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Policies

You can use policies to implement control gates and security checks in your attestations.

Operators can attach policies to workflow contracts. Those policies will be evaluated against the different materials and the statement metadata. The result of the evaluation is informed as a list of possible violations and added to the attestation statement before signing and sending it to Chainloop.

Currently, policy violations won't block attestation push commands. Instead, Chainloop will include them in the attestation so that they can be used for building server side control gates.

Policy specification​

A policy can be defined in a YAML document, like this:

cyclonedx-licenses.yaml
apiVersion: workflowcontract.chainloop.dev/v1
kind: Policy
metadata:
  name: cyclonedx-licenses
  description: Checks for components without licenses
  annotations:
    category: sbom
spec:
  policies:
  - kind: SBOM_CYCLONEDX_JSON
    embedded: |
      package main
  
      import rego.v1
      
      # Global result object
      result := {
        "skipped": skipped,
        "violations": violations,
        "skip_reason": skip_reason,
      }
  
      default skip_reason := ""
      
      skip_reason := m if {
        not valid_input
        m := "the file content is not recognized"
      }
      
      default skipped := true
      
      skipped := false if valid_input
      
      valid_input if {
        # expect at least 1 component in the SBOM
        count(input.components) > 0
      }
  
      violations contains msg if {
        count(without_license) > 0
        msg := sprintf("Missing licenses for %s", [components_str])
      }
  
      components_str := concat(", ", [comp.purl | some comp in without_license])
  
      without_license contains comp if {
        some comp in input.components
        not comp.licenses
      }

In this particular example, we see:

  • policies have a name (cyclonedx-licenses)
  • they can be optionally applied to a specific type of material (check the documentation for the supported types). If no type is specified, a material name will need to be explicitly set in the contract, through selectors.
  • they have a policy script that it's evaluated against the material (in this case a CycloneDX SBOM report). Currently, only Rego language is supported.
  • there can be multiple scripts, each associated with a different material type.

Policy scripts could also be specified in a detached form:

...
spec:
  policies:
  - kind: SBOM_CYCLONEDX_JSON
    path: my-script.rego

Supporting multiple material types​

Policies can accept multiple material types. This is specially useful when a material can be specified in multiple format types, but from the user perspective, we still want to maintain one single policy.

For example, this policy would check for vulnerabilities in SARIF, CycloneDX and CSAF formats:

...
apiVersion: workflowcontract.chainloop.dev/v1
kind: Policy
metadata:
  name: cve-policy
spec:
  policies:
    - kind: SBOM_CYCLONEDX_JSON
      path: cves-cyclonedx.rego
    - kind: CSAF_SECURITY_ADVISORY
      path: cves-csaf-sa.rego
    - kind: SARIF
      path: cves-sarif.rego

In these cases, Chainloop will choose the right script to execute, but externally it would be seen as a single policy. If more than one path is executed (because they might have the same kind), the evaluation result will be the sum of all evaluations.

Applying policies to contracts​

When defining a contract, a new policies section can be specified. Policies can be applied to any material, but also to the attestation statement as a whole.

schemaVersion: v1
materials:
  - name: sbom
    type: SBOM_CYCLONEDX_JSON
  - name: another-sbom
    type: SBOM_CYCLONEDX_JSON
  - name: my-image
    type: CONTAINER_IMAGE
policies:
  materials: # policies applied to materials
    - ref: file://cyclonedx-licenses.yaml # (1)
    # or optionally with the digest appended, see integrity checks below
    # - ref: file://cyclonedx-licenses.yaml@sha256:5b40425cb7bcba16ac47e3d8a8d3af7288afeeb632096994e741decedd5d38b3
  attestation: # policies applied to the whole attestation
    - ref: https://raw.githubusercontent.com/chainloop-dev/chainloop/main/docs/examples/policies/chainloop-commit.yaml # (2)

Here we can see that:

  • (1) materials will be validated against cyclonedx-licenses.yaml policy. But, since that policy has a type property set to SBOM_CYCLONEDX_JSON, only SBOM materials (sbom and another-sbom in this case) will be evaluated.

    If we wanted to only evaluate the policy against the sbom material, and skip the other, we should filter them by name:

    policies:
      materials:
        - ref: file://cyclonedx-licenses.yaml
          selector: # (3)
            name: sbom

    Here, in (3), we are making explicit that only sbom material must be evaluated by the cyclonedx-licenses.yaml policy.

  • (2) the attestation in-toto statement as a whole will be evaluated against the remote policy chainloop-commit.yaml, which has a type property set to ATTESTATION. This brings the opportunity to validate global attestation properties, like annotations, the presence of a material, etc. You can see this policy and other examples in the examples folder.

Finally, note that material policies are evaluated during chainloop attestation add commands, while attestation policies are evaluated in chainloop attestation push command.

Embedding or referencing policies​

There are two ways to attach a policy to a contract:

  • By referencing it, as it can be seen in the examples above. ref property admits a local file:// (filesystem) or remote reference https://. For example:

    policies:
      materials: 
        - ref: file://cyclonedx-licenses.yaml # local reference

    and

    policies:
      materials:
        - ref: https://raw.githubusercontent.com/chainloop-dev/chainloop/main/docs/examples/policies/sbom/cyclonedx-banned-licenses.yaml

    are both equivalent. The advantage of having remote policies is that they can be easily reused, allowing organizations to create policy catalogs.

  • If preferred, authors could create self-contained contracts embedding policy specifications. The main advantage of this method is that it ensures that the policy source cannot be changed, as it's stored and versioned within the contract:

      policies:
        materials:
          - embedded: # (1)
              # Put full policy spec here
              apiVersion: workflowcontract.chainloop.dev/v1
              kind: Policy
              metadata:
                name: cve-policy
              spec:
                policies:
                - kind: SBOM_CYCLONEDX_JSON
                  path: cves-cyclonedx.rego

In the example above, we can see that, when referenced by the embedded attribute (1), a full policy can be embedded in the contract.

Policy arguments​

Policies may accept arguments to customize its behaviour. If defined, the inputs section, will be used by Chainloop to know with inputs arguments are supported by the policy

For example, this policy matches a "quality" score against a "threshold" argument:

# quality.yaml
apiVersion: workflowcontract.chainloop.dev/v1
kind: Policy
metadata:
  name: quality
  description: Checks for components without licenses
  annotations:
    category: sbom
spec:
  inputs: #(1)
    - name: threshold
      description: quality threshold
      required: true
  policies:
    - kind: SBOM_CYCLONEDX_JSON
      embedded: |
        package main

        import rego.v1

        result := {
          "skipped": false,
          "violations": violations,
        }

        default threshold := 5
        threshold := to_number(input.args.threshold) # (2)

        violations contains msg if {
          input.score < threshold
          msg := sprintf("quality threshold not met %d < %d", [input.score, threshold])
        }
  • (1) the input section tells Chainloop which parameters should be expected. If missing, the argument will be ignored (an no value will be passed to the policy)
  • (2) input parametes are available in the input.args rego input field.

The above example can be instantiated with a custom threshold parameter, by adding a with property in the policy attachment in the contract:

policies:
  materials:
    - ref: file://quality.yaml
      with:
        threshold: 6 (1)

(1) This is interpreted as a string, that's why we need to add to_number in the policy script

Integrity Checks​

Optionally, you can append the sha256 hash of the policy file content to your policy attachment reference. By doing so, the policy engine will make sure the resolved policy matches the expected hash in the contract reference.

For example

policies:
  materials: 
    # append digest to optionally check the integrity of the policy file during evaluation
    - ref: file://cyclonedx-banned-licenses.yaml@sha256:5b40425cb7bcba16ac47e3d8a8d3af7288afeeb632096994e741decedd5d38b3
    # It also works for http references
    - ref: https://raw.githubusercontent.com/chainloop-dev/chainloop/main/docs/examples/policies/sbom/cyclonedx-banned-licenses.yaml@sha256:5b40425cb7bcba16ac47e3d8a8d3af7288afeeb632096994e741decedd5d38b3

How to write a Chainloop policy in Rego​

Check this how-to to know how you can write Chainloop policies in Rego language.

Policy Groups​

This feature allow operators to group related policies into one single entity that can be reused across the organization. With Policy Groups, materials and policies can be enforced in Chainloop contracts with little or no effort.

For example, they might want to create a "SBOM quality" group with some SBOM-related policies. The policy groups can be defined this way:

# sbom-quality.yaml
apiVersion: workflowcontract.chainloop.dev/v1
kind: PolicyGroup
metadata:
  name: sbom-quality
  description: This policy group applies a number of SBOM-related policies
  annotations:
    category: SBOM
spec:
  inputs:
    - name: bannedLicenses
      description: comma separated list of licenses to ban
      required: true
    - name: bannedComponents
      description: comma separated list of components to ban
      required: true
  policies:
    materials:
      - name: sbom
        type: SBOM_CYCLONEDX_JSON
        policies:
        - ref: sbom-banned-licenses
          with:
            licenses: {{ inputs.bannedLicenses }}
        - ref: sbom-banned-components
          with:
            components: {{ inputs.bannedComponents }}

Using Policy Groups​

This policy group could be applied to any contract:

schemaVersion: v1
materials: []
policyGroups:
  - ref: file://groups/sbom-quality-group.yaml
    with:
      bannedComponents: log4j@2.14.1
      bannedLicenses: AGPL-1.0-only, AGPL-1.0-or-later, AGPL-3.0-only, AGPL-3.0-or-later

As we introduced earlier, policy groups define both materials and policies applied to them. Once they are included to a contract, they become part of the contract. From this point of view, they can be seen as subcontracts.

Policy group parameters​

In the same way as policies, groups can accept arguments, which are specified in the inputs section. Then those inputs can be passed down to policies using interpolation.

In the example above, bannedComponents input parameter (which is mandatory) is passed to the underlying policy with the expression {{ inputs.banneComponents }}

        - ref: sbom-banned-components
          with:
            components: {{ inputs.bannedComponents }}