Assigning and referencing variables in Guard rules

You can assign variables in your AWS CloudFormation Guard rules files to store information that you want to reference in your Guard rules. Guard supports one-shot variable assignment. Variables are evaluated lazily, meaning that Guard only evaluates variables when rules are run.

Assigning variables

Use the let keyword to initialize and assign a variable. As a best practice, use snake case for variable names. Variables can store static literals or dynamic properties resulting from queries. In the following example, the variable ecs_task_definition_task_role_arn stores the static string value arn:aws:iam:123456789012:role/my-role-name.

let ecs_task_definition_task_role_arn = 'arn:aws:iam::123456789012:role/my-role-name'

In the following example, the variable ecs_tasks stores the results of a query that searches for all AWS::ECS::TaskDefinition resources in an AWS CloudFormation template. You could reference ecs_tasks to access information about those resources when you write rules.

let ecs_tasks = Resources.*[
    Type == 'AWS::ECS::TaskDefinition'
]

Referencing variables

Use the % prefix to reference a variable.

Based on the ecs_task_definition_task_role_arn variable example in Assigning variables, you can reference ecs_task_definition_task_role_arn in the query|value literal section of a Guard rule clause. Using that reference ensures that the value specified for the TaskDefinitionArn property of any AWS::ECS::TaskDefinition resources in a CloudFormation template is the static string value arn:aws:iam:123456789012:role/my-role-name.

Resources.*.Properties.TaskDefinitionArn == %ecs_task_definition_role_arn

Based on the ecs_tasks variable example in Assigning variables, you can reference ecs_tasks in a query (for example, %ecs_tasks.Properties). First, Guard evaluates the variable ecs_tasks and then uses the returned values to traverse the hierarchy. If the variable ecs_tasks resolves to non-string values, then Guard throws an error.

Variable scope

Scope refers to the visibility of variables defined in a rules file. A variable name can only be used once within a scope. There are three levels where a variable can be declared, or three possible variable scopes:

Examples of variables in Guard rules files

The following sections provide examples of both static and dynamic assignment of variables.

Static assignment

The following is an example CloudFormation template.

Resources:
  EcsTask:
    Type: 'AWS::ECS::TaskDefinition'
    Properties:
      TaskRoleArn: 'arn:aws:iam::123456789012:role/my-role-name'

Based on this template, you can write a rule called check_ecs_task_definition_task_role_arn that ensures that the TaskRoleArn property of all AWS::ECS::TaskDefinition template resources is arn:aws:iam::123456789012:role/my-role-name.

rule check_ecs_task_definition_task_role_arn
{
    let ecs_task_definition_task_role_arn = 'arn:aws:iam::123456789012:role/my-role-name'
    Resources.*.Properties.TaskRoleArn == %ecs_task_definition_task_role_arn
}

Within the scope of the rule, you can initialize a variable called ecs_task_definition_task_role_arn and assign to it the static string value 'arn:aws:iam::123456789012:role/my-role-name'. The rule clause checks whether the value specified for the TaskRoleArn property of the EcsTask resource is arn:aws:iam::123456789012:role/my-role-name by referencing the ecs_task_definition_task_role_arn variable in the query|value literal section.

Dynamic assignment

The following is an example CloudFormation template.

Resources:
  EcsTask:
    Type: 'AWS::ECS::TaskDefinition'
    Properties:
      TaskRoleArn: 'arn:aws:iam::123456789012:role/my-role-name'

Based on this template, you can initialize a variable called ecs_tasks within the scope of the file and assign to it the query Resources.*[ Type == 'AWS::ECS::TaskDefinition'. Guard queries all resources in the input template and stores information about them in ecs_tasks. You can also write a rule called check_ecs_task_definition_task_role_arn that ensures that the TaskRoleArn property of all AWS::ECS::TaskDefinition template resources is arn:aws:iam::123456789012:role/my-role-name

let ecs_tasks = Resources.*[
    Type == 'AWS::ECS::TaskDefinition'
]

rule check_ecs_task_definition_task_role_arn
{
    %ecs_tasks.Properties.TaskRoleArn == 'arn:aws:iam::123456789012:role/my-role-name'
}

The rule clause checks whether the value specified for the TaskRoleArn property of the EcsTask resource is arn:aws:iam::123456789012:role/my-role-name by referencing the ecs_task_definition_task_role_arn variable in the query section.

Enforcing AWS CloudFormation template configuration

Let’s walk through a more complex example of a production use case. In this example, we write Guard rules to ensure stricter controls on how Amazon ECS tasks are defined.

The following is an example CloudFormation template.

Resources:
  EcsTask:
    Type: 'AWS::ECS::TaskDefinition'
    Properties:
      TaskRoleArn: 
        'Fn::GetAtt': [TaskIamRole, Arn]
      ExecutionRoleArn:
        'Fn::GetAtt': [ExecutionIamRole, Arn]

  TaskIamRole:
    Type: 'AWS::IAM::Role'
    Properties:
      PermissionsBoundary: 'arn:aws:iam::123456789012:policy/MyExamplePolicy'

  ExecutionIamRole:
    Type: 'AWS::IAM::Role'
    Properties:
      PermissionsBoundary: 'arn:aws:iam::123456789012:policy/MyExamplePolicy'

Based on this template, we write the following rules to ensure that these requirements are met:

# Select all Amazon ECS task definition resources from the template
let ecs_tasks = Resources.*[
    Type == 'AWS::ECS::TaskDefinition'
]

# Select a subset of task definitions whose specified value for the TaskRoleArn property is an Fn::Gett-retrievable attribute
let task_role_refs = some %ecs_tasks.Properties.TaskRoleArn.'Fn::GetAtt'[0]

# Select a subset of TaskDefinitions whose specified value for the ExecutionRoleArn property is an Fn::Gett-retrievable attribute
let execution_role_refs = some %ecs_tasks.Properties.ExecutionRoleArn.'Fn::GetAtt'[0]

# Verify requirement #1
rule all_ecs_tasks_must_have_task_end_execution_roles 
    when %ecs_tasks !empty 
{
    %ecs_tasks.Properties {
        TaskRoleArn exists
        ExecutionRoleArn exists
    }
}

# Verify requirements #2 and #3
rule all_roles_are_local_and_type_IAM
    when all_ecs_tasks_must_have_task_end_execution_roles
{
    let task_iam_references = Resources.%task_role_refs
    let execution_iam_reference = Resources.%execution_role_refs

    when %task_iam_references !empty {
        %task_iam_references.Type == 'AWS::IAM::Role'
    }

    when %execution_iam_reference !empty {
        %execution_iam_reference.Type == 'AWS::IAM::Role'
    }
}

# Verify requirement #4
rule check_role_have_permissions_boundary
    when all_ecs_tasks_must_have_task_end_execution_roles
{
    let task_iam_references = Resources.%task_role_refs
    let execution_iam_reference = Resources.%execution_role_refs

    when %task_iam_references !empty {
        %task_iam_references.Properties.PermissionsBoundary exists
    }

    when %execution_iam_reference !empty {
        %execution_iam_reference.Properties.PermissionsBoundary exists
    }
}