Create a Linux AMI with custom UEFI Secure Boot keys

This procedure shows you how to create a Linux AMI with UEFI Secure Boot and custom-made private keys. Amazon Linux supports UEFI Secure Boot starting with AL2023 release 2023.1. For more information, see UEFI Secure Boot in the AL2023 User Guide.

Newly created instances without UEFI Secure Boot keys are created in SetupMode, which allows you to enroll your own keys. Some AMIs come preconfigured with UEFI Secure Boot and you cannot change the existing keys. If you want to change the keys, you must create a new AMI based on the original AMI.

You have two ways to propagate the keys in the variable store, which are described in Option A and Option B that follow. Option A describes how to do this from within the instance, mimicking the flow of real hardware. Option B describes how to create a binary blob, which is then passed as a base64-encoded file when you create the AMI. For both options, you must first create the three key pairs, which are used for the chain of trust.

Step 1

UEFI Secure Boot is based on the following three key databases, which are used in a chain of trust: the platform key (PK), the key exchange key (KEK), and the signature database (db).¹

You create each key on the instance. To prepare the public keys in a format that is valid for the UEFI Secure Boot standard, you create a certificate for each key. DER defines the SSL format (binary encoding of a format). You then convert each certificate into a UEFI signature list, which is the binary format that is understood by UEFI Secure Boot. And finally, you sign each certificate with the relevant key.

Prepare to create the key pairs

Before creating the key pairs, create a globally unique identifier (GUID) to be used in key generation.

Key pair 1: Create the platform key (PK)

The PK is the root of trust for UEFI Secure Boot instances. The private PK is used to update the KEK, which in turn can be used to add authorized keys to the signature database (db).

The X.509 standard is used for creating the key pair. For information about the standard, see X.509 on Wikipedia.

Key pair 2: Create the key exchange key (KEK)

The private KEK is used to add keys to the db, which is the list of authorized signatures to boot on the system.

Key pair 3: Create the signature database (db)

The db list contains authorized keys that are authorized to be booted on the system. To modify the list, the private KEK is necessary. Boot images will be signed with the private key that is created in this step.

Sign the boot image (kernel) with the private key

For Ubuntu 22.04, the following images require signatures.

/boot/efi/EFI/ubuntu/shimx64.efi
/boot/efi/EFI/ubuntu/mmx64.efi
/boot/efi/EFI/ubuntu/grubx64.efi
/boot/vmlinuz

To sign an image

Use the following syntax to sign an image.

sbsign --key db.key --cert db.crt --output /boot/vmlinuz /boot/vmlinuz

Refer to the documentation for your distribution to find out about your boot chain and required images.

¹ Thanks to the ArchWiki community for all of the work they have done. The commands for creating the PK, creating the KEK, creating the DB, and signing the image are from Creating keys, authored by the ArchWiki Maintenance Team and/or the ArchWiki contributors.

Step 2 (Option A): Add keys to the variable store from within the instance

After you have created the three key pairs, you can connect to your instance and add the keys to the variable store from within the instance by completing the following steps. Alternatively, complete the steps for Step 2 (Option B): Create a binary blob containing a pre-filled variable store.

Step 1: Launch an instance that will support UEFI Secure Boot

When you launch an instance with the following prerequisites, the instance will then be ready to be configured to support UEFI Secure Boot. You can only enable support for UEFI Secure Boot on an instance at launch; you can't enable it later.

After you’ve launched your instance, you can verify that it is ready to be configured to support UEFI Secure Boot (in other words, you can proceed to Step 2) by checking whether UEFI data is present. The presence of UEFI data indicates that non-volatile data is persisted.

To verify whether your instance is ready for Step 2

Use the get-instance-uefi-data command and specify the instance ID.

aws ec2 get-instance-uefi-data --instance-id i-0123456789example

The instance is ready for Step 2 if UEFI data is present in the output. If the output is empty, the instance cannot be configured to support UEFI Secure Boot. This can happen if your instance was launched before UEFI Secure Boot support became available. Launch a new instance and try again.

Step 2: Configure an instance to support UEFI Secure Boot

Enroll the key pairs in your UEFI variable store on the instance

After you create the signed UEFI signature lists (PK, KEK, and db), they must be enrolled into the UEFI firmware.

Writing to the PK variable is possible only if:

To enroll the keys in your UEFI variable store

The following commands must be run on the instance.

If SetupMode is enabled (the value is 1), the keys can be enrolled by running the following commands on the instance:

[ec2-user ~]$ efi-updatevar -f db.auth db

[ec2-user ~]$ efi-updatevar -f KEK.auth KEK

[ec2-user ~]$ efi-updatevar -f PK.auth PK

To verify that UEFI Secure Boot is enabled

To verify that UEFI Secure Boot is enabled, follow the steps in Verify whether an Amazon EC2 instance is enabled for UEFI Secure Boot.

You can now export your UEFI variable store with the get-instance-uefi-data CLI command, or you continue to the next step and sign your boot images to reboot into a UEFI Secure Boot-enabled instance.

Step 3: Create an AMI from the instance

To create an AMI from the instance, you can use the console or the CreateImage API, CLI, or SDKs. For the console instructions, see Create an Amazon EBS-backed AMI. For the API instructions, see CreateImage.

Step 2 (Option B): Create a binary blob containing a pre-filled variable store

After you have created the three key pairs, you can create a binary blob containing a pre-filled variable store containing the UEFI Secure Boot keys. Alternatively, complete the steps for Step 2 (Option A): Add keys to the variable store from within the instance.

Step 1: Create a new variable store or update an existing one

You can create the variable store offline without a running instance by using the python-uefivars tool. The tool can create a new variable store from your keys. The script currently supports the EDK2 format, the AWS format, and a JSON representation that is easier to edit with higher-level tooling.

Step 2: Upload the binary blob on AMI creation

Use register-image to pass your UEFI variable store data. For the --uefi-data parameter, specify your binary blob, and for the --boot-mode parameter, specify uefi.

aws ec2 register-image \
    --name uefi_sb_tpm_register_image_test \
    --uefi-data $(cat your_binary_blob.bin) \
    --block-device-mappings "DeviceName=/dev/sda1,Ebs= {SnapshotId=snap-0123456789example,DeleteOnTermination=true}" \
    --architecture x86_64 \
    --root-device-name /dev/sda1 \
    --virtualization-type hvm \
    --ena-support \
    --boot-mode uefi