Citadel Cookbook

Using a combination of IAM roles, S3 buckets, and EC2 it is possible to use AWS as a trusted-third-party for distributing secret or otherwise sensitive data.

Overview

IAM roles allow specifying snippets of IAM policies in a way that can be used from an EC2 virtual machine. Combined with a private S3 bucket, this can be used to authorize specific hosts to specific files.

IAM Roles can be created in the AWS Console. While the policies applied to a role can be changed later, the name cannot so be careful when choosing them.

Requirements

This cookbook requires Chef 12 or newer. It also requires the EC2 ohai plugin to be active. If you are using a VPC, this may require setting the hint file depending on your version of Ohai/Chef:

$ mkdir -p /etc/chef/ohai/hints
$ touch /etc/chef/ohai/hints/ec2.json

If you use knife-ec2 to start the instance, the hint file is already set for you.

IAM Policy

By default, your role will not be able to access any files in your private S3 bucket. You can create IAM policies that whitelist specific keys for each role:

{
  "Version": "2008-10-17",
  "Id": "<policy name>",
  "Statement": [
    {
      "Sid": "<statement name>",
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::<AWS account number>:role/<role name>"
      },
      "Action": "s3:GetObject",
      "Resource": "arn:aws:s3:::<bucket name>/<key pattern>"
    }
  ]
}

The key pattern can include * and ? metacharacters, so for example arn:aws:s3:::myapp.citadel/deploy_keys/* to allow access to all files in the deploy_keys folder.

This policy can be attached to either the IAM role or the S3 bucket with equal effect.

Limitations

Each EC2 VM can only be assigned a single IAM role. This can complicate situations where some secrets need to be shared by overlapping subsets of your servers. A possible improvement to this would be to make a script to create all needed composite IAM roles, possibly driven by Chef roles or other metadata.

Attributes

node['citadel']['bucket'] – The default S3 bucket to use.

Recipe Usage

You can access secret data via the citadel method.

file '/etc/secret' do
  owner 'root'
  group 'root'
  mode '600'
  content citadel['keys/secret.pem']
end

By default the node attribute node['citadel']['bucket'] is used to find the S3 bucket to query, however you can override this:

template '/etc/secret' do
  owner 'root'
  group 'root'
  mode '600'
  variables secret: citadel('mybucket')['id_rsa']
end

Developing with Vagrant

While developing in a local VM, you can use the node attributes node['citadel']['access_key_id'] and node['citadel']['secret_access_key'] to provide credentials. The recommended way to do this is via environment variables so that the Vagrantfile itself can still be kept in source control without leaking credentials:

config.vm.provision :chef_solo do |chef|
  chef.json = {
    citadel: {
      access_key_id: ENV['ACCESS_KEY_ID'],
      secret_access_key: ENV['SECRET_ACCESS_KEY'],
    },
  }
end

WARNING: Use of these attributes in production should be considered a likely security risk as they will end up visible in the node data, or in the role/environment/cookbook that sets them. This can be mitigated using Enterprise Chef ACLs, however such configurations are generally error-prone due to the defaults being wide open.

Testing with Test-Kitchen

Similarly you can use the same attributes with Test-Kitchen

provisioner:
  name: chef_solo
  attributes:
    citadel:
      access_key_id: <%= ENV['AWS_ACCESS_KEY_ID'] %>
      secret_access_key: <%= ENV['AWS_SECRET_ACCESS_KEY'] %>

Recommended S3 Layout

Within your S3 bucket I recommend you create one folder for each group of secrets, and in your IAM policies have one statement per group. Each group of secrets is a set of data with identical security requirements. Many groups will start out only containing a single file, however having the flexibility to change this in the future allows for things like key rotation without rewriting all of your IAM policies.

An example of an IAM policy resource would be:

"Resource": "arn:aws:s3:::mybucket/myfolder/*"

Creating and Updating Secrets

You can use any S3 client you prefer to manage your secrets, however make sure that new files are set to private (accessible only to the creating user) by default.

License

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.