Modeling GitHub permissions with OpenFGA

This tutorial explains how to model GitHub’s Organization permission model using OpenFGA. This article from the GitHub docs has links to all other articles we are going to be exploring in this document.

​

Before you start

In order to understand this guide correctly you must be familiar with some OpenFGA concepts and know how to develop the things that we will list below. It would be helpful to have an understanding of some concepts of OpenFGA before you start.

OpenFGA concepts

Modeling concentric relationships

You need to know how to update the authorization model to allow having nested relations such as all writers are readers. Learn more →

Modeling object-to-object relationships

You need to know how to create relationships between objects and how that might affect a user’s relationships to those objects. Learn more →Used here to indicate that users who have repo admin access on an organization, have admin access to all repositories owned by that organization.

Concepts & configuration language

• Some OpenFGA Concepts
• Configuration Language

​

What you will be modeling

GitHub is a system to develop and collaborate on code. In this tutorial, you will build a subset of the GitHub permission model (detailed below) in OpenFGA, using some scenarios to validate the model.

Note: For brevity, this tutorial will not model all of GitHub’s permissions. Instead, it will focus on modeling for the scenarios outlined below

​

Requirements

GitHub’s permission model is represented in their documentation. In this tutorial, you will be focusing on a subset of these permissions. Requirements:

• Users can be admins, maintainers, writers, triagers or readers of repositories (each level inherits all access of the level lower than it. e.g. admins inherit maintainer access and so forth)
• Teams can have members
• Organizations can have members
• Organizations can own repositories
• Users can have repository admin access on organizations, and thus have admin access to all repositories owned by that organization

​

Defined scenarios

There will be the following users:

• Anne
• Beth
• Charles, a member of the contoso/engineering team
• Diane, a member of the contoso/protocols team
• Erik, a member of the contoso org

And these requirements:

• members of the contoso/protocols team are members of the contoso/engineering team
• members of the contoso org are repo_admins on the org
• repo admins on the org are admins on all the repos the org owns

There will be a:

• contoso/tooling repository, owned by the contoso org and of which Beth is a writer and Anne is a reader and members of the contoso/engineering team are admins

​

Modeling GitHub’s permissions

​

01. Permissions For Individuals In An Org

GitHub has 5 different permission levels for repositories: At the end of this section we want to end up with the following permissions represented: To represent permissions in OpenFGA we use relations. For repository permissions we need to create the following authorization model: The OpenFGA service determines if a user has access to an object by checking if the user has a relation to that object. Let us examine one of those relations in detail: If we want to say anne is a reader of repository repo:contoso/tooling we create this relationship tuple: We can now ask OpenFGA “is anne a reader of repository repo:contoso/tooling?” We could also say that beth is a writer of the same repository: And ask some questions to OpenFGA: The first reply makes sense but the second one does not. Intuitively, if beth was writer, she was also be a reader. In fact, GitHub explains this in their documentation To make OpenFGA aware of this “concentric” permission model we need to update our definitions: Let us examine one of those relations in detail: With this simple update our model now supports nested definitions and now:

​

02. Permissions for teams in an org

GitHub also supports creating teams in an organization, adding members to a team and granting teams permissions, rather than individuals. At the end of this section we want to end up with the following permissions represented: To add support for teams and memberships all we need to do is add this object to the OpenFGA authorization model: In addition, the repo’s relations should have team member as a directly related user types. Let us now create a team, add a member to it and make it an admin of repo:contoso/tooling by adding the following relationship tuples: The last relationship tuple introduces a new OpenFGA concept. A userset. When the value of a user is formatted like this type:objectId#relation, OpenFGA will automatically expand the userset into all its individual user identifiers:

​

03. Permissions for child teams in an org

GitHub also supports team nesting, known as “child teams”. Child teams inherit the access permissions of the parent team. Let’s say we have a protocols team that is part of the engineering. The simplest way to achieve the aforementioned requirement is just adding this relationship tuple: which says that members of protocols are members of engineering.

Note: this is enough and valid for our current requirements, and for other read cases allows determining members of the direct team vs sub teams as the latter come from team:contoso/protocols#member. If the #member relation should not be followed for use cases a different approach could be taken.

We can now add a member to the protocols team and check that they are admins of the tooling repository. At the end of this section ended with the following permissions represented:

​

04. Base permissions for org members

In GitHub, “you can set base permissions that apply to all members of an organization when accessing any of the organization’s repositories”. For our purposes this means that if:

• User erik is a member of an organization contoso
• and contoso has a repository tooling
• and contoso has configured base permission to be “write”

then erik has write permissions to tooling. Let us model that! At the end of this section we want to end up with the following permissions represented: We need to introduce the notion of organization as a type, user organization membership and repository ownership as a relation. - It is worth calling that before this addition we were able to represent almost the entire GitHub repo permissions without adding the notion of organization to OpenFGA. Identifiers for users, repositories and teams were all that was necessary. Let us add support for organizations and membership. Hopefully this feels familiar by now: And support for repositories having owners: We can now make Erik a member of contoso and make contoso own contoso/tooling: What we still lack is the ability to create “default permissions” for the organization and have those be considered when determining if a user has a particular relation to a repository. Let’s start with the simplest case admin. We want to say that a user is an admin of a repo if either:

• [done] they have a repo admin relation (directly or through team membership)
• [pending] their organization is configured with repo_admin as the base permission

We need a way to consider the organization members, not just direct relations to the repo when getting a check for: More details on this technique can be found in the section Modeling Parent-Child Objects. We express it like this: What should the users in those relationship tuples with ??? be?

• Well:
  • If the base permission for org contoso is repo_admin then it should be organization:contoso#member.
  • If the base permission for org contoso is NOT repo_admin, then it should be empty (no relationship tuple).
• Whenever the value of this dropdown changes:
  • Delete the previous relationship tuple and create a new one:

The updated authorization model looks like this:

​

Summary

GitHub has a number of other permissions. You have organization billing managers, users that can manage specific apps, etc. We might explore those in the future, but hopefully this blog post has shown you how you could represent those cases using OpenFGA.