Asylo Remote Backend

Introduction

The Asylo Enclave Remote Backend (RemoteBackend) utilizes two gRPC peering agents, RemoteEnclaveProxyClient (ProxyClient) on the host machine running inside the Untrusted application, and RemoteEnclaveProxyServer (ProxyServer) on the target server wrapping the Enclave. All Enclave calls and Untrusted calls are still made with the Asylo API, but are forwarded from one gRPC agent to the other.

Local enclaves provide execution isolation by means of SGX or another Trusted Execution Environment (TEE). RemoteBackend provides execution isolation by placing the enclave on a separate target server. The target server can have the security properties required by the user application, whether that be SGX, AMD-SEV, or physical security.

The underlying technology, gRPC, allows users to place the user application and target server across any network routable space. For instance, the user application could be placed in the cloud, while the target server could be placed on-prem, or vice versa.

In this concepts guide we will do a deep technical dive into the inner workings of the RemoteBackend. This guide assumes technical knowledge of the Asylo API model.

Primary Components

RemoteEnclaveProxyClient

The RemoteEnclaveProxyClient implements primitives::Client and runs in the application on the host machine.

As part of configuration for the ProxyClient an implementation of RemoteProvision must be provided. RemoteProvision describes two functions:

• Provision() (required):

  Provision() provides the ProxyClient with a means to launch the requisite ProxyServer. This can be by means of the provided RemoteProxyProvisionServer, the provided LocalRemoteProxyProvision, or a custom implementation provided by the user.

• Finalize() (optional):

  Finalize() is called during the EnclaveManager::Destroy call, and performs final actions prior to destroying the ProxyClient. Finalize() must be safe to call more than once.

RemoteEnclaveProxyServer

The RemoteEnclaveProxyServer runs on the machine with the TEE. The ProxyServer loads an enclave, handles entry calls, and will finally terminate the enclave upon request from the ProxyClient. All untrusted exit calls made by the loaded enclave will be forwarded to the untrusted application and performed on the non-TEE machine.

RemoteProxyClientConfig

The RemoteProxyClientConfig provides the client with the Provision object, necessary to launch a ProxyServer, and the proper credentials to connect to it. It is held by the EnclaveLoadConfig and passed to the ProxyClient via the EnclaveManager during the LoadEnclave call.

Communicator

Communicator performs all calls between the ProxyClient and ProxyServer and handles connection and disconnection between the two. Currently Communicator only supports IPv6.

Local vs Remote Enclave Lifecycle

The lifecycle of an Asylo Enclave is largely unchanged in the Remote case. The enclave is still loaded and initialized on an EnclaveManager::LoadEnclave call, and will live and respond to EnterAndRun calls until an EnclaveManager::DestroyEnclave call is made.

There are two key differences to the lifecycle:

1. During the EnclaveManager::LoadEnclave call two additional steps are performed:

   • The ProxyClient will call RemoteProvision::Provision() in order to start a ProxyServer and inform it of the address and port the ProxyClient is running on.
   • The ProxyClient will make a blocking call to WaitForEndPointAddress(), where it will wait for the ProxyServer to initiate connection.

2. Termination of ProxyClient or ProxyServer outside of normal operations (i.e. kill -9, or power loss) may cause the application to terminate or behave unexpectedly.

Utilities

RemoteBackend requires additional steps for setup, configuration, and monitoring. To aid in this, Asylo has added several utilities.

LocalRemoteProxyProvision and RemoteProxyProvisionServer

Provisioning a ProxyServer is one of the key steps to starting the RemoteBackend loading an enclave remotely. In addition to RemoteProvision, where an Asylo user can implement their own Provisioning service, Asylo provides two sample RemoteProvision implementations. It is likely that neither provisioning service is sufficient for production use cases, but they can be a guide in developing your own provisioning service.

LocalRemoteProxyProvision launches a ProxyServer on the local machine. Its primary use is for testing. The user flow is to start a ProxyClient configured to use LocalRemoteProxyProvision, and use the --remote_proxy flag to pass the path to the desired remote proxy. LocalRemoteProxyProvision will take care of all the provisioniong and connection.

RemoteProxyProvisionServer launches a ProxyServer on a remote machine. The user flow is to start a RemoteProxyProvisionServer on the remote machine, and then start a ProxyClient on a local machine. The ProxyClient will contact the RemoteProxyProvisionServer, send the enclave across the wire, and request a ProxyServer. The ProxyServer will start, and the RemoteProxyProvisionServer will pass back the address for the ProxyServer. The ProxyClient then communicates with the ProxyServer per normal operations.

GrpcChannelBuilder

gRPC channels are relatively simple to setup, but managing configuration and credentialing between numerous gRPC servers and clients can become burdensome. To aid in this Asylo has developed the GrpcChannelBuilder. It’s purpose is to allow users to easily configure each gRPC server at runtime.

If used, GrpcChannelBuilder introduces 3 commandline flags:

1. --security_type:

   Can be one of either local or ssl and defaults to local.

   • local: Utilizes gRPC::Insecure*Credentials(), these credentials provide no authentication and data could be spoofed or intercepted.

   • ssl: Utilizes gRPC::Ssl*Credentials(), these credentials provide the added benefit of secure authentication, mitigating the ability of spoof and intercept attacks.

2. --ssl_server_cred:

   Local path to a self-signed ssl credential. Required if --security_type == ssl.

3. --ssl_server_cert:

   Local path to a self-signed ssl certificate. Required if --security_type == ssl.

These options might be insufficient for RemoteBackend usage in a specific production environment; other security types and additional authentication would need to be implemented in those cases. As such, custom credentialling can be provided via RemoteEnclaveConfig::Create(). Keep in mind that the default RemoteEnclaveConfig uses GrpcChannelBuilder with --security_type=local.

OpenCensus Metrics Collection

Metrics are an important aspect for any distributed application. To aid in the collection effort, Asylo provides metrics via OpenCensus. OpenCensus collects the metrics locally and can export them to several backends, currently OpenCensus supports exporting metrics to Prometheus and StackDriver, custom exporters can also be built.

To use the OpenCensus metrics collection, an Asylo User would add an OpenCensusMetricConfig to the RemoteEnclaveConfig, and register an exporter via the instructions provided by OpenCensus.