Re: [PATCH 02/13] qcrypto-luks: implement encryption key management

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

On Thu, Jan 30, 2020 at 03:23:10PM +0100, Kevin Wolf wrote:
> Am 30.01.2020 um 13:53 hat Daniel P. Berrangé geschrieben:
> > On Thu, Jan 30, 2020 at 01:38:47PM +0100, Kevin Wolf wrote:
> > > Am 28.01.2020 um 18:32 hat Daniel P. Berrangé geschrieben:
> > > > On Tue, Jan 28, 2020 at 05:11:16PM +0000, Daniel P. Berrangé wrote:
> > > > > On Tue, Jan 21, 2020 at 03:13:01PM +0200, Maxim Levitsky wrote:
> > > > > > On Tue, 2020-01-21 at 08:54 +0100, Markus Armbruster wrote:
> > > > > > 
> > > > > > <trimmed>
> > > > > > 
> > > > > > > > +##
> > > > > > > > +# @LUKSKeyslotUpdate:
> > > > > > > > +#
> > > > > > > > +# @keyslot:         If specified, will update only keyslot with this index
> > > > > > > > +#
> > > > > > > > +# @old-secret:      If specified, will only update keyslots that
> > > > > > > > +#                   can be opened with password which is contained in
> > > > > > > > +#                   QCryptoSecret with @old-secret ID
> > > > > > > > +#
> > > > > > > > +#                   If neither @keyslot nor @old-secret is specified,
> > > > > > > > +#                   first empty keyslot is selected for the update
> > > > > > > > +#
> > > > > > > > +# @new-secret:      The ID of a QCryptoSecret object providing a new decryption
> > > > > > > > +#                   key to place in all matching keyslots.
> > > > > > > > +#                   null/empty string erases all matching keyslots
> > > > > > > 
> > > > > > > I hate making the empty string do something completely different than a
> > > > > > > non-empty string.
> > > > > > > 
> > > > > > > What about making @new-secret optional, and have absent @new-secret
> > > > > > > erase?
> > > > > > 
> > > > > > I don't remember already why I and Keven Wolf decided to do this this way, but I think that you are right here.
> > > > > > I don't mind personally to do this this way.
> > > > > > empty string though is my addition, since its not possible to pass null on command line.
> > > > > 
> > > > > IIUC this a result of using  "StrOrNull" for this one field...
> > > > > 
> > > > > 
> > > > > > > > +# Since: 5.0
> > > > > > > > +##
> > > > > > > > +{ 'struct': 'LUKSKeyslotUpdate',
> > > > > > > > +  'data': {
> > > > > > > > +           '*keyslot': 'int',
> > > > > > > > +           '*old-secret': 'str',
> > > > > > > > +           'new-secret' : 'StrOrNull',
> > > > > > > > +           '*iter-time' : 'int' } }
> > > > > 
> > > > > It looks wierd here to be special casing "new-secret" to "StrOrNull"
> > > > > instead of just marking it as an optional string field
> > > > > 
> > > > >    "*new-secret": "str"
> > > > > 
> > > > > which would be possible to use from the command line, as you simply
> > > > > omit the field.
> > > > > 
> > > > > I guess the main danger here is that we're using this as a trigger
> > > > > to erase keyslots. So simply omitting "new-secret" can result
> > > > > in damage to the volume by accident which is not an attractive
> > > > > mode.
> > > 
> > > Right. It's been a while since I discussed this with Maxim, but I think
> > > this was the motivation for me to suggest an explicit null value.
> > > 
> > > As long as we don't support passing null from the command line, I see
> > > the problem with it, though. Empty string (which I think we didn't
> > > discuss before) looks like a reasonable enough workaround to me, but if
> > > you think this is too much magic, then maybe not.
> > > 
> > > > Thinking about this again, I really believe we ought to be moire
> > > > explicit about disabling the keyslot by having the "active" field.
> > > > eg
> > > > 
> > > > { 'struct': 'LUKSKeyslotUpdate',
> > > >   'data': {
> > > >           'active': 'bool',
> > > >           '*keyslot': 'int',
> > > >           '*old-secret': 'str',
> > > >           '*new-secret' : 'str',
> > > >           '*iter-time' : 'int' } }
> > > > 
> > > > "new-secret" is thus only needed when "active" == true.
> > > 
> > > Hm. At the very least, I would make 'active' optional and default to
> > > true, so that for adding or updating you must only specify 'new-secret'
> > > and for deleting only 'active'.
> > 
> > Is that asymmetry really worth while ? It merely saves a few
> > characters of typing by omitting "active: true", so I'm not
> > really convinced.
> > 
> > > 
> > > > This avoids the problem with being unable to specify a null for
> > > > StrOrNull on the command line too.
> > > 
> > > If we ever get a way to pass null on the command line, how would we
> > > think about a struct like this? Will it still feel right, or will it
> > > feel like we feel about simple unions today (they exist, we would like
> > > to get rid of them, but we can't because compatibility)?
> > 
> > Personally I really don't like the idea of using "new-secret:null"
> > as a way to request deletion of a keyslot. That's too magical
> > for an action that is so dangerous to data IMhO.
> > 
> > I think of these operations as activating & deactivating keyslots,
> > hence my suggestion to use an explicit "active: true|false" to
> > associate the core action being performed, instead of inferring
> > the action indirectly from the secret.
> 
> The general idea of the amend interface is more that you describe a
> desired state rather than operations to achieve it.
> 
> > I think this could lend itself better to future extensions too.
> > eg currently we're just activating or deactivating a keyslot.
> > it is conceivable in future (LUKS2) we might want to modify an
> > existing keyslot in some way. In that scenario, "active" can
> > be updated to be allowed to be optional such that:
> > 
> >  - active: true ->  activate a currently inactive keyslot
> >  - active: false -> deactivate a currently active keyslot
> >  - active omitted -> modify a currently active keyslot
> 
> This distinction feels artificial to me. All three operations just
> change the content of a keyslot. Whether it contained a key or not in
> the old state shouldn't make a difference for how to get a new value
> (which could be a new key or just an empty keyslot) written to it.

There is an explicit "active" state associated with keyslots on disk
and in the LUKS crypto data structures in QEMU. So this is simply
exposing "active" as a field in the amend interface, directly.

This also matches up with the "qemu-img info" output for a luks
volume which reports the "active" state of each key slot.

> Making an omitted key mean something different from the other options so
> that it's not just defaulting to one of them is problematic, too. We
> have at least one place where it works like this (backing files) and it
> tends to give us headaches.

Omitting "active" in my example above, just means that we're doing
something that is not changing the "active" state in the keyslot
on disk. 

Regards,
Daniel
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