libvirt driver support for flavor and image defined ephemeral encryption

This spec outlines the specific libvirt virt driver implementation to support the Flavor and Image defined ephemeral storage encryption [1] spec.

Problem description

The libvirt virt driver currently provides very limited support for ephemeral disk encryption through the LVM imagebackend and the use of the PLAIN encryption format provided by dm-crypt.

As outlined in the Flavor and Image defined ephemeral storage encryption [1] spec this current implementation is controlled through compute host configurables and is transparent to end users, unlike block storage volume encryption via Cinder.

With the introduction of the Flavor and Image defined ephemeral storage encryption [1] spec we can now implement support for encrypting ephemeral disks via images and flavors, allowing support for newer encryption formats such as LUKSv1. This also has the benefit of being natively supported by QEMU, as already seen in the libvirt driver when attaching LUKSv1 encrypted volumes provided by Cinder.

Use Cases

  • As a user of a cloud with libvirt based computes I want to request that all of my ephemeral storage be encrypted at rest through the selection of a specific flavor or image.

  • As a user of a cloud with libvirt based computes I want to be able to pick how my ephemeral storage be encrypted at rest through the selection of a specific flavor or image.

  • As a user I want each encrypted ephemeral disk attached to my instance to have a separate unique secret associated with it.

  • As an operator I want to allow users to request that the ephemeral storage of their instances is encrypted using the flexible LUKSv1 encryption format.

Proposed change

Deprecate the legacy implementation within the libvirt driver

The legacy implementation using dm-crypt within the libvirt virt driver needs to be deprecated ahead of removal in a future release, this includes the following options:

  • [ephemeral_storage_encryption]/enabled

  • [ephemeral_storage_encryption]/cipher

  • [ephemeral_storage_encryption]/key_size

Limited support for dm-crypt will be introduced using the new framework before this original implementation is removed.

Populate disk_info with encryption properties

The libvirt driver has an additional disk_info dict built from the contents of the previously mentioned block_device_info and image metadata associated with an instance. With the introduction of the DriverImageBlockDevice within the Flavor and Image defined ephemeral storage encryption [1] spec we can now avoid the need to look again at image metadata while also adding some ephemeral encryption related metadata to the dict.

This dict currently contains the following:


The default bus used by disks


The default bus used by cd-rom drives


A nested dict keyed by disk name including information about each disk.

Each item within the mapping dict containing following keys:


The bus for this disk


The device name for this disk as known to libvirt


A type from the BlockDeviceType enum (‘disk’, ‘cdrom’,’floppy’, ‘fs’, or ‘lun’)

It can also contain the following optional keys:


Used to format swap/ephemeral disks before passing to instance (e.g. ‘swap’, ‘ext4’)


The 1-based boot index of the disk.

In addition to the above this spec will also optionally add the following keys for encrypted disks:


The encryption format used by the disk


A dict of encryption options


The UUID of the encryption secret associated with the disk

Handle ephemeral disk encryption within imagebackend

With the above in place we can now add encryption support within each image backend. As highlighted at the start of this spec this initial support will only be for the LUKSv1 encryption format.

Generic key management code will be introduced into the base nova.virt.libvirt.imagebackend.Image class and used to create and store the encryption secret within the configured key manager. The initial LUKSv1 support will store a passphrase for each disk within the key manager. This is unlike the current ephemeral storage encryption or encrypted volume implementations that currently store a symmetric key in the key manager. This remains a long running piece of technical debt in the encrypted volume implementation as LUKSv1 does not directly encrypt data with the provided key.

The base nova.virt.libvirt.imagebackend.Image class will also be extended to accept and store the optional encryption details provided by disk_info above including the format, options and secret UUID.

Each backend will then be modified to encrypt disks during nova.virt.libvirt.imagebackend.Image.create_image using the provided format, options and secret.


Finally, with the above support in place the COMPUTE_EPHEMERAL_ENCRYPTION and COMPUTE_EPHEMERAL_ENCRYPTION_LUKS traits can be enabled when using a backend that supports LUKSv1. This will in turn enable scheduling to the compute of any user requests asking for ephemeral storage encryption using the format.


Continue to use the transparent host configurables and expand support to other encryption formats such as LUKS.

Data model impact

As discussed above the ephemeral encryption keys will be added to the disk_info for individual disks within the libvirt driver.

REST API impact


Security impact

This should hopefully be positive given the unique secret per disk and user visible choice regarding how their ephemeral storage is encrypted at rest.

Notifications impact


Other end user impact

Users will now need to opt-in to ephemeral storage encryption being used by their instances through their choice of image or flavors.

Performance Impact

QEMU will natively decrypted these LUKSv1 ephemeral disks for us using the libgcrypt library. While there have been performance issues with this in the past workarounds [2] can be implemented that use dm-crypt instead.

Other deployer impact


Developer impact

This spec will aim to implement LUKSv1 support for all imagebackends but in the future any additional encryption formats supported by these backends will need to ensure matching traits are also enabled.

Upgrade impact

The legacy implementation is deprecated but will continue to work for the time being. As the new implementation is separate there is no further upgrade impact.



Primary assignee:


Other contributors:


Feature Liaison

Feature liaison:


Work Items

  • Populate the individual disk dicts within disk_info with any ephemeral encryption properties.

  • Provide these properties to the imagebackends when creating each disk.

  • Introduce support for LUKSv1 based encryption within the imagebackends.

  • Enable the COMPUTE_EPHEMERAL_ENCRYPTION_LUKS trait when the selected imagebackend supports LUKSv1.


  • Flavor and Image defined ephemeral storage encryption [1]


Unlike the parent spec once imagebackends support LUKSv1 and enable the required trait we can introduce Tempest based testing of this implementation in addition to extensive functional and unit based tests.

Documentation Impact

  • New user documentation around the specific LUKSv1 support for ephemeral encryption within the libvirt driver.

  • Reference documentation around the changes to the virt block device layer.



Release Name