Update policies with GitHub actions

When a TEE-protected application is part of a continuous delivery (CD) workflow, the policies used to attest to the application's security often must also be a part of the CD workflow. Some of the application's attributes (for example, the mrenclave measurement) change every time an application is built. If updating the policy is not part of the CD workflow, the policy uses the attributes of the previous version to attest to the application's security. Appraisals of the unmodified policy will not match the new evidence. These instructions describe a sample GitHub Actions workflow that will automate the policy-updating process. This can be included in your CD workflow to keep your attestation policies up-to-date.

Sample policy

These instructions assume familiarity with GitHub Actions.

Note

This example assumes that you have an existing application that uses Intel® Software Guard Extensions (Intel® SGX) and that you have the mrenclave value for the enclave measurement available. An unsigned sample policy is provided below. We assume the sample policy is uploaded, and you know the policy ID. This policy will evaluate a number of claims during attestation, including the mrenclave, mrsigner, and other Intel® SGX claims. In this example workflow, we will update the mrenclave value only and leave the other evaluated values unchanged, as the mrenclave value will change every time the enclave code is rebuilt.

 {

 "policy": "default matches_sgx_policy = false \n\n matches_sgx_policy = true { \n input.sgx_is_debuggable == false \n input.sgx_isvsvn == 0 \n input.sgx_isvprodid == 0 \n input.sgx_mrsigner == \"d412a4f07ef83892a5915fb2ab584be31e186e5a4f95ab5f6950fd4eb8694d7b\" \n \n input.sgx_mrenclave == \"bab91f200038076ac25f87de0ca67472443c2ebe17ed9ba95314e609038f51ab\" \n }"
  "user_id": "f04971b7-fb41-4a9e-a06e-4bf6e71f98b3",
  "policy_name": "Sample_Policy_SGX",
  "policy_type": "Appraisal policy",
  "service_offer_name": "SGX Attestation",
  "service_offer_id": "b04971b7-fb41-4a9e-a06e-4bf6e71f98bd"
  }

GitHub actions workflow

The following steps explain how to write a GitHub Actions workflow to update the mrenclave value in an existing policy. This example illustrates retrieving TEE information for an Intel® SGX application running in a Gramine Shielded Container (GSC) with the integrated Intel® Trust Authority client. It uses the Docker integration for Gramine gsc/gsc info-image action to get the latest mrenclave value from a newly-built container image. This new value is used to update an existing appraisal policy so that the Intel® Trust Authority can attest the GSC enclave with the correct, updated value.

While this sample uses a Gramine Shielded container, it still represents a workflow useful for images not using a Gramine Shielded container.

The first step needs to acquire the Intel® Trust Authority tenant management CLI utility. Replace run:wget path/to/executable with the path and file name of the trustauthorityctl tool.
```
name: get Intel Trust Authority tenant CLI
run:  wget path/to/trustauthorityctl
```
This job retrieves the policy to be updated and temporarily writes it to policy.txt. The GitHub Actions variable vars.APPRAISAL_POLICY_ID should be configured with the UUID of the appraisal policy to be updated. The variable secrets.TENANT_ADMIN_API_KEY should contain an Intel® Trust Authority Tenant Admin API key value. Be sure this is configured as a "secret" value, as a Tenant Admin API key has expansive permissions beyond only updating a policy.
```
name: retrieve policy
run:  trustauthorityctl list policy -a ${{ secrets.TENANT_ADMIN_API_KEY }} -p ${{ vars.APPRAISAL_POLICY_ID }} > policy.txt
```
Retrieve the updated mrenclave value from the GSC image.

In the example below, the ${repo} variable represents the path to the image, including the registry URL (for example, container-registry-sample.com/sample-imagename). The ${tag} variable represents the image tag.
```
name: Update mrenclave hash
run:  info=($(gsc/gsc info-image ${repo}:${tag}))
     if [ $? -ne 0 ]; then
       echo "Failed to collect gsc info from ${repo}:${tag}"
       exit 1
     fi

     mrenclave=$(echo ${info[2]} | tr -d '"'| xxd -r -p | base64)
```

Replace the mrenclave in the policy.

name: Replace mrenclave value
run:  sed -ri 's:mrenclave == "[a-zA-Z0-9]+":mrenclave == "'"$mrenclave"'":' policy.txt

Update the policy in Intel® Trust Authority. This will overwrite the existing policy content, including the updated mrenclave value.

name: update the policy in Intel Trust Authority
run:  trustauthorityctl update policy  -a ${{ secrets.TENANT_ADMIN_API_KEY }} -i ${{ vars.APPRAISAL_POLICY_ID }} -f policy.txt