Re: QAPI schema for desired state of LUKS keyslots (was: [PATCH 02/13] qcrypto-luks: implement encryption key management)

[Maxim Levitsky]

On Mon, 2020-02-17 at 11:37 +0100, Kevin Wolf wrote:
> Am 15.02.2020 um 15:51 hat Markus Armbruster geschrieben:
> > Review of this patch led to a lengthy QAPI schema design discussion.
> > Let me try to condense it into a concrete proposal.
> > 
> > This is about the QAPI schema, and therefore about QMP.  The
> > human-friendly interface is out of scope.  Not because it's not
> > important (it clearly is!), only because we need to *focus* to have a
> > chance at success.
> > 
> > I'm going to include a few design options.  I'll mark them "Option:".
> > 
> > The proposed "amend" interface takes a specification of desired state,
> > and figures out how to get from here to there by itself.  LUKS keyslots
> > are one part of desired state.
> > 
> > We commonly have eight LUKS keyslots.  Each keyslot is either active or
> > inactive.  An active keyslot holds a secret.
> > 
> > Goal: a QAPI type for specifying desired state of LUKS keyslots.
> > 
> > Proposal:
> > 
> >     { 'enum': 'LUKSKeyslotState',
> >       'data': [ 'active', 'inactive' ] }
> > 
> >     { 'struct': 'LUKSKeyslotActive',
> >       'data': { 'secret': 'str',
> >                 '*iter-time': 'int } }
> > 
> >     { 'struct': 'LUKSKeyslotInactive',
> >       'data': { '*old-secret': 'str' } }
> > 
> >     { 'union': 'LUKSKeyslotAmend',
> >       'base': { '*keyslot': 'int',
> >                 'state': 'LUKSKeyslotState' }
> >       'discriminator': 'state',
> >       'data': { 'active': 'LUKSKeyslotActive',
> >                 'inactive': 'LUKSKeyslotInactive' } }
> > 
> > LUKSKeyslotAmend specifies desired state for a set of keyslots.
> 
> Though not arbitrary sets of keyslots, it's only a single keyslot or
> multiple keyslots containing the same secret. Might be good enough in
> practice, though it means that you may have to issue multiple amend
> commands to get to the final state that you really want (even if doing
> everything at once would be safe).
> 
> > Four cases:
> > 
> > * @state is "active"
> > 
> >   Desired state is active holding the secret given by @secret.  Optional
> >   @iter-time tweaks key stretching.
> > 
> >   The keyslot is chosen either by the user or by the system, as follows:
> > 
> >   - @keyslot absent
> > 
> >     One inactive keyslot chosen by the system.  If none exists, error.
> > 
> >   - @keyslot present
> > 
> >     The keyslot given by @keyslot.
> > 
> >     If it's already active holding @secret, no-op.  Rationale: the
> >     current state is the desired state.
> > 
> >     If it's already active holding another secret, error.  Rationale:
> >     update in place is unsafe.
> > 
> >     Option: delete the "already active holding @secret" case.  Feels
> >     inelegant to me.  Okay if it makes things substantially simpler.
> > 
> > * @state is "inactive"
> > 
> >   Desired state is inactive.
> > 
> >   Error if the current state has active keyslots, but the desired state
> >   has none.
> > 
> >   The user choses the keyslot by number and/or by the secret it holds,
> >   as follows:
> > 
> >   - @keyslot absent, @old-secret present
> > 
> >     All active keyslots holding @old-secret.  If none exists, error.
> > 
> >   - @keyslot present, @old-secret absent
> > 
> >     The keyslot given by @keyslot.
> > 
> >     If it's already inactive, no-op.  Rationale: the current state is
> >     the desired state.
> > 
> >   - both @keyslot and @old-secret present
> > 
> >     The keyslot given by keyslot.
> > 
> >     If it's inactive or holds a secret other than @old-secret, error.
> > 
> >     Option: error regardless of @old-secret, if that makes things
> >     simpler.
> > 
> >   - neither @keyslot not @old-secret present
> > 
> >     All keyslots.  Note that this will error out due to "desired state
> >     has no active keyslots" unless the current state has none, either.
> > 
> >     Option: error out unconditionally.
> > 
> > Note that LUKSKeyslotAmend can specify only one desired state for
> > commonly just one keyslot.  Rationale: this satisfies practical needs.
> > An array of LUKSKeyslotAmend could specify desired state for all
> > keyslots.  However, multiple array elements could then apply to the same
> > slot.  We'd have to specify how to resolve such conflicts, and we'd have
> > to code up conflict detection.  Not worth it.
> > 
> > Examples:
> > 
> > * Add a secret to some free keyslot:
> > 
> >   { "state": "active", "secret": "CIA/GRU/MI6" }
> > 
> > * Deactivate all keyslots holding a secret:
> > 
> >   { "state": "inactive", "old-secret": "CIA/GRU/MI6" }
> > 
> > * Add a secret to a specific keyslot:
> > 
> >   { "state": "active", "secret": "CIA/GRU/MI6", "keyslot": 0 }
> > 
> > * Deactivate a specific keyslot:
> > 
> >   { "state": "inactive", "keyslot": 0 }
> > 
> >   Possibly less dangerous:
> > 
> >   { "state": "inactive", "keyslot": 0, "old-secret": "CIA/GRU/MI6" }
> > 
> > Option: Make use of Max's patches to support optional union tag with
> > default value to let us default @state to "active".  I doubt this makes
> > much of a difference in QMP.  A human-friendly interface should probably
> > be higher level anyway (Daniel pointed to cryptsetup).
> > 
> > Option: LUKSKeyslotInactive member @old-secret could also be named
> > @secret.  I don't care.
> > 
> > Option: delete @keyslot.  It provides low-level slot access.
> > Complicates the interface.  Fine if we need lov-level slot access.  Do
> > we?
> > 
> > I apologize for the time it has taken me to write this.
> > 
> > Comments?
> 
> Works for me (without taking any of the options).
> 
> The unclear part is what the human-friendly interface should look like
> and where it should live. I'm afraid doing only the QMP part and calling
> the feature completed like we do so often won't work in this case.

IMHO, I think that the best way to create human friendly part is to implement
luks specific commands for qemu-img and use interface very similar
to what cryptsetup does.

Best regards,
        Maxim Levitsky
> 
> Kevin