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

[Daniel P . Berrangé]

On Sat, Feb 15, 2020 at 03:51:46PM +0100, Markus Armbruster wrote:
> 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.

OK

> 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.
> 
> 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?

This is all fine with me. I have no strong opinion on the handful of
options listed above, so fine with any choices out of them.

Regards,
Daniel
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