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Re: API definition for LUKS key management

From: Max Reitz
Subject: Re: API definition for LUKS key management
Date: Tue, 12 Nov 2019 11:47:00 +0100
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On 12.11.19 10:12, Kevin Wolf wrote:
> Am 11.11.2019 um 19:34 hat Daniel P. Berrangé geschrieben:
>> On Mon, Nov 11, 2019 at 05:58:20PM +0200, Maxim Levitsky wrote:
>>> One of the concerns that was raised during the review was that amend 
>>> interface for luks that I propose is
>>> different from the amend inteface used currently for qcow2.
>>> qcow2 amend interface specifies all the format options, thus overwrites the 
>>> existing options.
>>> Thus it seems natural to make the luks amend interface work the same way, 
>>> that it receive an array
>>> of 8 slots, and for each slot specify if it is active, and if true what 
>>> password to put in it.
>>> This does allow to add and erase the keyslots, but it doesn't allow:
>>>    * add a password without knowing all other passwords that exist in 
>>> existing keyslots
>>>      this can be mitigated by specifying which keyslots to modify for 
>>> example by omitting the
>>>      keyslots that shouldn't be touched from the array (passing null 
>>> placeholder instead)
>>>      but then it already doesn't follow the 'specify all the options each 
>>> time' principle.
>> I think this is highly undesirable, as we must not assume that the
>> mgmt app has access to all the passwords currently set.
> And I think this shows the problem that we realy have with the crypto
> driver and amend: For every other driver, if you must, you can query the
> current settings and just write them back.
> The difference here is that crypto doesn't allow to directly query or
> specify the content of some options (the keyslots), but provides only a
> way to derives that content from a secret, and obviously there is no way
> back from the stored data to the secret (that's what it's for).
> I think we have two options here:
> 1. Add a special "don't touch this" value for keyslots. Normally, just
>    leaving out the value would be suitable syntax for this. Here,
>    however, we have a list of keyslots, so we can't leave anything out.
>    We could use something like an alternate between str (new secret ID),
>    null (erase keyslot) and empty dict (leave it alone) - the latter
>    feels a bit hackish, but maybe it's not too bad.

I thought of something similar, but how would that look on the command line?

Though I suppose if the worst thing were how it looks on the command
line, we could introduce a new qemu-img subcommand that then internally
translates into the right amend syntax.

>    If the list is
>    shorter than 8 entries, the rest is assumed to mean "leave it alone",
>    too.
> 2. Allow to query and set the raw key, which doesn't require a password
>> The two key use cases for having multiple key slots are
>>   - To enable a two-phase change of passwords to ensure new password
>>     is safely written out before erasing the old password
>>   - To allow for multiple access passwords with different controls
>>     or access to when each password is made available.
>>     eg each VM may have a separate "backup password" securely
>>     stored off host that is only made available for use when
>>     doing disaster recovery.
>> the second use case is doomed if you need to always provide all
>> current passwords when changing any key slots.
> That providing all current passwords doesn't work is obvious.
>>>    * erase all keyslots matching a password - this is really hard to do 
>>> using this approach,
>>>      unless we give user some kind of api to try each keyslot with given 
>>> password,
>>>      which is kind of ugly and might be racy as well.
>>> So what do you think?
>> The point of using "amend" is that we already have some of the boilerplate
>> supporting framework around that, so it saves effort for both QEMU and
>> our users. If the semantics of "amend" don't fit nicely though, then the
>> benefit of re-using "amend" is cancelled out and we should go back to
>> considering a separate "key-manage" command.
> This wouldn't solve the fundamental problem that the crypto block
> driver, as it currently is, isn't able to provide a blockdev-amend
> callback. It's worse for qcow2 because qcow2 already implements amend.

Hm, well, I would have assumed this is only bad on the premise that we
want to have amend be complete at some point.  Do we?

While I do think it might be nice to be able to change e.g. cluster_size
especially for the upcoming subcluster extension (in addition to
enabling subclusters on an existing image), I seriously doubt anyone’s
going to implement it.  (Maybe enabling subclusters, but not changing

> I think we need to find a solution for the amend API.

I do think it’s weird to look for non-amend solutions when it clearly
looks like an amend problem, but OTOH I don’t think it would be that bad
to disregard amend.  (Provided there are good reasons for disregarding it.)


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