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Access Control: Open Policy Agent

Gautier DI FOLCO January 07, 2024 [dev] #access control #security #draft concepts

After introducing DAC, we have proposed capabilities.

Our implementation was really simple and purely in-memory.

Hopefully there are implementations which can hold rules and data such as Open Policy Agent (OPA).

OPA allows to decouple application code and policy expression/enforcement. So, we are able to deploy, review and tests both part independently.

OPA is able to implement many access control schemes.

OPA can be run as a CLI, a server (with a REST API), a library.

To run, OPA needs a policy, data and inputs.

Let's say we want (again) to represent an UNIX-like file access control.

We have our data:

{
    "files": [
        {
            "path": "/home/alice/afile.txt",
            "owner": "alice",
            "group": "users",
            "permissions": {
                "owner": ["read", "write", "execute"],
                "group": ["read"],
                "others": []
            }
        },
        {
            "path": "/home/bob/afile.txt",
            "owner": "bob",
            "group": "users",
            "permissions": {
                "owner": ["read", "write"],
                "group": [],
                "others": []
            }
        }
    ],
    "users": [
        {
            "name": "alice",
            "groups": ["users"]
        },
        {
            "name": "bob",
            "groups": ["users"]
        },
        {
            "name": "charlie",
            "groups": ["root"]
        }
    ]
}

Then our rules:

package rules

import rego.v1

default allow := false

access_request := input.access_request

# Owner
allow if {
	some file in data.files
	file.path == access_request.path
	file.owner == access_request.user
	some allowed_action in file.permissions.owner
	allowed_action == access_request.action
}

# Group
allow if {
	some file in data.files
	file.path == access_request.path
	some user in data.users
	user.name == access_request.user
	some group in user.groups
	file.group == group
	some allowed_action in file.permissions.group
	allowed_action == access_request.action
}

# Others
allow if {
	some file in data.files
	file.path == access_request.path
	some user in data.users
	user.name == access_request.user
	every group in user.groups {
		file.group != group
	}
	some allowed_action in file.permissions.others
	allowed_action == access_request.action
}

# Root
allow if {
	some file in data.files
	file.path == access_request.path
	some user in data.users
	user.name == access_request.user
	some group in user.groups
	group == "root"
}

We can vary our inputs:

{
    "access_request": {
        "path": "/home/bob/afile.txt",
        "user": "charlie",
        "action": "read"
    }
}

Which gives this kind of outputs:

{
    "access_request": {
        "action": "read",
        "path": "/home/bob/afile.txt",
        "user": "charlie"
    },
    "allow": true
}