Re: [RFC PATCH v2 2/2] dm-inlinecrypt: add target for inline block device encryption

[Milan Broz <gmazyland@gmail.com>]

On 10/17/24 9:44 PM, Eric Biggers wrote:
> On Wed, Oct 16, 2024 at 04:27:48PM -0700, Eric Biggers wrote:
>> Add a new device-mapper target "dm-inlinecrypt" that is similar to
>> dm-crypt but uses the blk-crypto API instead of the regular crypto API.
>> This allows it to take advantage of inline encryption hardware such as
>> that commonly built into UFS host controllers.
> 
> A slight difference in behavior vs. dm-crypt that I just became aware of:
> dm-crypt allows XTS keys whose first half equals the second half, i.e.
> cipher key == tweak key.  dm-inlinecrypt typically will not allow this.  Inline
> encryption hardware typically rejects such keys, and blk-crypto-fallback rejects
> them too because it uses CRYPTO_TFM_REQ_FORBID_WEAK_KEYS.
> 
> IMO, rejecting these weak keys is desirable, and the fact that dm-inlinecrypt
> fixes this issue with dm-crypt will just need to be documented.

Hm, I thought this is already rejected in crypto API (at least in FIPS mode)...

It should be rejected exactly as you described even for dm-crypt,
just the check should be (IMO) part of crypto API (set keys), not dm-crypt itself.

And here I think we should not be backward "compatible" as it is security issue,
both XTS keys just must not be the same.

Milan