On Tue, 2020-03-03 at 11:18 +0200, Maxim Levitsky wrote:
> On Sat, 2020-02-15 at 15:51 +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.
> >
> > 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?
>
> I tried today to implement this but I hit a very unpleasant roadblock:
>
> Since QCrypto is generic (even though it only implements currently luks for
> raw/qcow2 usage,
> and legacy qcow2 aes encryption), I still can't assume that this is always
> the case.
> Thus I implemented the Qcrypto amend API in this way:
>
> ##
> # @QCryptoBlockAmendOptions:
> #
> # The options that are available for all encryption formats
> # when amending encryption settings
> #
> # Since: 5.0
> ##
> { 'union': 'QCryptoBlockAmendOptions',
> 'base': 'QCryptoBlockOptionsBase',
> 'discriminator': 'format',
> 'data': {
> 'luks': 'QCryptoBlockAmendOptionsLUKS' } }
>
> However the QCryptoBlockAmendOptionsLUKS is a union too to be in line with
> the API proposal,
> but that is not supported on QAPI level and after I and Markus talked about
> we are not sure
> that it is worth it to implement this support only for this case.
>
> So far I see the following solutions
>
>
> 1. Drop the QCryptoBlockAmendOptionsLUKS union for now.
> This will bring the schema pretty much to be the same as my original proposal,
> however the API will be the same thus once nested unions are implemented this
> union
> can always be introduced again.
>
> 2. Drop the QCryptoBlockAmendOptions union. Strictly speaking this union is
> not needed
> since it only has one member anyway, however this union is used both by qcow2
> QAPI scheme,
> so that it doesn't hardcode an encryption format for amend just like it
> doesn't for creation,
> (this can be hardcoded for now as well for now as long as we don't have more
> amendable encryption formats).
> However I also use the QCryptoBlockAmendOptions in C code in QCrypto API thus
> it will be ugly to use the
> QCryptoBlockAmendOptionsLUKS instead.
>
>
> 3. Make QCryptoBlockAmendOptionsLUKS a struct and add to it a nested member
> with new union type
> (say QCryptoBlockAmendOptionsLUKS1) which will be exactly as
> QCryptoBlockAmendOptionsLUKS was.
>
> This IMHO is even uglier since it changes the API (which we can't later fix)
> and adds both a dummy struct
> field and a dummy struct name.
>
> I personally vote 1.
Any update?
>
> Best regards,
> Maxim Levitsky
>
>
>
