AWS CloudFormation Guard as a Lambda

The Lambda version of the tool is a lightweight wrapper around the core cfn-guard code that can simply be invoked as a Lambda. We currently support 2 methods for deploying the Lambda.

Table of Contents

• Method 1: Installation using AWS CLI
• Method 2: Installation using SAM CLI
• Calling the AWS Lambda Function
• FAQs

Method 1: Installation using AWS CLI

Dependencies

• AWS CLI configured with permissions to deploy and invoke Lambdas
• Rust (See the installation instructions in the top-level README)
• If building on a Mac, you'll need Homebrew.
• If building on Ubuntu, you'll need to run sudo apt-get update; sudo apt install build-essential if you haven't already
• If building on CentOS/RHEL you'll need to add the musl-libc package repository to your yum config (see https://copr.fedorainfracloud.org/coprs/ngompa/musl-libc/)

Mac/Ubuntu

1. Install and configure the dependencies.
2. Run rustup target add x86_64-unknown-linux-musl.
3. If you're on a Mac, add the following to ~/.cargo/config:

   [target.x86_64-unknown-linux-musl]
   linker = "x86_64-linux-musl-gcc"
   

4. Ensure you're in the guard-lambda directory.
5. Run cargo build --release --target x86_64-unknown-linux-musl. For a custom runtime, AWS Lambda looks for an executable called bootstrap in the deployment package zip. Rename the generated cfn-lambda executable to bootstrap and add it to a zip archive.
6. Run cp ./../target/x86_64-unknown-linux-musl/release/cfn-guard-lambda ./bootstrap && zip lambda.zip bootstrap && rm bootstrap.
7. Initialize the following shell variables with values corresponding to your account:

   LAMBDA_FUNCTION_NAME=CloudFormationGuardLambda
   AWS_ACCOUNT_ID=111111111111
   REGION=us-east-1
   ROLE_NAME="${LAMBDA_FUNCTION_NAME}Role"
   

8. Create an execution role for Lambda function. Refer the linked documentation for updated instructions. Alternatively, use the following command:

   aws iam create-role \
      --role-name $ROLE_NAME \
      --assume-role-policy-document '{"Version": "2012-10-17","Statement": [{ "Effect": "Allow", "Principal": {"Service": "lambda.amazonaws.com"}, "Action": "sts:AssumeRole"}]}'
   
   aws iam attach-role-policy \
      --role-name $ROLE_NAME \
      --policy-arn arn:aws:iam::aws:policy/service-role/AWSLambdaBasicExecutionRole
   

9. Run the following command to submit cfn-guard as an AWS Lambda function to your account:

    aws lambda create-function \
    --function-name $LAMBDA_FUNCTION_NAME \
    --handler guard.handler \
    --zip-file fileb://./lambda.zip \
    --runtime provided.al2023 \
    --role "arn:aws:iam::${AWS_ACCOUNT_ID}:role/${ROLE_NAME}" \
    --environment Variables={RUST_BACKTRACE=1} \
    --tracing-config Mode=Active \
    --region $REGION
   

Method 2: Installation using SAM CLI

Dependencies

• SAM CLI installed

• AWS CLI installed and configured with permissions to deploy via CloudFormation. SAM CLI will internally use the credentials you setup AWS CLI with. You may use the following IAM policy as a reference for least privileged access.

  IAM Policy for SAM CLI User

  {
      "Version": "2012-10-17",
      "Statement":
      [
          {
              "Effect": "Allow",
              "Action":
              [
                  "cloudformation:CreateChangeSet",
                  "cloudformation:CreateStack",
                  "cloudformation:DeleteChangeSet",
                  "cloudformation:DeleteStack",
                  "cloudformation:DescribeChangeSet",
                  "cloudformation:DescribeStackEvents",
                  "cloudformation:DescribeStackResource",
                  "cloudformation:DescribeStackResources",
                  "cloudformation:DescribeStacks",
                  "cloudformation:ExecuteChangeSet",
                  "cloudformation:GetTemplate",
                  "cloudformation:GetTemplateSummary",
                  "cloudformation:ListStackResources",
                  "cloudformation:SetStackPolicy",
                  "cloudformation:UpdateStack",
                  "cloudformation:UpdateTerminationProtection",
                  "iam:AttachRolePolicy",
                  "iam:CreateRole",
                  "iam:DeleteRole",
                  "iam:DetachRolePolicy",
                  "iam:GetRole",
                  "iam:PassRole",
                  "lambda:CreateFunction",
                  "lambda:DeleteFunction",
                  "lambda:GetFunction",
                  "lambda:TagResource",
                  "s3:GetObject",
                  "s3:PutObject"
              ],
              "Resource": "*"
          }
      ]
  }
  

• Docker installed

Building and deploying

1. Make sure docker is running
2. Navigate to guard-lambda directory and run sam build --use-container to build the code for the Lambda function
3. Run sam deploy --guided and complete the interactive workflow. This workflow will create a CloudFormation changeset and deploy it
4. Once it succeeds, the name of the function will be shown in the CloudFormationGuardLambdaFunctionName output
5. For subsequent updates, build the code again (step 2) and run sam deploy (without --guided)

Calling the AWS Lambda Function

Payload Structure

The payload JSON to cfn-guard-lambda requires the following two fields:

• data - (Mandatory, string) Infrastructure as code template data in YAML or JSON structure.
• rules - (Mandatory, list of strings) List of rules that you want to run your YAML or JSON structured data against.
• verbose - (Optional, boolean) A flag when set to false makes Lambda emit a shorter version of the output. This is set to true by default for backward compatibility.

Invoking cfn-guard-lambda

Initialize the variable LAMBDA_FUNCTION_NAME to the name of the deployed AWS Lambda Function, and invoke it using the following syntax:

aws lambda invoke \
--function-name $LAMBDA_FUNCTION_NAME \
--cli-binary-format raw-in-base64-out \
--payload "{"data": "<input data>", "rules" : ["<input rules 1>", "<input rules 2>", ...], "verbose": <true|false>}" \
output.json

Note: --cli-binary-format option is only required to override the default configuration setting to perform the parsing of JSON input. If the command doesn't work with this option, try running it without this configuration override. Your current AWS CLI version may have this configuration set to the required value.

Example

aws lambda invoke \
--function-name $LAMBDA_FUNCTION_NAME \
--cli-binary-format raw-in-base64-out \
--payload '{"data":"{\"Resources\":{\"NewVolume\":{\"Type\":\"AWS::EC2::Volume\",\"Properties\":{\"Size\":500,\"Encrypted\":true,\"AvailabilityZone\":\"us-west-2b\"}},\"NewVolume2\":{\"Type\":\"AWS::EC2::Volume\",\"Properties\":{\"Size\":50,\"Encrypted\":true,\"AvailabilityZone\":\"us-west-2c\"}}}}","rules":["let ec2_volumes = Resources.*[ Type == /EC2::Volume/ ]\nrule EC2_ENCRYPTION_BY_DEFAULT when %ec2_volumes !empty {\n    %ec2_volumes.Properties.Encrypted == true \n      <<\n            Violation: All EBS Volumes should be encrypted \n            Fix: Set Encrypted property to true\n       >>\n}"],"verbose":false}' \
output.json

Note: --cli-binary-format option is only required to override the default configuration setting to perform the parsing of JSON input. If the command doesn't work with this option, try running it without this configuration override. Your current AWS CLI version may have this configuration set to the required value.

FAQs

Q: How do I troubleshoot a lambda call returning an opaque error message like:

{"errorType": "Runtime.ExitError", "errorMessage": "RequestId: 1c0c0620-0f83-40bc-8eca-3cf2cf24820f Error: Runtime exited with error: exit status 101"}

A: Run the same rule set and template locally with cfn-guard to get a better message, such as:

Parsing error handling template file, Error = while parsing a flow mapping, did not find expected ',' or '}' at line 21 column 1

cfn-guard-lambda is just a wrapper for the cfn-guard code and each can be used to test the other.