool AreLegacyECDSACiphersEnabled() {
+  return base::FeatureList::IsEnabled(kLegacyECDSACiphersFeature);
+}
+#endif
+
 }  // namespace
 
 class SSLClientSocketImpl::SSLContext {
@@ -943,16 +959,14 @@ int SSLClientSocketImpl::Init() {
   }
 
   // Use BoringSSL defaults, but disable HMAC-SHA256 and HMAC-SHA384 ciphers
-  // (note that SHA256 and SHA384 only select legacy CBC ciphers). Additionally
-  // disable HMAC-SHA1 ciphers in ECDSA. Also disable
+  // (note that SHA256 and SHA384 only select legacy CBC ciphers). Also disable
   // DHE_RSA_WITH_AES_256_GCM_SHA384. Historically, AES_256_GCM was not
-  // supported. As DHE is being deprecated, don't add a cipher only to remove it
-  // immediately.
+  // supported. As DHE is being deprecated, don't add a cipher only to remove
+  // it immediately.
   //
   // TODO(davidben): Remove the DHE_RSA_WITH_AES_256_GCM_SHA384 exclusion when
   // the DHEEnabled administrative policy expires.
-  command.append(
-      "ALL:!SHA256:!SHA384:!ECDSA+SHA1:!DHE-RSA-AES256-GCM-SHA384:!aPSK:!RC4");
+  command.append("ALL:!SHA256:!SHA384:!DHE-RSA-AES256-GCM-SHA384:!aPSK:!RC4");
 
   if (ssl_config_.require_ecdhe)
     command.append(":!kRSA:!kDHE");
@@ -962,6 +976,11 @@ int SSLClientSocketImpl::Init() {
     command.append(":!kDHE");
   }
 
+  // Additionally disable HMAC-SHA1 ciphers in ECDSA. These are the remaining
+  // CBC-mode ECDSA ciphers.
+  if (!AreLegacyECDSACiphersEnabled())
+    command.append("!ECDSA+SHA1");
+
   // Remove any disabled ciphers.
   for (uint16_t id : ssl_config_.disabled_cipher_suites) {
     const SSL_CIPHER* cipher = SSL_get_cipher_by_value(id);