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Re: [Qemu-block] [Qemu-devel] [PATCH RFC 0/2] Limit support for encrypte

From: Markus Armbruster
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH RFC 0/2] Limit support for encrypted images to qemu-img
Date: Wed, 11 Mar 2015 09:55:16 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

"Daniel P. Berrange" <address@hidden> writes:

> On Tue, Mar 10, 2015 at 06:26:38PM +0100, Markus Armbruster wrote:
>> RFC because the series only covers open [PATCH 1], but not create.
>> Also missing: make qemu-img print a warning when it creates an
>> encrypted image.  Finally, some of the material in the cover letter
>> should be worked into the commit messages.
>> We've steered users away from QCOW/QCOW2 encryption for a while,
>> because it's a flawed design (commit 136cd19 Describe flaws in
>> qcow/qcow2 encryption in the docs).
>> In addition to flawed crypto, we have comically bad usability, and
>> plain old bugs.  Let me show you.
>> = Usability issues =
>> == Confusing startup ==
>> == Incorrect passwords not caught ==
>> == Need to stop the guest to add an encrypted image ==
>> == Use without key is not always caught ==
>> == QMP device_add of usb-storage fails when it shouldn't ==
> So there's really two separate root cuase problems we're facing
> here. One of the usability issues is inherant artifact of the
> qcow design. The other 4 issues are all due to the badly designed
> block driver / monitor key management approach.

Yup.  The latter is fixable, but not worth fixing as long as the
underlying crypto isn't worth using.

>> If people become sufficiently interested in encrypted images to
>> contribute a cryptographically sane implementation for QCOW2 (or
>> whatever other format), then rewriting the necessary support around it
>> from scratch will likely be easier and yield better results than
>> fixing up the existing mess.
>> Let's drop the mess and move on.  Keep qemu-img convert working, of
>> course, to let users rescue their data.
> Once I've got through the current work i'm doing on TLS support
> for migrate/nbd/chardev/etc, my intention is to work on adding
> support for the LUKS format to QEMU. We really need this natively
> in OpenStack since we're increasingly using the QEMU native client
> for nbd, iscsi, nfs, etc but at the same time don't want to sacrifice
> encryption which we currently do via LUKS. It is probably a good
> 4-6 months though before I get on to working on this.


> I agree with all your points about the usability being fubar. This
> clearly needs to be fixed for encryption support to be viable in
> QEMU, regardless of the actual encryption format used.
> I guess my question is whether it is worth trying to fix the blockdev
> integration part of things now, or to rip it out now and reimplement
> it from scratch later ?  I think I probably agree with killing it
> now, since it might actually make doing a sensible impl easier later
> on.

In your shoes, I'd certainly prefer to drop the current mess and start
over.  Not just because I think it'll make the work easier, but also
because I'd want to break out of the back-compat strait-jacket, to be
free to create the best user interface I can.

> And lets assume we do eventually have a fixed blockdev layer and a
> sane LUKS encryption driver, would we still want to kill off qcow2
> encryption ?  Given the way subpar encryption is being actively
> attacked by everyone & their dog, I think mandatory retirement of
> qcow2 encryption is a good idea sooner rather than later.

I strongly believe retiring it is a public service.

Except for qemu-img.  I'm prepared to keep it there for a long time,
maybe forever.

> My only concern here is whether we've given users enough prior
> warning. While we added that doc change a year ago, what are the
> odds that anyone has actually read those docs & noticed the warning.
> Should we have one major release where we log a deprecation warning
> on stderr, informing users of an explicit timeframe for its removal,
> before we actually use the big hammer of disabling it permanently ?

I'm fine with that.  In fact, I could agree to pretty much any
deprecation schedule, as long as we start it *now*.

> FWIW, I could see an improved interaction scheme working as follows
> First, introduce a new monitor command for setting named passwords,
>     add_key mykey1 SECRETDATA
> Now, extend the blockdev_add so that you can provide key names
> by adding
>     'keyname': 'mykey1'
> as a parameter in the json args.

Can you explain why that's better than sticking 'key': SECRETDATA right
into blockdev-add's arguments?

> If an attempt is made to add a blockdev without having provided a
> key, the attempt should just fail. This avoids all the insanity
> around delayed opening of files, as well as avoiding need to stop
> the guest to add devices.

Yes, we need to exterminate the awkward and dangerous intermediate state
"image opened, but lacks decryption key".  This series avoids it (except
in qemu-img, but there it's safely confined to img_open()).  Future work
should not bring it back.

> For cold plug, have a command line arg '--add-keys prompt' to
> indicate the user should be prompted on TTY to enter keys, which
> is good for interactive usage. For managed usage we could allow
> '--add-keys fd=FDNUM' and just read keys from the file descriptor.

I guess sugared command line like "-hda geheim.qcow2" should simply
prompt for the key right in the tty.  None of this nonsense with not
starting the guest, and having "cont" prompt in the monitor.

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