Configure permissions of GITHUB_TOKEN in workflows

[per1234]

GITHUB_TOKEN is an access token that is automatically generated and made accessible for use in GitHub Actions workflow runs. The global default permissions of this token for workflow runs in a trusted context (i.e., not triggered by a pull_request event from a fork) are set in the GitHub enterprise/organization/repository's administrative settings, giving it either read-only or write permissions in all scopes.

In the case of a read-only default configuration, any workflow operations that require write permissions would fail with an error like:

403: Resource not accessible by integration

In the case of a write default configuration, workflows have unnecessary permissions, which violates the security principle of least privilege.

For this reason, GitHub Actions now allows fine grained control at a per-workflow or per-workflow job scope of the permissions provided to the token. This is done using the permissions workflow key, which is used here to configure the workflows for only the permissions required by each individual job:

https://docs.github.com/en/actions/using-jobs/assigning-permissions-to-jobs

Configuration Granularity

I chose to always configure permissions at the job level even though in some cases the same permissions configuration could be used for all jobs in a workflow. Even if functionally equivalent, I think it is semantically more appropriate to always set the permissions at the job scope since the intention is to make the most granular possible permissions configuration. Hopefully this approach will increase the likelihood that appropriate permissions configurations will be made in any additional jobs that are added to the workflows in the future.

Security Implications

The automatic permissions downgrade from write to read for workflow runs in an untrusted context (e.g., triggered by a pull_request event from a fork) is unaffected by this change.

API Request Implications

Even when all permissions are withheld (permissions: {}), the token still provides the authenticated API request rate limiting allowance (authenticating API requests to avoid rate limiting is a one of the uses of the token in these workflows).

Excess Permissions

Read permissions are required in the contents scope in order to checkout private repositories. Even though those permissions are not required when the workflows are installed in public repositories, the templates are intended to be applicable in public and private repositories both and so a small excess in permissions was chosen instead of the alternative of having to maintain separate variants of each workflow for use in public or private repos.

Testing

I have applied the changes to downstream repositories:

• Configure permissions of GITHUB_TOKEN in workflows ArduinoCore-renesas#4
• Configure permissions of GITHUB_TOKEN in workflows arduino-lint#566

And equivalent changes some time ago to another collection of reusable workflows: per1234/.github@1105264