Data exists in three states, at rest, in-flight, and in-use. Over recent years the security industry has done an excellent job of providing solutions for securing data at rest, such as data stored on a hard drive and in-flight think web pages via HTTPS. Unfortunately, those looking to steal data are very familiar with these advances, so they probe the entire system searching for new vulnerabilities. They’ll look at code that hasn’t been touched in years, or even decades (Shellshock), and architectural elements of the system which were previously trusted like memory (Meltdown), cache, and CPU registers (Spectre). Confidential Computing address this third state, data in-use, by providing a hardware-based trusted execution environment (TEE). Last spring, the Linux Foundation realized that extensive reliance on public clouds demanded a more advanced holistic approach to security. Hence, they launched the Confidential Computing Consortium.
The key to Confidential Computing is building a TEE entirely in hardware. The three major CPU platforms all support some form of a TEE, and they are Intel’s Software Guard Extensions (SGX), AMD’s Secure Encrypted Virtualization (SEV), and ARM’s TrustZone. Developers can leverage these TEE platforms, but each is different, so code written for SGX will not work on an AMD processor. To defeat a TEE and access your most sensitive data, an attacker will need to profile the server hardware to determine which processor environment is in use. They will then need to find and deploy the appropriate vulnerability for that platform if one exists. They also need to ensure that their exploit has no digital or architectural fingerprints that would make attribution back to them a possibility when the exploit is eventually discovered.
Creating a trusted execution environment in hardware requires the host CPU to be intimately involved in the process. AMD, ARM, and Intel each provide their own hardware support for building a TEE, and each has its benefits. Two security researchers, one from Wayne State University and the other from the University of Houston, produced an excellent comparison of AMD and Intel’s platforms. For Intel, they stated:
“We conclude that Intel SGX is suited for highly security-sensitive but small workloads since it enforces the memory integrity protection and has a limited amount of secure resources.”
“AMD SME and SEV do not provide memory integrity protection. However, providing a greater amount of secure resources to applications, performing faster than Intel SGX (when an application requires a large amount of secure memory), and no code refactoring, make them more suitable for complex or legacy applications and services.”
Based on the work of these researches it would seem that AMD has a more comprehensive platform and that their solution is considerably more performant than Intel’s SGX.
So how does confidential computing establish a trusted execution environment? Today the Confidential Computing Consortium has three contributed projects, and each has its take on this as their objective:
- Software Guard Extensions (SGX) from Intel
- Open Enclave SDK
For the past two decades, the US and the UK have used the construct of a Sensitive Compartmented Information Facility (SCIF but pronounced SKIF) to manage classified data. A SCIF is an enclave, a private space surrounded by public space, with a very well-defined set of procedures for securely using data within this private space and moving data into and out of the private space. Intel adopted some of these same concepts when they defined the Software Guard Extensions (SGX). SGX is a new set of processor instructions that first appeared in Skylake. When SGX instructions are used, it enables the processor to build a private enclave in memory where all the data and code in that memory region is encrypted. That region is further zoned off from all other processes, so they don’t have access to it, even those with a higher privilege. As the processor fetches instructions or data from that enclave, it then decrypts them in-flight, and if a result is to be stored back in the enclave, it will then be encrypted in-flight before it is stored.
When Intel rolled out SGX in 2015, it immediately became the safe that all safe crackers wanted to defeat. In computer science, safe crackers are security researchers, and in the five years since SGX was released, we’ve seen seven well-documented exploits. The two that exposed the most severe flaws in SGX were Prime+Probe and Foreshadow. Prime+Probe was able to grab the RSA keys that secured the encrypted code and data in the enclave. Within six months, a countermeasure was published to disable Prime+Probe. Foreshadow was a derivative of Spectre, and used flaws in speculative execution and buffer overflow to attack the secure enclave. SGX is a solid start with regard to building a trusted execution environment in hardware. WolfSSL also adopted SGX and tied it to its popular TLS/SSL stack to provide a secure connection into and out of an SGX enclave.
The Open Enclave SDK claims it is hardware agnostic, a software-only platform for creating a trusted execution environment. The Open Enclave SDK requires SGX with Flexible Launch Control (FLC) as a prerequisite for installation. It is an extension of SGX and only runs on Intel hardware. Recently, a technology preview was made available for the Open Portable Trusted Execution Environment OS available on ARM that leverages TrustZone. At this point, there appears to be no support for AMD’s platform.
Enarx is also a hardware-agnostic, but it is an application launcher designed to support Intel’s SGX and AMD’s Secure Encrypted Virtualization (SEV) platform. It does not require that applications be modified to use these trusted execution environments. When delivered, this would be a game-changer. “Enarx aims to make it simple to deploy workloads to a variety of different TEEs in the cloud, on your premises or elsewhere, and to allow you to have confidence that your application workload is as secure as possible.” At this point, Enarx hasn’t mentioned support for ARM’s TrustZone technology. There is tremendous promise in the work the Enarx team is doing, and they appear to be making some substantial progress.
The Confidential Computing Consortium is still less than a year old, and it has attracted all the major CPU and data center players as members. Their goal is an ambitious one, but with projects like Enarx well underway, it’s hopeful that securing data in-use will soon become commonplace throughout on-premises and cloud environments.
*Note this story was originally written for Linkedin on July 12, 2020.