# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   cloudfront_origin_access_control_enabled_check
# 
# Description:
#   This control checks whether your Amazon CloudFront distributions backed by Amazon S3 are configured to use an origin access control.
# 
# Reports on:
#   AWS::CloudFront::Distribution
# 
# Evaluates:
#   AWS CloudFormation, AWS CloudFormation hook
# 
# Rule Parameters:
#   None
# 
# Scenarios:
#   Scenario: 1
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document does not contain any CloudFront distribution resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: No S3 backed 'Origins' are provided on the CloudFront distribution resource or 'Origins' is not present on
#            the CloudFront distribution resource or is present and an empty list
#      Then: SKIP
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: 'S3Origin' is present on the CloudFront distribution resource
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: One or more S3 backed 'Origins' are configured on the CloudFront distribution resource
#       And: 'OriginAccessControlId' is not present for the 'Origin' or is an empty string or invalid local reference
#      Then: FAIL
#   Scenario: 5
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains a CloudFront distribution resource
#       And: One or more S3 backed 'Origins' are provided on the CloudFront distribution resource
#       And: 'OriginAccessControlId' is present for each S3 backed 'Origin' and is a non-empty string or valid local
#            reference
#      Then: PASS

#
# Constants
#
let CLOUDFRONT_DISTRIBUTION_TYPE = "AWS::CloudFront::Distribution"
let CLOUDFRONT_ORIGIN_ACCESS_CONTROL_TYPE = "AWS::CloudFront::OriginAccessControl"
let S3_BUCKET_DNS_NAME_PATTERN = /(.*)\.s3(-external-\d|[-\.][a-z]*-[a-z]*-[0-9])?\.amazonaws\.com(\.cn)?$/
let INPUT_DOCUMENT = this

#
# Assignments
#
let cloudfront_distributions = Resources.*[ Type == %CLOUDFRONT_DISTRIBUTION_TYPE ]

#
# Primary Rules
#
rule cloudfront_origin_access_control_enabled_check when is_cfn_template(%INPUT_DOCUMENT)
                                                          %cloudfront_distributions not empty {
    check(%cloudfront_distributions.Properties)
        <<
        [CT.CLOUDFRONT.PR.10]: Require any Amazon CloudFront distributions with Amazon S3 backed origins to have origin access control configured
        [FIX]: The 'Origins' property configures origins backed by Amazon S3. For each origin backed by Amazon S3, configure an origin access control identifier using the 'OriginAccessControlId' property.
        >>
}

rule cloudfront_origin_access_control_enabled_check when is_cfn_hook(%INPUT_DOCUMENT, %CLOUDFRONT_DISTRIBUTION_TYPE) {
    check(%INPUT_DOCUMENT.%CLOUDFRONT_DISTRIBUTION_TYPE.resourceProperties)
        <<
        [CT.CLOUDFRONT.PR.10]: Require any Amazon CloudFront distributions with Amazon S3 backed origins to have origin access control configured
        [FIX]: The 'Origins' property configures origins backed by Amazon S3. For each origin backed by Amazon S3, configure an origin access control identifier using the 'OriginAccessControlId' property.
        >>
}

#
# Parameterized Rules
#
rule check(cloudfront_distribution) {
    %cloudfront_distribution[
        filter_cloudfront_distribution_with_legacy_s3_origins(this)
    ] {
        DistributionConfig {
            # Scenario 3
            S3Origin not exists
        }
    }

    %cloudfront_distribution[
        # Scenario 2
        filter_cloudfront_distribution_with_origins(this)
    ] {
        DistributionConfig {
            Origins [
                # Scenario 4
                DomainName == %S3_BUCKET_DNS_NAME_PATTERN or
                check_origin_domain_name_get_att(DomainName)
            ] {
                # Scenario 3 and 5
                OriginAccessControlId exists
                check_is_string_and_not_empty(OriginAccessControlId) or
                check_local_references(%INPUT_DOCUMENT, OriginAccessControlId, %CLOUDFRONT_ORIGIN_ACCESS_CONTROL_TYPE)
            }
        }
    }
}

rule filter_cloudfront_distribution_with_legacy_s3_origins(cloudfront_distribution) {
    %cloudfront_distribution {
        DistributionConfig exists
        DistributionConfig is_struct

        DistributionConfig {
            S3Origin exists
        }
    }
}

rule filter_cloudfront_distribution_with_origins(cloudfront_distribution) {
    %cloudfront_distribution {
        DistributionConfig exists
        DistributionConfig is_struct

        DistributionConfig {
            Origins exists
            Origins is_list
            Origins not empty
        }
    }
}

rule check_origin_domain_name_get_att(domain) {
  %domain {
    'Fn::GetAtt' {
        this is_list
        this not empty
        this[1] == "DomainName" or
        this[1] == "RegionalDomainName"
    }
    check_local_references(%INPUT_DOCUMENT, this, "AWS::S3::Bucket")
  }
}

#
# Utility Rules
#
rule check_is_string_and_not_empty(value) {
    %value {
        this is_string
        this != /\A\s*\z/
    }
}

rule is_cfn_template(doc) {
    %doc {
        AWSTemplateFormatVersion exists  or
        Resources exists
    }
}

rule is_cfn_hook(doc, RESOURCE_TYPE) {
    %doc.%RESOURCE_TYPE.resourceProperties exists
}

rule check_local_references(doc, reference_properties, referenced_resource_type) {
    %reference_properties {
        'Fn::GetAtt' {
            query_for_resource(%doc, this[0], %referenced_resource_type)
                <<Local Stack reference was invalid>>
        } or Ref {
            query_for_resource(%doc, this, %referenced_resource_type)
                <<Local Stack reference was invalid>>
        }
    }
}

rule query_for_resource(doc, resource_key, referenced_resource_type) {
    let referenced_resource = %doc.Resources[ keys == %resource_key ]
    %referenced_resource not empty
    %referenced_resource {
        Type == %referenced_resource_type
    }
}