Changes/IntelSGX

Intel SGX Software Stack

This is a proposed Change for Fedora Linux.
This document represents a proposed Change. As part of the Changes process, proposals are publicly announced in order to receive community feedback. This proposal will only be implemented if approved by the Fedora Engineering Steering Committee.

Summary

The Intel SGX technology enables creation of execution enclaves, whose memory is encrypted and thus protected from all other code running on the machine, including SMM, firmware, kernel and userspace. This proposal is to introduce the SGX host software stack to Fedora, to enable applications and features which have a dependency on SGX technology.

Owner

• Name: Daniel Berrange
• Email: berrange@redhat.com

Current status

• Targeted release: Fedora Linux 42
• Last updated: 2024-10-23
• [Announced]
• [<will be assigned by the Wrangler> Discussion thread]
• FESCo issue: <will be assigned by the Wrangler>
• Tracker bug: <will be assigned by the Wrangler>
• Release notes tracker: <will be assigned by the Wrangler>

Detailed Description

The Intel SGX technology enables creation of execution enclaves, whose memory is encrypted and thus protected from all other code running on the machine, including SMM, firmware, kernel and userspace. While it has many potential use cases, this proposal is focused around the infrastructure needed to enable support for attestation of TDX confidential virtual machines.

The SGX software stack compromises a number of components

• Support for developing new enclaves
  • Header files for the enclave code (a minimalist C library, C++ library, crypto and some other misc libraries)
  • Static library archives for the enclave code.
  • Build helper tools (for signing enclaves, generating code enclave API entrypoints)
• Support for developing applications that use enclaves
  • Header files for platform code
  • Dynamic libraries for platform code
• Support for deploying applications that use enclaves
  • Enclave service daemon - assists unprivileged applications in loading enclaves
  • Registration tools - assists platform administrator in acquiring certificates to identity the platform
  • Quote generation daemon - assists QEMU in acquiring signed attestation reports for TDX VMs.

Enclave classes

Broadly speaking there are considered to be two classes of enclave:

1. Architectural enclaves - these are a small set of common enclaves, created and signed by Intel, which ship as standard in any SGX deployment. They provide a hardware enablement role, bootstrapping the cryptographic root of trust on a platform.
2. Application enclaves - this is an arbitrary set of custom enclaves, written by independent software vendors / providers to perform application specific tasks. They are signed by whichever party is to be relied upon to validate trustworthiness of the enclave code.

Both a4rchitectural and application enclaves are build and deployed in the same way, using the same runtime libraries. Their only difference is where they sit in the chain of trust.

Given their use in bootstrapping the SGX platform root of trust, the architectural enclaves would normally be considered to be serving the same role as platform firmware would. They establish an association with the physical platform hardware, which all other SGX uses relies upon.

Application enclaves, meanwhile are clearly just userspace code, with a special execution context. They are created by ordinary application developers as needed to handle some specialized tasks.

What often confuses people is that all enclaves run in ring 3, which is traditionally the realm of userspace, while the enclaves' jobs look like either firmware or userspace jobs depending on context.

Their execution context, while using a region of memory aallocated by & accounted to the loading application, is securely isolated from everything else on the system. The memory assigned for use by an enclave is encrypted and unable to be accessed directly by the application thereafter. Nothing in ring 0 (firmware/kernel), ring 3 (userspace), or even psuedo-ring -1 (SMM) can interact with the enclaves, except via their published API entrypoints which are called via the EENTER instructions using a separate stack. IOW, the traditional view of hardware execution contexts being a simple stack of rings has been rather distorted by the capabilities of SGX.

The Fedora licensing guidelines describe the requirements for a pre-built binary to quality for the "firmware exception"

Benefit to Fedora

As a general purpose infrastructure technology, SGX can be applied to / used by a wide variety of scenarios / applications.

The primary goal in introducing SGX into Fedora, however, is to support the Fedora KVM virtualization stack when it introduces confidential virtual machines running with Intel TDX. The TDX attestation implementation in currently integrated with Intel CPUs is built on the SGX technology. Attestation is the means by which a guest VM owner, can prove that their VM machine is running in confidential mode on genuine Intel hardware, as opposed to being in "blue pill" environment.

Scope

Proposal owners

Add the following packages to Fedora

• CppMicroServices - a C++ runtime library for building microservices daemons
• sgx-srpm-macros - define some common macros for where SGX content will live in the filesystem tree
• sgx-compat-gccXXX - one (or more) specific GCC versions, built with targetted configure arguments, to match the GCC configuration required for enclave reproducible build.
• sgx-compat-binutilsXXX - one (or more) specific binutils versions, built with targetted configure arguments, to match the GCC configuration required for enclave reproducible build.
• sgx-compat-nasmXXX - one (or more) specific NASM versions, built with targetted configure arguments, to match the GCC configuration required for enclave reproducible build.
• sgx-compat-glibc-headersXXX - one (or more) specific GCC versions, built with targetted configure arguments, to match the GCC configuration required for enclave reproducible build.
• sgx-compat-kernel-headersXXX - one (or more) specific GCC versions, built with targetted configure arguments, to match the GCC configuration required for enclave reproducible build.
• linux-sgx-enclavesXXX - one (or more) packages for performing a reproducible build of architectural enclaves
• linux-sgx - provide the SGX platform development headers & libraries, runtime libraries, and supporting daemons

The pre-built, signed architectural enclaves are not always re-created on each SGX release. New binaries are only issued if there was a required feature change, or a security fix required. Thus the package for the reproducible enclave builds is separated out from the general SGX package. Furthermore, to perform a reproducible build of the full set of enclaves, requires potentially more than one linux-sgx-enclavesXXX package. For example SGX 2.23 and 2.24 releases only updated the 'qve' enclave, all other enclaves remained on version 2.22. This in turn creates the requirement for multiple GCC/binutils/nasm versions, at some points. The current newest 2.25 release has updated all enclaves, so initially only a single 'linux-sgx-enclaves-2_25' is expected to be required.

NB, an upstream bug has resulted in different parts of the enclave code being built with different GCC/binutils versions. This should be resolved in future releases:

NB, The requirement for glibc and kernel header packages is likely another upstream bug. The enclave code should be build exclusively against the SGX runtime, which is a completely custom C library. This is being investigated upstream.

Other developers

The kernel functionality required for SGX is already present in Fedora kernel packages, so no work by other maintainers is anticipated.

Release engineering

N/A - does not impact deliverables for releng

Policies and guidelines

Define policy requirements wrt shipping of pre-built SGX architectural enclaves (AEs) with cryptographic signatures from Intel.

Two options

• Declare SGX architectural enclaves can be considered firmware, and thus exempt from build-from-source policy requirements.

• Define a policy explicitly for SGX enclaves, that permits shipping pre-built signed binaries for architectural enclaves only, if-and-only-if, a reproducible build can validate the binaries.

NB, in all options, the proposed policy is ONLY intended to application to architectural enclave shipped by Intel. Any application that creates its own SGX enclaves would NOT be covered by this. ie this is NOT proposing to allow arbitrary 3rd party binary enclaves providing application code in Fedora, only the low level hardware enablement/bootstrapping enclave.

Application enclaves are considered out of scope for this feature proposal, but if required in the future, it is suggested that a Fedora signing key would be required for any application enclaves.

The distinction between architectural enclaves (signed by Intel to bootstrap trust) and application enclaves (signed by Fedora, with trust chained from the AEs), is considered conceptually similar to the distinction between shim (signed by Microsoft to bootstrap trust), vs kernel (signed by Fedora, with trust chained from shim).

Trademark approval

N/A

Alignment with the Fedora Strategy

This aligns with

• "Reaching the world". Including SGX will make the Fedora support for hosting Intel TDX confidential virtual machines feature complete, by enabling attestation by the guest owner
• "Innovation & leadership in technology". SGX is a general purpose infrastructure technology which enables application developers to build systems to securely run sensitive workloads.

Upgrade/compatibility impact

This is a new package set which should not have any upgrade impact, as it will not initially be a dependency of other software. In future it may be pulled in automatically as a dependency in certain KVM deployment scenarios. Even when installed, using anything related to SGX first requires host firmware changes to enable use of the technology. The systemd services provided have their unit files conditionalized on the existence of /dev/sgx_enclave device nodes.

Early Testing (Optional)

Do you require 'QA Blueprint' support? N

The proposed new packages are available for testing via Copr:

 https://copr.fedorainfracloud.org/coprs/berrange/sgx-ng/

How To Test

• Document how to validate a single-socket system by obtaining PCK certificates automatically
• Document how to configure a multi-socket system to enable its registration with Intel services, and request a PCK certificate

User Experience

Initially minimal user experience impact, since on its own it doesn't deliver noticable end user features. Only once a followup proposal for integrating Intel TDX into KVM is done will the user experience changes.

Dependencies

No existing packages will have a dependency on this change initially. In future, some deployments of QEMU may change to pull in certain SGX packages, to support Intel TDX.

Kernel support for SGX already exists in Fedora:

 $ git grep CONFIG_X86_SGX=y
 kernel-x86_64-debug-fedora.config:CONFIG_X86_SGX=y
 kernel-x86_64-debug-rhel.config:CONFIG_X86_SGX=y
 kernel-x86_64-fedora.config:CONFIG_X86_SGX=y
 kernel-x86_64-rhel.config:CONFIG_X86_SGX=y
 kernel-x86_64-rt-debug-rhel.config:CONFIG_X86_SGX=y
 kernel-x86_64-rt-rhel.config:CONFIG_X86_SGX=y

Contingency Plan

• Contingency mechanism: (What to do? Who will do it?) N/A (not a System Wide Change)
• Contingency deadline: N/A (not a System Wide Change)
• Blocks release? N/A (not a System Wide Change), Yes/No

Documentation

N/A (not a System Wide Change)