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: 2024-10-30
• [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 linking into the enclave binaries.
  • Build helper tools (for signing enclaves, generating code enclave API entrypoints)
• Support for developing applications that use enclaves, eg to be able to load and communicate with 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.

The code and binaries related to the host OS platform components will be installed in the normal filesystem locations common to all Fedora packages. e.g. libraries in /usr/lib64, headers in /usr/include and binaries in /usr/bin.

For the purposes of packaging, the enclaves will be treated as cross-compilation target. While the compiler build architecture target is x86_64, the runtime has custom C / C++ libraries that must be used, and requires a separate code loader. Enclaves cannot be directly linked to applications, they are strictly independent libraries.

With this in mind, all enclave related headers and libraries are proposed to be installed at /usr/x86_64-intel-sgx, specifically under lib64 and include sub-directories. There are no binaries for enclaves, only libraries, so no bin dir is required.

The generated binary packages will generally all have an 'sgx-' prefix to their name, with enclave related packages having an more specialized 'sgx-enclave-' name prefix.

Feedback

• Objection: The SGX technology can be used as a way to implement DRM. Notable example has been BluRay playback.. Answer: As with many technologies, it is possible to use SGX in ways that are both positive and negative, wrt the owner / users of a machine. Use of SGX for DRM in BluRay playback is hostile to the owner/user of a machine. This change is NOT proposing to introduce / support any such usage/applications in Fedora. The fact that bad uses of SGX exist outside of Fedora, must not block the use of SGX in in Fedora for scenarios where it can offer features that benefit Fedora's users.

• Objection: The SGX enclave code is not open source, because it requires a vendor signature on output binaries. Answer: 100% of the SGX code is made available under a variety open source licenses (Apache, BSD, MIT, GPL & more). One of the architectural enclaves, pce, requires an Intel signature because it is used to establish the root of trust with the hardware. In their role of bootstrapping use of SGX hardware, the architectural enclaves are a type of firmware, and it is normal for firmware to have a vendor signature. In contrast to almost all firmware which is proprietary, the architectural enclave code is all under Fedora approved OSS licenses. All of the architectural enclaves, can be rebuilt from source by a user, and signed with a user specified key if desired. If doing so, the limitation is that if the pce enclave is not signed by an Intel key, it will not be possible to obtain a certificate for the host from Intel's public PCS API service, and thus the integrity of SGX signed quotes cannot be validated back to the Intel root CA certificate.

Benefit to Fedora

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

In the context of this change proposal, no application usage is intended to be introduced. This proposal is focused on infrastructure enablement.

A followup change proposal will be made to introduce Intel TDX confidential virtual machines, which is anticipated to be the first end user facing usage of 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 a "blue pill" environment. All currently shipping Intel CPUs which support TDX build attestation on top of SGX with OSS enclave code (the tdqe enclave), as opposed to embedding attestation in proprietary firmware.

NB, the 'tdqe enclave will be included in this proposal, but will remain unused until the followup TDX change proposal introduces the QEMU changes that depend on it.

Scope

Proposal owners

Add at least the following source 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
• linux-sgx - provide the SGX platform development headers & libraries, runtime libraries, and supporting daemons

Assuming that pre-built, signed SGX architectural enclaves will be shipped, the following additional package is proposed:

• linux-sgx-enclavesXXX - one (or more) packages containing architectural enclaves

Not every architectural enclave binary is re-issued on every release. New signed builds are only made available when there is a CVE fix, or a feature enhancement. At time of writing the SGX 2.25 release included new builds for every enclave. A future 2.26 release may only update a subset. Hence at some points in time, Fedora may have more than one linux-sgx-encalvesXXX package present.

If it is required to perform a reproducible build of the above architectural enclave binaries, to prove the binaries match the claimed sources, the following toolchain packages are needed:

• 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.

At time of writing, with the SGX 2.25 release, the versions required to perform a fully reproducible build will be

• gcc: 8.5.0, 9.5.0
• binutils: 2.38, 2.40
• nasm: 2.16.01
• glibc: 2.38
• kernel: 5.17

Note, while Fedora already ships builds fo gcc, binutils, etc, performing reproducible builds requires specific versions, configured with particular choice of build options. Hence the intention to ship parallel packages, which are exclusively intended for use in reproducing enclaves. There will be no support for using these toolchain packages for any other situation in Fedora.

Note: the fact that two versions are needed for binutils and GCC, is an upstream an upstream bug

Note: the enclaves are supposed to be built exclusively against the SGX SDK which provides its own C runtime headers. Thus the fact that it has a requirement for header files from glibc & kernel is also likely another upstream bug.

The full set of proposed source RPMs and their corresponding binary RPM outputs can be see from Copr:

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

Note, the pre-built, signed architectural enclaves are not always re-created on each SGX release.

Other developers

It is not anticipated that other package maintainers need do anything to support introduction of SGX. The kernel is already built with the SGX feature enabled.

systemd ships with udev rules to enable creation of some of the required device nodes. The SGX package contains addon udev rules for remaining device nodes. The latter may be contributed upstream to systemd in future, and would flow back into Fedora in a normal systemd update.

Release engineering

N/A - does not impact deliverables for releng

Policies and guidelines

A decision is needed around the handling of the pre-built, signed SGX enclaves. There are some relevant parallels in the guidelines for firmware as well as current practice wrt shim and CPU microcode.

Normal Fedora practice requires building everything from source. There is a general exception to this for firmware blobs, which don't need to be built from source and don't need to be under an OSS compliant license as long as the binaries can be freely distributed. Hardware firmware binaries almost always include a digital signature from the vendor, to ensure that only trusted firmware is loaded onto the device. Thus even if source code is available for a given firmware binary, Fedora would still need to be shipping it as a pre-built vendor supplied signed binary.

Another example of shipping pre-built vendor binaries would be CPU microcode. This could be considered firmware for the core CPU, and again are vendor supplied, signed binaries, with no source code available.

A slightly different reference point is in the handling of shim, where Fedora builds a binary from known good source, possibly with local pathes, and sends it off for signing by Microsoft, packaging the binary that is sent back. Microsoft has various requirements before it will permit signing a vendor's shim.

Consider if Fedora built a pristine shim release, with a designated toolchain version, using a reproducible build process. There would be no need to send the binary to Microsoft for signing, as the binary built in Fedora would match a standard pre-published signed binary from that shim release + toolchain. This is the situation SGX architectural enclaves are in. The main difference between the shim & SGX scenarios is that Fedora has the ability to add arbitrary local patches to shim & build with arbitrary toolchain versions of its choosing. This would not be possible when following a vendor specified reproducible build process.

In the shim case, it is often (but not always) possible for users to enroll their own certificates in UEFI to bless shim signatures from vendors other than Microsoft. IOW, a user often has the ability to build & customize shim themselves signing with their own keys. In the case of SGX enclaves, this is possible for 7, out of the 8 architectural enclaves, however the most fundamental enclave (pce) must always have an Intel signature to bootstrap the hardware root of trust.

In summary, the architectural enclaves meet the definition of firmware documented in the Fedora licensing guidelines:

• https://docs.fedoraproject.org/en-US/legal/license-approval/#_technical_firmware_requirements

Thus it should be permissible to ship the pre-built, signed binaries with no policy changes.

The shim model also gives impetus to the idea of it being acceptable to ship pre-built binaries, provided a reproducible build process can prove the binaries match the source under Fedora approved licenses.

Ultimately it is suggested to consider a hybrid between the two views. Treat the SGX enclaves as firmware, but none the less require a reproducible build process, to prove the binaries correspond to the claim OSS code.

It is further suggested that this not be limited to SGX enclaves, instead expand the existing firmware exception guidance, to require a reproducible build is performed if the pre-built signed firmware binaries have complete & corresponding soruce available with a supported reproducible build process.

The intent is to prove that the signed vendor firmware actually matches the published code whenever practical. This would apply to all SGX architectural enclaves, but also potentially to other distributed firmware with available source.

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, until such time as they are reviewed & built in Fedora koji:

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

These should work on any Intel Xeon class platform

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 noticeable end user features, as it is not believed that any existing applications in Fedora are able to leverage SGX.

The initial user benefit will be that users can bootstrap trust in SGX on their Fedora host. This will facilitate users in deploying 3rd party applications of their choosing that utilize SGX.

At a later time, when support for Intel TDX is integrated into KVM and QEMU, the immediate Fedora user benefit will significantly expand.

Dependencies

The primary functional dependency for use of SGX is kernel support, which has existed in Fedora for some time. See "CONFIG_X86_SGX=y" in the kconfig files.

This change proposal does not rely on work by another other package maintainers, but new packages will require reviewer attention.

Contingency Plan

• Contingency mechanism: The new packages have no ill effects on existing Fedora usage. Any outstanding work can be postponed to a later release if required.
• Contingency deadline: Beta free
• Blocks release? No

Documentation

Documentation will be provided to describe:

• Configuring hardware UEFI to enable use of SGX
• Obtaining PCK certificates for a host from the Intel trusted services API

A subsequent change proposal will cover usage of SGX with TDX confidential virtual machines.