libvirt driver support for flavor and image defined ephemeral encryption

https://blueprints.launchpad.net/nova/+spec/ephemeral-encryption-libvirt

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:

disk_bus

The default bus used by disks

cdrom_bus

The default bus used by cd-rom drives

mapping

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

Each item within the mapping dict containing following keys:

bus

The bus for this disk

dev

The device name for this disk as known to libvirt

type

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

It can also contain the following optional keys:

format

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

boot_index

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:

encryption_format

The encryption format used by the disk

encryption_options

A dict of encryption options

encryption_secret_uuid

The UUID of the encryption secret associated with the disk

backing_encryption_secret_uuid

The UUID of the encryption secret for the backing file associated with the disk in the case of qcow2.

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 raw keys in the key manager due to their support of the dm-crypt PLAIN encryption format. With LUKSv1 it is not necessary to store raw keys as it does not directly encrypt data with the provided key [1].

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(s).

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

Enable the COMPUTE_EPHEMERAL_ENCRYPTION_LUKS trait

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.

Alternatives

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

N/A

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

N/A

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 decrypt 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

N/A

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.

Implementation

Assignee(s)

Primary assignee:

melwitt

Other contributors:

lyarwood

Feature Liaison

Feature liaison:

melwitt

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.

Dependencies

  • Flavor and Image defined ephemeral storage encryption [1]

Testing

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.

  • Document that for the raw imagebackend, both [libvirt]images_type = raw and [DEFAULT]use_cow_images = False must be configured in order for resize to work. This is also true without encryption but it may still be helpful to users.

  • Document that a user must have policy permission to create secrets in Barbican in order for encryption to work for that user. Secrets are created in Barbican using the user’s auth token. Admins have permission to create secrets in Barbican by default.

References

Revisions

Release Name

Description

Wallaby

Introduced

Yoga

Reproposed

Zed

Reproposed

2023.1 Antelope

Reproposed

2023.2 Bobcat

Reproposed

2024.1 Caracal

Reproposed

2024.2 Dalmatian

Reproposed