Re: [PATCH v3] block/rbd: Add support for layered encryption

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Nov 16, 2022 at 09:03:31AM +0000, Or Ozeri wrote:
> > -----Original Message-----
> > From: Daniel P. Berrangé <berrange@redhat.com>
> > Sent: 15 November 2022 19:47
> > To: Or Ozeri <ORO@il.ibm.com>
> > Cc: qemu-devel@nongnu.org; qemu-block@nongnu.org; Danny Harnik
> > <DANNYH@il.ibm.com>; idryomov@gmail.com
> > Subject: [EXTERNAL] Re: [PATCH v3] block/rbd: Add support for layered
> > encryption
> > 
> > AFAICT, supporting layered encryption shouldn't require anything other than
> > the 'parent' addition.
> > 
> 
> Since the layered encryption API is new in librbd, we don't have to
> support "luks" and "luks2" at all.
> In librbd we are actually deprecating the use of "luks" and "luks2",
> and instead ask users to use "luks-any".

Deprecating that is a bad idea. The security characteristics and
feature set of LUKSv1 and LUKSv2 can be quite different. If a mgmt
app is expecting the volume to be protected with LUKSv2, it should
be stating that explicitly and not permit a silent downgrade if
the volume was unexpectedly using LUKSv1.

> If we don't add "luks-any" here, we will need to implement
> explicit cases for "luks" and "luks2" in the qemu_rbd_encryption_load2.
> This looks like a kind of wasteful coding that won't be actually used
> by users of the rbd driver in qemu.

It isn't wasteful - supporting the formats explicitly is desirable
to prevent format downgrades.

> Anyhow, we need the "luks-any" option for our use-case, so if you
> insist, I will first submit a patch to add "luks-any", before this
> patch.

I'm pretty wary of any kind of automatic encryption format detection
in QEMU. The automatic block driver format probing has been a long
standing source of CVEs in QEMU and every single mgmt app above QEMU.

What is the problem with specifying the desired LUKS format explicitly.
The mgmt app should know what formats it wants to be using. It should
be possible to query format for existing volumes too.

With regards,
Daniel
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