Re: [PATCH v6 11/12] crypto: x86/aes-kl - Support AES algorithm using Key Locker instructions

Milan Broz
2023-05-12T17:53:01Z

On 5/6/23 02:01, Eric Biggers wrote:
...
> This does not correctly describe what is going on. Actually, this patchset
> registers the AES-KL XTS algorithm with the usual name "xts(aes)". So, it can
> potentially be used by any AES-XTS user. It seems that you're actually relying
> on the algorithm priorities to prioritize AES-NI, as you've assigned priority
> 200 to AES-KL, whereas AES-NI has priority 401. Is that what you intend, and if
> so can you please update your explanation to properly explain this?
>
> The alternative would be to use a unique algorithm name, such as
> "keylocker-xts(aes)". I'm not sure that would be better, given that the
> algorithms are compatible. However, that actually would seem to match the
> explanation you gave more closely, so perhaps that's what you actually intended?

Sorry to be late in-game, but as this is intended for LUKS/dm-crypt use,
I have a comment here:

LUKS2 will no longer support algorithms with the dash in the name for dm-crypt
(iow "aes-generic" or something like that will no longer work, and I am afraid
you will need aes-kl/keylocker-xts here to force to use AES-KL for dm-crypt).

One reason is described in https://gitlab.com/cryptsetup/cryptsetup/-/issues/809,
but the major problem is that cryptsetup used CIPHER-MODE-IV syntax (that mixes
badly with the dash in algorithm names). And we still rely on internal conversions
of common modes to that syntax (currently it worked only by a luck).

When I added the "capi" format for dm-crypt for algorithms specification,
I made a mistake in that it allows everything, including crypto driver
platform-specific names.
The intention was to keep the kernel to decide which crypto driver will be used.
So, this is perhaps fine for dm-crypt now but LUKS is a portable format, and a generic
algorithm (like AES) should not depend on a specific driver or CPU feature.

IOW, implement xts(aes) and let the user prioritize the driver (no changes
needed for LUKS header then, AES-KL is loaded automatically) or/and create a wrapper
(similar to paes, that we already support) that will force to use AES-KL
(...but without the dash in the name, please :)

If there is a problem with it, please create an issue for cryptsetup upstream
to discuss it there (before the kernel part is merged!), so we can find some
solution - I would like to avoid incompatibilities later.

Thanks,
Milan