Verifying build platforms

One of SLSA’s guiding principles is to “trust platforms, verify artifacts”. However, consumers cannot trust platforms to produce Build L3 artifacts and provenance unless they have some proof that the provenance is unforgeable and the builds are isolated.

This page describes the parts of a build platform that consumers SHOULD assess and provides sample questions consumers can ask when assessing a build platform. See also Threats & mitigations and the build model.

Threats

Adversary goal

The SLSA Build track defends against an adversary whose primary goal is to inject unofficial behavior into a package artifact while avoiding detection. Remember that verifiers only accept artifacts whose provenance matches expectations. To bypass this, the adversary tries to either (a) tamper with a legitimate build whose provenance already matches expectations, or (b) tamper with an illegitimate build’s provenance to make it match expectations.

More formally, if a build with external parameters P would produce an artifact with binary hash X and a build with external parameters P’ would produce an artifact with binary hash Y, they wish to produce provenance indicating a build with external parameters P produced an artifact with binary hash Y.

See threats D, E, F, and G for examples of specific threats.

Note: Platform abuse (e.g. running non-build workloads) and attacks against builder availability are out of scope of this document.

Adversary profiles

Consumers SHOULD also evaluate the build platform’s ability to defend against the following types of adversaries.

1. Project contributors, who can:
   • Create builds on the build platform. These are the adversary’s controlled builds.
   • Modify one or more controlled builds’ external parameters.
   • Modify one or more controlled builds’ environments and run arbitrary code inside those environments.
   • Read the target build’s source repo.
   • Fork the target build’s source repo.
   • Modify a fork of the target build’s source repo and build from it.
2. Project maintainer, who can:
   • Do everything listed under “project contributors”.
   • Create new builds under the target build’s project or identity.
   • Modify the target build’s source repo and build from it.
   • Modify the target build’s configuration.
3. Build platform administrators, who can:
   • Do everything listed under “project contributors” and “project maintainers”.
   • Run arbitrary code on the build platform.
   • Read and modify network traffic.
   • Access the control plane’s cryptographic secrets.
   • Remotely access build environments (e.g. via SSH).

Build platform components

Consumers SHOULD consider at least these five elements of the build model when assessing build platforms for SLSA conformance: external parameters, control plane, build environments, caches, and outputs.

The following sections detail these elements of the build model and give prompts for assessing a build platform’s ability to produce SLSA Build L3 provenance. The assessment SHOULD take into account the security model used to identify the transitive closure of the builder.id for the [provenance model], specifically around the platform’s boundaries, actors, and interfaces.

External parameters

External parameters are the external interface to the builder and include all inputs to the build process. Examples include the source to be built, the build definition/script to be executed, user-provided instructions to the control plane for how to create the build environment (e.g. which operating system to use), and any additional user-provided strings.

Prompts for assessing external parameters

• How does the control plane process user-provided external parameters? Examples: sanitizing, parsing, not at all
• Which external parameters are processed by the control plane and which are processed by the build environment?
• What sort of external parameters does the control plane accept for build environment configuration?
• How do you ensure that all external parameters are represented in the provenance?
• How will you ensure that future design changes will not add additional external parameters without representing them in the provenance?

Control plane

The control plane is the build platform component that orchestrates each independent build execution. It is responsible for setting up each build and cleaning up afterwards. At SLSA Build L2+ the control plane generates and signs provenance for each build performed on the build platform. The control plane is operated by one or more administrators, who have privileges to modify the control plane.

Prompts for assessing the control plane

• Administration

  • What are the ways an employee can use privileged access to influence a build or provenance generation? Examples: physical access, terminal access, access to cryptographic secrets
  • What controls are in place to detect or prevent the employee from abusing such access? Examples: two-person approvals, audit logging, workload identities
  • Roughly how many employees have such access?
  • How are privileged accounts protected? Examples: two-factor authentication, client device security policies
  • What plans do you have for recovering from security incidents and platform outages? Are they tested? How frequently?

• Provenance generation

  • How does the control plane observe the build to ensure the provenance’s accuracy?
  • Are there situations in which the control plane will not generate provenance for a completed build? What are they?

• Development practices

  • How do you track the control plane’s software and configuration? Example: version control
  • How do you build confidence in the control plane’s software supply chain? Example: SLSA L3+ provenance, build from source
  • How do you secure communications between builder components? Example: TLS with certificate transparency.
  • Are you able to perform forensic analysis on compromised build environments? How? Example: retain base images indefinitely

• Creating build environments

  • How does the control plane share data with build environments? Example: mounting a shared file system partition
  • How does the control plane protect its integrity from build environments? Example: not mount its own file system partitions on build environments
  • How does the control plane prevent build environments from accessing its cryptographic secrets? Examples: dedicated secret storage, not mounting its own file system partitions to build environments, hardware security modules

• Managing cryptographic secrets

  • How do you store the control plane’s cryptographic secrets?
  • Which parts of the organization have access to the control plane’s cryptographic secrets?
  • What controls are in place to detect or prevent employees abusing such access? Examples: two-person approvals, audit logging
  • How are secrets protected in memory? Examples: secrets are stored in hardware security modules and backed up in secure cold storage
  • How frequently are cryptographic secrets rotated? Describe the rotation process.
  • What is your plan for remediating cryptographic secret compromise? How frequently is this plan tested?

Build environment

The build environment is the independent execution context where the build takes place. In the case of a distributed build, the build environment is the collection of all execution contexts that run build steps. Each build environment must be isolated from the control plane and from all other build environments, including those running builds from the same tenant or project. Tenants are free to modify the build environment arbitrarily. Build environments must have a means to fetch input artifacts (source, dependencies, etc).

Prompts for assessing build environments

• Isolation technologies

  • How are build environments isolated from the control plane and each other? Examples: VMs, containers, sandboxed processes
  • How is separation achieved between trusted and untrusted processes?
  • How have you hardened your build environments against malicious tenants? Examples: configuration hardening, limiting attack surface
  • How frequently do you update your isolation software?
  • What is your process for responding to vulnerability disclosures? What about vulnerabilities in your dependencies?
  • What prevents a malicious build from gaining persistence and influencing subsequent builds?

• Creation and destruction

  • What operating system and utilities are available in build environments on creation? How were these elements chosen? Examples: A minimal Linux distribution with its package manager, OSX with HomeBrew
  • How long could a compromised build environment remain active in the build platform?

• Network access

  • Are build environments able to call out to remote execution? If so, how do you prevent them from tampering with the control plane or other build environments over the network?
  • Are build environments able to open services on the network? If so, how do you prevent remote interference through these services?

Cache

Builders may have zero or more caches to store frequently used dependencies. Build environments may have either read-only or read-write access to caches.

Prompts for assessing caches

• What sorts of caches are available to build environments?
• How are those caches populated?
• How are cache contents validated before use?

Output storage

Output Storage holds built artifacts and their provenance. Storage may either be shared between build projects or allocated separately per-project.

Prompts for assessing output storage

• How do you prevent builds from reading or overwriting files that belong to another build? Example: authorization on storage
• What processing, if any, does the control plane do on output artifacts?

Builder evaluation

Organizations can either self-attest to their answers or seek certification from a third-party auditor. Evidence for self-attestation should be published on the internet and can include information such as the security model defined as part of the provenance. Evidence submitted for third-party certification need not be published.