Tag Archives: SLSA

Flutter Cocoon Achieves SLSA Level 3

As part of Dart and Flutter's ongoing mission to harden security, we have achieved Supply Chain Levels for Software Artifacts (SLSA) Level 3 security on Flutter’s Cocoon application. Flutter achieved SLSA Level 2 on Cocoon in 2022.

Highlights

Flutter’s Cocoon application provides continuous integration orchestration for Flutter Infrastructure. Cocoon also helps integrate several CI services with GitHub and provides tools to make GitHub development easier. Achieving SLSA Level 3 for Cocoon means we have addressed all the security concerns of levels 1, 2, and 3 across the application. SLSA Level 3 is all about improving security and ensuring Cocoon’s source code and build artifacts are valid. This is done by adding increased validation to the artifact’s provenance.

What is Provenance?

Provenance is a description of how an artifact is built. Provenance includes information about the type of machine used to build the artifact, the location of the source code, and the instructions used to build the artifact. While SLSA Level 2 requires provenance to exist, SLSA Level 3 goes further by requiring proof that the provenance is valid and legitimate.

Cocoon uses the SLSA Verifier tool to verify provenance using a single-line command. With SLSA Verifier, all Cocoon builds will only be accepted if they have valid provenance. The provenance must prove the artifact was built on a Cloud Build instance with the source code coming from the Cocoon repository on GitHub. This ensures the Cocoon artifacts generated are safe and securely generated.

SLSA Verifier is easy to add into an existing workflow. For Cocoon, this required adding a provenance verification script to run SLSA Verifier on an artifact’s provenance. This script is configured in our Cloud Build template and must run and pass verification before an artifact can be deployed.

Future Work

In addition to improving SLSA Levels for Cocoon, the Flutter team is working towards improving our SLSA levels across some of our larger and more complex projects, such as Flutter and Engine. These projects have their own set of challenges, such as overhauling our artifact generation process to use Gn and Ninja (Engine), and hardening security requirements for our release workflows (Flutter). We are excited to continue to deliver new security updates and solutions!

Getting To SLSA Level 2 with Tekton and Tekton Chains

Overview

As application developers, we achieve amazing results quickly by leveraging a rich ecosystem of freely available libraries, modules and frameworks that provide ready-to-use capabilities and abstract away from underlying complexity. This is so foundational to how we work that we'll nonchalantly build and publish an app that pulls in hundreds of dependencies without even thinking about it. And it's only fairly recently, in the wake of some very high profile and high impact compromises, that we've started to reckon with the fact that this wonderful ecosystem is also a security quagmire. All of the dependencies that feed into your build make up your software supply chain, and supply chains need to be secured. In this post, we'll show how an increasingly popular open source CI/CD system, Tekton, implements the OpenSSF SLSA framework to provide you with supply chain security guarantees.

Software Supply Chain Security

A software supply chain is anything that goes into or affects your code from development, through your CI/CD pipeline, until it gets deployed into production. Increasingly, the software supply chain has become a vector for attacks. The recent Log4j, SolarWinds, Kaseya, and Codecov hacks highlight vulnerable surface areas exposed by an insecure software supply chain.

Between 2020 and 2021, there has been a 650% Surge in OSS supply chain attacks, and Gartner projects that 45% of organizations worldwide will have experienced software supply chain attacks by 2025.

Supply Chains Levels for Software Artifacts (SLSA)

The Supply chain Levels for Software Artifacts (SLSA) framework is a check-list of controls to prevent tampering, improve integrity, and increase security in the packages and infrastructure used by projects, businesses or enterprises. SLSA formalizes criteria around software supply chain integrity to help the industry and open source ecosystem secure the software development life cycle at all stages.

As part of the framework, SLSA has multiple levels of assurances. These levels contain industry-recognized best practices to create four levels of increasing assurance.

Supply-Chain Levels for Software Artifacts (SLSA) Levels 1 through 4

SLSA provides a set of requirements that needs to be met for an artifact to be considered for a particular SLSA level.

Tekton + Tekton Chains

Tekton is a powerful and flexible, open source, cloud-native framework for creating CI/CD systems, allowing developers to build, test, and deploy across cloud providers and on-premise systems. Tekton consists of several subprojects which are relevant to SLSA:

  • Pipelines: A system that allows one to define a pipeline of CI/CD tasks and have it be orchestrated by the Tekton controller.
  • Chains: A standalone system which observes Pipelines and generates provenance for the artifacts built by Pipelines.

Tekton build processes are defined as tasks and pipelines. A Task is a collection of Steps that are defined and arranged in a specific order of execution as part of a continuous integration flow.

A Pipeline is a collection of Tasks defined and arranged in a specific order of execution as part of a continuous integration flow.

A TaskRun is an instantiation of a Task with specific inputs, outputs and execution parameters while a PipelineRun is an instantiation of a Pipeline.

A Task/Pipeline can define a set of Results. TaskRuns and PipelineRuns create Results as defined in the Task/Pipeline. Results are used to communicate to Tekton Chains run specifics like the uri and the digest of the built artifact.

Tasks, Pipelines, TaskRuns and PipelineRuns are defined through yaml files. The entire build is defined by the set of yaml files which define Tekton Tasks, Pipelines, TaskRuns and PipelineRuns. These yaml files can be checked in as code and run directly from the code repository.

Getting to SLSA L1: Automation + Provenance

For an artifact to be SLSA L1 compliant it should satisfy the following:

  1. Scripted build: All build steps are fully defined in some sort of “build script”. The only manual command, if any, is to invoke the build script.
  2. Provenance: The provenance is available to the consumer in a format that the consumer accepts. The format SHOULD be in-toto SLSA Provenance, but another format MAY be used if both producer and consumer agree and it meets all the other requirements.

Tekton Tasks, TaskRuns, Pipelines and PipelineRuns are specified in yaml files. These yaml files can be considered as scripts and can even be checked in into a code repository. These could also be run from code repositories. Tekton Chains provides a way to generate provenance in in-toto SLSA format. As such, Tekton can easily make builds which satisfy the SLSA L1 requirements.

Let's follow through with an example, which has the following files:

  • setup.sh: Sets up Google cloud to run an instance of the build specified in pipeline_run.yaml. It also installs Tekton Pipeline and Tekton Chains. In the production environment, this would be run once to set up the environment and all builds would use the same environment.
  • pipeline_run.yaml: This file is the actual build file that is run by Tekton Pipelines. The build here first clones a Github repo, builds the container specified in the source and uploads it to a Docker repository.
A workflow diagram depicting how Tekton can be used to acheive SLSA L2 requirements
The build script pipeline.yaml is the definition of the script while pipeline_run.yaml defines an instance of the build. It provides instance specific parameters for the build. Though both pipeline_run.yaml and pipeline.yaml are in source control for this example, the build definition is in pipeline.yaml and as such pipeline.yaml being in source control would satisfy the requirement of a source controlled build script.

kubectl create -f

https://raw.githubusercontent.com/google/tekton-slsa-demo/main/pipeline_run.yaml

Tekton Chains for Provenance Generation

Provenance is metadata about how an artifact was built, including the build process, top-level source, and dependencies. Knowing the provenance allows software consumers to make risk-based security decisions.

Tekton Chains observes TaskRuns and PipelineRuns in a Kubernetes cluster. Once the runs are done, Chains collects information (provenance) about the Run or the build process and the artifact created by the Run. It signs the provenance and stores the signed provenance. The provenance generated for the example build complies to the SLSA provenance schema and is explained further below.

Note that every step of the build has been recorded and can be reconstructed by following the steps in the provenance.

Next Steps: CI/CD @ SLSA L2

SLSA requires that for a build to be SLSA L2 compliant it should satisfy the following

  1. Every change to the source is tracked in a version control system
  2. All build steps were fully defined in some sort of “build script”. The only manual command, if any, was to invoke the build script.
  3. All build steps ran using some build service, not on a developer’s workstation.
  4. The provenance is available to the consumer in a format that the consumer accepts. The format SHOULD be in-toto SLSA Provenance, but another format MAY be used if both producer and consumer agree and it meets all the other requirements.
  5. The provenance’s authenticity and integrity can be verified by the consumer. This SHOULD be through a digital signature from a private key accessible only to the service generating the provenance.
  6. The data in the provenance MUST be obtained from the build service (either because the generator is the build service or because the provenance generator reads the data directly from the build service). Regular users of the service MUST NOT be able to inject or alter the contents.

Every change to the source is tracked in a version control system

Tekton does not explicitly enforce that the source is version controlled. Tekton users can enforce that the source is version controlled by writing an appropriate Task which will check for version control. The source should also be communicated by Tekton Pipelines to Tekton Chains through a result variable that is suffixed with -ARTIFACT_INPUTS.

All build steps were fully defined in some sort of “build script”. The only manual command, if any, was to invoke the build script.

This is a requirement for SLSA L1 as well and as explained above, Tekton provides a way to script the build through yaml files. The build is defined as a Pipeline (or Task) which can be saved as a yaml file and submitted into source control. The build instance which is defined as a PipelineRun (or TaskRun) can resolve the Pipeline (or Task) yaml from source control and use it for the current instance of the build.

All build steps ran using some build service, not on a developer’s workstation.

Tekton can be hosted on a cloud provider or on a hosted Kubernetes cluster and run as a build service. The build scripts can be submitted into source control (like GitHub) and Tekton can read the scripts directly from source control.

Provenance should be available

This is a requirement for SLSA L1 and as explained above Tekton Chains provides build provenance.

Provenance should be signed and Authenticated

As can be seen in the example, Tekton Chains creates and signs the build provenance. The signature can be verified anytime to ensure that the provenance has not been tampered after the build and the provenance is really created by the build process that claims to have built it. The signing is done according to the SLSA specification using the DSSE format.

Tekton Chains creates the provenance and signs it using a secure private key. Chains then uploads the signed provenance to a user-specified location, one of which is Google Cloud’s Container Analysis, which implements the open standard Grafeas API for storing provenance.

An annotated block of code depicting how Tekton Chains create provenance and signs it

Provenance should be generated by a Service

Note that the provenance in the example is generated by the Tekton Chains service and it cannot be modified after it has been generated, which is guaranteed by the signature.

SLSA requirements for the contents of the provenance, for the build to be considered L2.

All images below are extracted from the provenance of the example build. These can be verified by re-running the example.

1. Identifies artifact: The provenance MUST identify the output artifact via at least one cryptographic hash. The subject field in the SLSA provenance captures the location of the built artifact and the cryptographic hash associated with it. To be able to capture the artifact, Tekton Pipelines should populate the result variable -ARTIFACT_OUTPUTS with the location and the digest of the artifact.
A block of code extracted from the provenance of the example build
2. Identifies builder: The provenance identifies the entity that performed the build and generated the provenance. The builder.id field captures the builder that built the artifact.
A block of code extracted from the provenance of the example build
3. Identifies build instructions: The provenance identifies the top-level instructions used to execute the build. In our example, the build script is in source control. Recording the repo, the path in the repo and the commit hash will uniquely identify the build instructions used to build the artifact.
A block of code extracted from the provenance of the example build
4. Identifies source code: The provenance identifies the repository origin(s) for the source code used in the build. The materials field records all the dependencies used to build the artifact, one of which is the source code. In the example the source used is in a GitHub repo, and as such the repo name and the commit hash will uniquely identify the source code.
A block of code extracted from the provenance of the example build

Conclusion

SLSA aims to secure the software supply chain by providing guidelines on how the software build should be done. Tekton pipelines and Tekton chains implement those guidelines and help in securing the software supply chain.

By Prakash Jagatheesan (team: TektonCD), Brandon Lum (team: GOSST)

Flutter SLSA Progress & Identity and Access Management through Infrastructure As Code

We are excited to announce several new achievements in Dart and Flutter's mission to harden security. We have achieved Supply Chain Levels for Software Artifacts (SLSA) Level 2 security on Flutter’s Cocoon application, reduced our Identity and Access Management permissions to the minimum required access, and implemented Infrastructure-as-Code to manage permissions for some of our applications. These achievements follow our recent success to enable Allstar and Security Scorecards.

Highlights

Achieving Flutter’s Cocoon SLSA level 2: Cocoon application provides continuous integration orchestration for Flutter Infrastructure. Cocoon also helps integrate several CI services with Github and provides tools to make Github development easier. Achieving SLSA Level 2 for Cocoon means we have addressed all the security concerns of levels 1 and 2 across the application. Under SLSA Level 2, Cocoon has “extra resistance to specific threats” to its supply chain. The Google Open Source Security team has audited and validated our achievement of SLSA Level 2 for Cocoon.


Implementing Identity & Access Management (IAM) via Infrastructure-as-Code: We have implemented additional security hardening features by onboarding docs-flutter-dev, master-docs-flutter-dev, and flutter-dashboard to use Identity and Access Management through an Infrastructure-as-Code system. These projects host applications, provide public documentation for Flutter, and contain a dashboard website for Flutter build status.

Using our Infrastructure-as-Code approach, security permission changes require code changes, ensuring approval is granted before the change is made. This also means that changes to security permissions are audited through source control and contain associated reasoning for the change. Existing IAM roles for these applications have been pared so that the applications follow the Principle of Least Privilege.

Advantages

  • Achieving SLSA Level 2 for Cocoon means we have addressed all the security concerns of levels 1 and 2 across the application. Under SLSA Level 2, Cocoon has “extra resistance to specific threats” to its supply chain.
  • Provenance is now generated for both, flutter-dashboard and auto-submit, artifacts through Cocoon’s automated build process. Provenance on these artifacts shows proof of their code source and tamper-proof build evidence. This work helps harden the security on the multiple tools used during the Cocoon build process: Google Cloud Platform, Cloudbuild, App Engine, and Artifact Registry.
  • Overall we addressed 83% of all SLSA requirements across all levels for the Cocoon application. We have identified the work across the application which will need to be completed for each level and category of SLSA compliance. Because of this, we know we are well positioned to continue future work toward SLSA Level 4.

Learnings and Best Practices

  1. Relatively small changes to the Cocoon application’s build process significantly increased the security of its supply chain. Google Cloud Build made this simple, since provenance metadata is created automatically during the Cloud Build process.

  2. Regulating IAM permissions through code changes adds many additional benefits and can make granting first time access simpler.

  3. Upgrading the SLSA level of an application sometimes requires varying efforts depending on the different factors of the application build process. Working towards SLSA level 4 will likely necessitate different configuration and code changes than required for SLSA level 2.

Coming Soon

Since this is the beginning of the Flutter and Dart journey toward greater SLSA level accomplishments, we hope to apply our learnings to more applications. We hope to begin work toward SLSA level 2 and beyond for more complex repositories like Flutter/flutter. Also, we hope to achieve an even higher level of SLSA compliance for the Cocoon application.

References

Supply Chain Levels for Software Artifacts (SLSA) is a security framework which outlines levels of supply chain security for an application as a checklist.

By Jesse Seales, Software Engineer – Dart and Flutter Security Working Group