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

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> Am 06.02.2020 um 14:36 hat Daniel P. Berrangé geschrieben:
>> On Thu, Feb 06, 2020 at 02:20:11PM +0100, Markus Armbruster wrote:
>> > One more question regarding the array in
>> > 
>> >     { 'struct': 'QCryptoBlockAmendOptionsLUKS',
>> >       'data' : {
>> >                 'keys': ['LUKSKeyslotUpdate'],
>> >                  '*unlock-secret' : 'str' } }
>> > 
>> > Why an array?  Do we really need multiple keyslot updates in one amend
>> > operation?
>> 
>> I think it it is unlikely we'd use this in libvirt. In the case of wanting
>> to *change* a key, it is safer to do a sequence of "add key" and then
>> "remove key". If you combine them into the same operation, and you get
>> an error back, it is hard to know /where/ it failed ? was the new key
>> added or not ?
>
> I think the array came in because of the "describe the new state"
> approach. The state has eight keyslots, so in order to fully describe
> the new state, you would have to be able to pass multiple slots at once.

I see.

Of course, it can also describe multiple new states for the same slot.

Example:

    [{'state': 'active', 'keyslot': 0, 'secret': 'sec0'},
     {'state': 'active', 'keyslot': 0, 'secret': 'sec1'}]

    where slot 0's old state is 'inactive'.

Which one is the new state?

If we execute the array elements one by one, this first makes slot 0
active with secret 'sec0', then tries to make it active with secret
'sec1', which fails.  Simple enough, but it's not really "describe the
new state", it's still "specify a series of state transitions".

If we merge the array elements into a description of the new state of
all eight slots, where a slot's description can be "same as old state",
then this makes slot 0 active with either secret 'sec0' or 'sec1',
depending on how we resolve the conflict.  We could even make conflicts
an error, and then this would fail without changing anything.

What do we want?

Is this worth the trouble?