LUKS volume with detached header — QEMU documentation

# LUKS volume with detached header

## Introduction

This document gives an overview of the design of LUKS volume with detached header and how to use it.

## Background

The LUKS format has ability to store the header in a separate volume from the payload. We could extend the LUKS driver in QEMU to support this use case.

Normally a LUKS volume has a layout:

            +-----------------------------------------------+
            |         |                |                    |
    disk    | header  |  key material  |  disk payload data |
            |         |                |                    |
            +-----------------------------------------------+

With a detached LUKS header, you need 2 disks so getting:

            +--------------------------+
    disk1   |   header  | key material |
            +--------------------------+
            +---------------------+
    disk2   |  disk payload data  |
            +---------------------+

There are a variety of benefits to doing this:

> - Secrecy - the disk2 cannot be identified as containing LUKS

>     volume since there’s no header

> - Control - if access to the disk1 is restricted, then even

>     if someone has access to disk2 they can’t unlock it. Might be useful if you have disks on NFS but want to restrict which host can launch a VM instance from it, by dynamically providing access to the header to a designated host

> - Flexibility - your application data volume may be a given

>     size and it is inconvenient to resize it to add encryption.You can store the LUKS header separately and use the existing storage volume for payload

> - Recovery - corruption of a bit in the header may make the

>     entire payload inaccessible. It might be convenient to take backups of the header. If your primary disk header becomes corrupt, you can unlock the data still by pointing to the backup detached header

## Architecture

Take the qcow2 encryption, for example. The architecture of the LUKS volume with detached header is shown in the diagram below.

There are two children of the root node: a file and a header. Data from the disk payload is stored in the file node. The LUKS header and key material are located in the header node, as previously mentioned.

                         +-----------------------------+
    Root node            |          foo[luks]          |
                         +-----------------------------+
                            |                       |
                       file |                header |
                            |                       |
                 +---------------------+    +------------------+
    Child node   |payload-format[qcow2]|    |header-format[raw]|
                 +---------------------+    +------------------+
                            |                       |
                       file |                 file  |
                            |                       |
                 +----------------------+  +---------------------+
    Child node   |payload-protocol[file]|  |header-protocol[file]|
                 +----------------------+  +---------------------+
                            |                       |
                            |                       |
                            |                       |
                       Host storage            Host storage

## Usage

### Create a LUKS disk with a detached header using qemu-img

Shell commandline:

    # qemu-img create --object secret,id=sec0,data=abc123 -f luks \
      -o cipher-alg=aes-256,cipher-mode=xts -o key-secret=sec0 \
      -o detached-header=true test-header.img
    # qemu-img create -f qcow2 test-payload.qcow2 200G
    # qemu-img info 'json:{"driver":"luks","file":{"filename": \
      "test-payload.img"},"header":{"filename":"test-header.img"}}'

### Set up a VM’s LUKS volume with a detached header

Qemu commandline:

    # qemu-system-x86_64 ... \
      -object '{"qom-type":"secret","id":"libvirt-3-format-secret", \
      "data":"abc123"}' \
      -blockdev '{"driver":"file","filename":"/path/to/test-header.img", \
      "node-name":"libvirt-1-storage"}' \
      -blockdev '{"node-name":"libvirt-1-format","read-only":false, \
      "driver":"raw","file":"libvirt-1-storage"}' \
      -blockdev '{"driver":"file","filename":"/path/to/test-payload.qcow2", \
      "node-name":"libvirt-2-storage"}' \
      -blockdev '{"node-name":"libvirt-2-format","read-only":false, \
      "driver":"qcow2","file":"libvirt-2-storage"}' \
      -blockdev '{"node-name":"libvirt-3-format","driver":"luks", \
      "file":"libvirt-2-format","header":"libvirt-1-format","key-secret": \
      "libvirt-3-format-secret"}' \
      -device '{"driver":"virtio-blk-pci","bus":XXX,"addr":YYY,"drive": \
      "libvirt-3-format","id":"virtio-disk1"}'

### Add LUKS volume to a VM with a detached header

1. object-add the secret for decrypting the cipher stored in LUKS header above:

       # virsh qemu-monitor-command vm '{"execute":"object-add", \
         "arguments":{"qom-type":"secret", "id": \
         "libvirt-4-format-secret", "data":"abc123"}}'

2. block-add the protocol node for LUKS header:

       # virsh qemu-monitor-command vm '{"execute":"blockdev-add", \
         "arguments":{"node-name":"libvirt-1-storage", "driver":"file", \
         "filename": "/path/to/test-header.img" }}'

3. block-add the raw-drived node for LUKS header:

       # virsh qemu-monitor-command vm '{"execute":"blockdev-add", \
         "arguments":{"node-name":"libvirt-1-format", "driver":"raw", \
         "file":"libvirt-1-storage"}}'

4. block-add the protocol node for disk payload image:

       # virsh qemu-monitor-command vm '{"execute":"blockdev-add", \
         "arguments":{"node-name":"libvirt-2-storage", "driver":"file", \
         "filename":"/path/to/test-payload.qcow2"}}'

5. block-add the qcow2-drived format node for disk payload data:

       # virsh qemu-monitor-command vm '{"execute":"blockdev-add", \
         "arguments":{"node-name":"libvirt-2-format", "driver":"qcow2", \
         "file":"libvirt-2-storage"}}'

6. block-add the luks-drived format node to link the qcow2 disk with the LUKS header by specifying the field “header”:

       # virsh qemu-monitor-command vm '{"execute":"blockdev-add", \
         "arguments":{"node-name":"libvirt-3-format", "driver":"luks", \
         "file":"libvirt-2-format", "header":"libvirt-1-format", \
         "key-secret":"libvirt-2-format-secret"}}'

7. hot-plug the virtio-blk device finally:

       # virsh qemu-monitor-command vm '{"execute":"device_add", \
         "arguments": {"driver":"virtio-blk-pci", \
         "drive": "libvirt-3-format", "id":"virtio-disk2"}}

## TODO

1. Support the shared detached LUKS header within the VM.