Fixing AWS Packer Build Failures: Encryption & SCP Guide

by Axel Sørensen 57 views

Having trouble with your AWS Packer builds failing due to encryption and SCP (Service Control Policy) issues? You're not alone, guys! This article dives into a common problem where Packer builds for non-Amazon-owned AMIs (like Ubuntu or RHEL) fail because of SCPs that enforce encryption. We'll break down the issue, explore the root cause, and provide a comprehensive solution to get your Packer builds running smoothly again. Let's get started!

Understanding the AWS Packer and SCP Challenge

When diving into infrastructure automation, tools like HashiCorp Packer become indispensable for creating consistent and repeatable machine images. However, integrating Packer with AWS environments that have strict security policies, such as Service Control Policies (SCPs), can introduce challenges. Specifically, SCPs designed to enforce encryption on EBS volumes can interfere with Packer builds, especially when creating AMIs from non-Amazon-owned base images, such as Ubuntu or RHEL. Let's see how this situation typically manifests and understand the core issues.

The Problem: SCPs Blocking Packer Builds

The core problem lies in the interaction between SCPs that deny the creation of unencrypted EBS volumes and Packer's process of launching temporary instances to build AMIs. When creating custom images, Packer often needs to launch an EC2 instance, attach volumes, perform configurations, and then create an AMI from this instance. If an SCP is in place that prevents the creation or attachment of unencrypted volumes, the Packer build process can fail, even if encryption is enabled in the Packer configuration. The error messages typically indicate an UnauthorizedOperation due to an explicit deny in a service control policy, specifically related to ec2:RunInstances and resources of type arn:aws:ec2:*:*:volume/*.

This issue is frequently encountered in environments where security is a top priority, and organizations implement SCPs to ensure all EBS volumes are encrypted at rest. While Amazon Linux AMIs might work without issues due to default encryption settings, other AMIs like Ubuntu or RHEL require explicit encryption configurations to comply with these SCPs. Therefore, even if you've set encrypted = true in your Packer build configuration, the build may still fail if the underlying mechanisms for ensuring encryption aren't correctly configured or understood by AWS during the instance launch phase.

Real-World Scenario: Ubuntu 24.04 AMI Build Failure

Consider a scenario where you're trying to create an Ubuntu 24.04 AMI using Packer in an AWS environment governed by strict SCPs. Your Packer configuration includes settings to encrypt the EBS volume, but the build process fails with an error message indicating that your IAM role is not authorized to perform the ec2:RunInstances operation on a volume because of an SCP. The error message looks something like this:

Error launching source instance: UnauthorizedOperation: You are not authorized to perform this operation. User: arn:aws:sts::9876543210:assumed-role/packer-iac-execution-role/packer-build-iac-role-assume is not authorized to perform: ec2:RunInstances on resource: arn:aws:ec2:us-east-1:9876543210:volume/* with an explicit deny in a service control policy.

This error occurs even though you've set encrypted = true and specified a KMS key ID in your Packer configuration. The reason is that the SCP is evaluating the request at a lower level, specifically when the instance is being launched and the volume is being attached. If the volume isn't explicitly encrypted at this stage, the SCP will deny the operation.

To further illustrate, let's examine the relevant parts of the Packer configuration and the SCP that cause this issue. The Packer configuration might look like this:

source "amazon-ebs" "packer-ubuntu-2404" {
  region     = var.region
  subnet_id  = var.subnet_id
  instance_type = "t3.small"
  ssh_username  = "ubuntu"
  ami_name        = "packer-ubuntu-2404-{{timestamp}}"

  source_ami_filter {
    filters = {
      name                = "ubuntu/images/hvm-ssd-gp3/ubuntu-noble-24.04-amd64-server-20250731"
      virtualization-type = "hvm"
      root-device-type    = "ebs"
    }
    owners      = ["099720109477"] # Canonical
    most_recent = true
  }

  launch_block_device_mappings {
    device_name           = "/dev/xvda"
    volume_size           = 20
    volume_type           = "gp3"
    delete_on_termination = true
    encrypted             = true
    kms_key_id            = "alias/aws/ebs"
  }
}

The SCP that causes the denial might look like this:

{
  "Sid": "PreventUnencryptedEBSVolumeAttachment",
  "Effect": "Deny",
  "Action": ["ec2:RunInstances"],
  "Resource": "arn:aws:ec2:*:*:volume/*",
  "Condition": {
    "Bool": {
      "ec2:Encrypted": "false"
    }
  }
}

This SCP explicitly denies the ec2:RunInstances action if the EBS volume being attached is not encrypted. Even with encrypted = true in the Packer configuration, the SCP evaluates the encryption status during the instance launch, leading to the failure.

Why Amazon Linux Works (Sometimes)

So, why do Amazon Linux 2 or Amazon Linux 2023 builds sometimes work without any issues when the same SCP is enabled? The key difference often lies in the default encryption settings and the way these AMIs are configured. Amazon Linux AMIs frequently have default encryption enabled at the account level, or the base AMIs themselves might be pre-configured to use encryption. This means that when Packer launches an instance from an Amazon Linux base AMI, the EBS volume is encrypted by default, satisfying the SCP requirements.

However, Ubuntu and RHEL AMIs do not always have the same default settings. They often require explicit configuration to enable encryption, which is where the Packer configuration comes into play. If the encryption settings are not correctly propagated or understood during the instance launch phase, the SCP will still block the operation. This inconsistency highlights the need for a robust solution that ensures encryption is correctly handled for all AMI types.

Decoding the Error Messages: What's Really Going On?

Error messages are your best friends when troubleshooting, guys! They might seem cryptic at first, but they hold the key to understanding what's going wrong. In the context of Packer builds failing due to SCPs, the error messages often point directly to authorization issues related to EBS volume encryption. Let's dissect these messages and understand what they're trying to tell us.

Common Error Message Components

When a Packer build fails due to an SCP denying the creation or attachment of an unencrypted EBS volume, the error message typically includes the following components:

  1. UnauthorizedOperation: This is the primary indicator that the operation was denied due to insufficient permissions. It means that the IAM role or user Packer is using doesn't have the necessary authorization to perform the requested action.
  2. Specific Action Denied: The message will specify the exact AWS action that was denied, such as ec2:RunInstances or ec2:CreateVolume. This helps you narrow down the scope of the issue.
  3. Resource Affected: The error will identify the AWS resource that the action was performed on, often in the format of an ARN (Amazon Resource Name). In this case, it's usually arn:aws:ec2:*:*:volume/*, indicating that the issue is related to EBS volumes.
  4. Explicit Deny in a Service Control Policy: This is the crucial part. It confirms that the denial is not due to a missing IAM permission but rather because an SCP explicitly prohibits the action. SCPs act as guardrails at the AWS Organizations level, overriding IAM policies.
  5. Encoded Authorization Failure Message: AWS sometimes includes an encoded message that provides more detailed information about the authorization failure. While this message can be difficult to decipher directly, it can be valuable for AWS support or advanced troubleshooting.
  6. Status Code and Request ID: These provide additional context for debugging and can be helpful when contacting AWS support. The status code is typically 403, indicating a forbidden request.

Example Error Message Breakdown

Let's look at an example error message and break it down:

Error launching source instance: UnauthorizedOperation: You are not authorized to perform this operation. User: arn:aws:sts::9876543210:assumed-role/packer-iac-execution-role/packer-build-iac-role-assume is not authorized to perform: ec2:RunInstances on resource: arn:aws:ec2:us-east-1:9876543210:volume/* with an explicit deny in a service control policy. Encoded authorization failure message: [encoded scp message]
	status code: 403, request id: 2eb0941f-9498-4d81-bec4-45a6f31c16e0

Here's what each part tells us:

  • Error launching source instance: This indicates the error occurred while Packer was trying to launch the EC2 instance that it uses to build the AMI.
  • UnauthorizedOperation: The operation was denied due to insufficient permissions.
  • User: arn:aws:sts::9876543210:assumed-role/packer-iac-execution-role/packer-build-iac-role-assume: This specifies the IAM role that Packer was using when the error occurred. It's the role that needs to be checked for permissions and SCP restrictions.
  • ec2:RunInstances: The specific action that was denied is launching an EC2 instance. This is a critical action for Packer, as it needs to launch an instance to build the AMI.
  • resource: arn:aws:ec2:us-east-1:9876543210:volume/*: The denial is related to an EBS volume in the us-east-1 region. The volume/* part indicates that the SCP is likely affecting all volumes in the account.
  • with an explicit deny in a service control policy: This confirms that the issue is not an IAM permission problem but an SCP restriction. An SCP is actively preventing the action.
  • Encoded authorization failure message: [encoded scp message]: This contains a more detailed, but encoded, message about the SCP denial. It's often used for advanced debugging.
  • status code: 403: The HTTP status code 403 means