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+================================
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+LUKS volume with detached header
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+================================
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+
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+Introduction
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+============
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+
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+This document gives an overview of the design of LUKS volume with detached
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+header and how to use it.
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+
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+Background
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+==========
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+
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+The LUKS format has ability to store the header in a separate volume from
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+the payload. We could extend the LUKS driver in QEMU to support this use
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+case.
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+
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+Normally a LUKS volume has a layout:
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+
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+::
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+
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+ +-----------------------------------------------+
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+ | | | |
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+ disk | header | key material | disk payload data |
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+ | | | |
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+ +-----------------------------------------------+
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+
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+With a detached LUKS header, you need 2 disks so getting:
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+
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+::
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+
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+ +--------------------------+
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+ disk1 | header | key material |
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+ +--------------------------+
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+ +---------------------+
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+ disk2 | disk payload data |
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+ +---------------------+
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+
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+There are a variety of benefits to doing this:
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+
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+ * Secrecy - the disk2 cannot be identified as containing LUKS
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+ volume since there's no header
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+ * Control - if access to the disk1 is restricted, then even
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+ if someone has access to disk2 they can't unlock
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+ it. Might be useful if you have disks on NFS but
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+ want to restrict which host can launch a VM
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+ instance from it, by dynamically providing access
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+ to the header to a designated host
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+ * Flexibility - your application data volume may be a given
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+ size and it is inconvenient to resize it to
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+ add encryption.You can store the LUKS header
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+ separately and use the existing storage
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+ volume for payload
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+ * Recovery - corruption of a bit in the header may make the
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+ entire payload inaccessible. It might be
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+ convenient to take backups of the header. If
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+ your primary disk header becomes corrupt, you
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+ can unlock the data still by pointing to the
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+ backup detached header
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+
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+Architecture
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+============
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+
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+Take the qcow2 encryption, for example. The architecture of the
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+LUKS volume with detached header is shown in the diagram below.
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+
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+There are two children of the root node: a file and a header.
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+Data from the disk payload is stored in the file node. The
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+LUKS header and key material are located in the header node,
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+as previously mentioned.
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+
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+::
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+
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+ +-----------------------------+
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+ Root node | foo[luks] |
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+ +-----------------------------+
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+ | |
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+ file | header |
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+ | |
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+ +---------------------+ +------------------+
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+ Child node |payload-format[qcow2]| |header-format[raw]|
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+ +---------------------+ +------------------+
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+ | |
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+ file | file |
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+ | |
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+ +----------------------+ +---------------------+
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+ Child node |payload-protocol[file]| |header-protocol[file]|
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+ +----------------------+ +---------------------+
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+ | |
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+ | |
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+ | |
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+ Host storage Host storage
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+
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+Usage
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+=====
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+
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+Create a LUKS disk with a detached header using qemu-img
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+--------------------------------------------------------
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+
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+Shell commandline::
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+
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+ # qemu-img create --object secret,id=sec0,data=abc123 -f luks \
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+ -o cipher-alg=aes-256,cipher-mode=xts -o key-secret=sec0 \
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+ -o detached-header=true test-header.img
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+ # qemu-img create -f qcow2 test-payload.qcow2 200G
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+ # qemu-img info 'json:{"driver":"luks","file":{"filename": \
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+ "test-payload.img"},"header":{"filename":"test-header.img"}}'
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+
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+Set up a VM's LUKS volume with a detached header
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+------------------------------------------------
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+
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+Qemu commandline::
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+
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+ # qemu-system-x86_64 ... \
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+ -object '{"qom-type":"secret","id":"libvirt-3-format-secret", \
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+ "data":"abc123"}' \
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+ -blockdev '{"driver":"file","filename":"/path/to/test-header.img", \
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+ "node-name":"libvirt-1-storage"}' \
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+ -blockdev '{"node-name":"libvirt-1-format","read-only":false, \
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+ "driver":"raw","file":"libvirt-1-storage"}' \
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+ -blockdev '{"driver":"file","filename":"/path/to/test-payload.qcow2", \
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+ "node-name":"libvirt-2-storage"}' \
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+ -blockdev '{"node-name":"libvirt-2-format","read-only":false, \
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+ "driver":"qcow2","file":"libvirt-2-storage"}' \
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+ -blockdev '{"node-name":"libvirt-3-format","driver":"luks", \
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+ "file":"libvirt-2-format","header":"libvirt-1-format","key-secret": \
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+ "libvirt-3-format-secret"}' \
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+ -device '{"driver":"virtio-blk-pci","bus":XXX,"addr":YYY,"drive": \
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+ "libvirt-3-format","id":"virtio-disk1"}'
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+
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+Add LUKS volume to a VM with a detached header
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+----------------------------------------------
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+
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+1. object-add the secret for decrypting the cipher stored in
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+ LUKS header above::
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+
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+ # virsh qemu-monitor-command vm '{"execute":"object-add", \
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+ "arguments":{"qom-type":"secret", "id": \
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+ "libvirt-4-format-secret", "data":"abc123"}}'
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+
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+2. block-add the protocol node for LUKS header::
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+
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+ # virsh qemu-monitor-command vm '{"execute":"blockdev-add", \
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+ "arguments":{"node-name":"libvirt-1-storage", "driver":"file", \
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+ "filename": "/path/to/test-header.img" }}'
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+
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+3. block-add the raw-drived node for LUKS header::
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+
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+ # virsh qemu-monitor-command vm '{"execute":"blockdev-add", \
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+ "arguments":{"node-name":"libvirt-1-format", "driver":"raw", \
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+ "file":"libvirt-1-storage"}}'
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+
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+4. block-add the protocol node for disk payload image::
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+
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+ # virsh qemu-monitor-command vm '{"execute":"blockdev-add", \
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+ "arguments":{"node-name":"libvirt-2-storage", "driver":"file", \
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+ "filename":"/path/to/test-payload.qcow2"}}'
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+
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+5. block-add the qcow2-drived format node for disk payload data::
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+
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+ # virsh qemu-monitor-command vm '{"execute":"blockdev-add", \
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+ "arguments":{"node-name":"libvirt-2-format", "driver":"qcow2", \
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+ "file":"libvirt-2-storage"}}'
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+
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+6. block-add the luks-drived format node to link the qcow2 disk
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+ with the LUKS header by specifying the field "header"::
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+
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+ # virsh qemu-monitor-command vm '{"execute":"blockdev-add", \
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+ "arguments":{"node-name":"libvirt-3-format", "driver":"luks", \
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+ "file":"libvirt-2-format", "header":"libvirt-1-format", \
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+ "key-secret":"libvirt-2-format-secret"}}'
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+
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+7. hot-plug the virtio-blk device finally::
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+
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+ # virsh qemu-monitor-command vm '{"execute":"device_add", \
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+ "arguments": {"driver":"virtio-blk-pci", \
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+ "drive": "libvirt-3-format", "id":"virtio-disk2"}}
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+
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+TODO
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+====
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+
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+1. Support the shared detached LUKS header within the VM.
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Hyman Huang