Re: RBD encryption support in libvirt
On Thu, Sep 02, 2021 at 05:55:20AM +0000, Or Ozeri wrote:
Hi,
I wanted to get your advice on a patch I'm preparing for libvirt.
It touches the code-path that allows using LUKS encryption on top of an RBD image.
We recently added LUKS and LUKS2 encryption support in Ceph's librbd.
We exposed this in qemu in a recent patch by adding new optional
"encrypt" member to BlockdevOptionsRbd.
This patch was included in the recent release of qemu 6.1.
To enable libvirt users to use librbd encryption, we need libvirt
to use this new "encrypt" when it builds the blockdev options for RBD.
The interesting question is how to define the libvirt XML syntax that
will trigger the use of librbd encryption.
My thought was to use the already existing <encryption> tag.
In that case, we just need to add a new format VIR_STORAGE_ENCRYPTION_FORMAT_LUKS2 to the
enum virStorageEncryptionFormatType.
This type will be checked in qemuBlockStorageSourceGetRBDProps.
I don't think we want to switch impls based on luks1 vs luks2,
because that will create trouble in future when/if QEMU's native
impl gains Luksv2 support. So I think we need an explicit way
to request librbd encryption, even for luks2.
The problem with this approach is that it only works for LUKS2.
librbd encryption also supports LUKS1.
We want to allow the user to choose between the qemu LUKS implementation and the librbd
one.
One reason to keep support both is that on the one hand librbd only supports XTS mode.
On the other hand, qemu implementation will not support a chain of uniquely encrypted RBD
images (each serving as a backing store for the previous one).
I think I asked this before on qemu-devel, but I can't find the
answer, but can you explain the RBB backing store chain problem ?
QEMUs' LUKS driver can be layered on top of any QEMU block protocol
driver. So if there are multiple RBD layers in the qemu -blockdev
config, we can layer LUKS over each one independantly.
In any case, I agree we need a way to request a specific luks
impl.
So we need a way in the XML API to support both implementations.
Our current thought is to add a new "engine" attribute to the encryption tag.
By default, encryption will use the QEMU LUKS implementation, unless <encryption
engine='rbd' ...> is specified.
To make this more general, we can have engine='backend' instead of
engine='rbd' to denote that the encryption is to be delegated to the backend
storage properties (instead of the format properties).
I don't think we'll ever do it, but conceptually libvirt could even
integrate with host kernel cryptsetup tools. Having an 'engine'
attribute feels like a decent enough idea.
Maybe engine='host|qemu|builtin'
- host - the cryptosetup/kernel driver
- qemu - qemu's custom luks driver
- builtin - the librbd (or equiv) builtin driver
Regards,
Daniel
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