Re: [Qemu-devel] [PATCH v1 00/15] Convert QCow[2] to QCryptoBlock & add LUKS support

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

On Wed, Jan 25, 2017 at 05:41:59PM +0100, Max Reitz wrote:
> On 25.01.2017 17:29, Daniel P. Berrange wrote:
> > On Wed, Jan 25, 2017 at 04:58:32PM +0100, Max Reitz wrote:
> >> On 03.01.2017 19:27, Daniel P. Berrange wrote:
> >>> This series is a continuation of previous work to support LUKS in
> >>> QEMU. The existing merged code supports LUKS as a standalone
> >>> driver which can be layered over/under any other QEMU block device
> >>> driver. This works well when using LUKS over protocol drivers (file,
> >>> rbd, iscsi, etc, etc), but has some downsides when combined with
> >>> format drivers like qcow2.
> >>
> >> When trying out whether compressed images are actually encrypted (which
> >> they are not, as I wrote in my last reply to patch 12), I noticed that
> >> the user interface still has some flaws:
> > 
> > The original code explicitly forbids this combination
> > 
> >  "qemu-img: Compression and encryption not supported at the same time"
> > 
> > but I guess we lost the error check due to changing to use
> > encryption-format as the option name
> 
> Yes, not supporting it is completely fine, but qemu-img should refuse
> that combination then (or at least warn about it).

Yep, I'll make sure the existing error message covers this scenario.

> 
> >> One is that you actually can't convert to encrypted images any more, or
> >> if you can, it doesn't seem obvious to me:
> >>
> >> $ ./qemu-img convert -O qcow2 --object secret,id=sec0,data=12345 \
> >>     -o encryption-format=luks,luks-key-secret=sec0 \
> >>     foo.qcow2 bar.qcow2
> >> qemu-img: Could not open 'bar.qcow2': Parameter 'key-secret' is required
> >> for cipher
> >>
> >> The issue is that you have to specify the key secret as a runtime
> >> parameter in addition to the creation option. Not only is that a bit
> >> cumbersome, but it's also impossible because --image-opts doesn't work
> >> for the output image.
> > 
> > Yeah, this is a problem I've not figured out a solutiuon for yet - it
> > also affects the previously merged bare luks format code.
> > 
> > Somehow qemu-img needs to know which create options are also required
> > to be passed when opening the newly created image.
> > 
> > Perhaps the BlockDriver struct needs a new callback like
> > 
> >   bdrv_create_opts_to_runtime_opts(QemuOpts *copts, QemuOpts *ropts);
> 
> Yeah, it's tough. For the moment, I'd be fine with qemu-img convert
> working at all, though, even if that means having to specify the secret
> twice. But I don't know if fixing the --image-opts issue is any easier
> -- maybe we can allow image-opts syntax for the targets when -n is
> given? Then you'd have to call qemu-img create and qemu-img convert
> separately (or call qemu-img convert twice, once without -n (which
> successfully creates the image but then fails when trying to open it)
> and once without...), but at least there'd be a way to make it work.
> 
> I have to admit that I personally wouldn't mind a hack in qemu-img
> convert like "copy every option ending in 'key-secret' to the runtime
> opts" too much. But I don't know how much that might infuriate some
> other people.
> 
> It's ugly, yes, but it would work perfectly well for now. I don't think
> it would hurt us in the future. If other parameters appear that we have
> to copy over, we can still implement the well-engineered solution.

This turns out to be pretty trivial to implement. So although it is
a bit ugly, it feels like reasonable approach to take in the short
term.

Regards,
Daniel
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