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

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Oct 17, 2024 at 10:17:04PM +0200, Milan Broz wrote:
> 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.
> 

In "FIPS mode" such keys are always rejected, but otherwise it is opt-in via the
flag CRYPTO_TFM_REQ_FORBID_WEAK_KEYS.  dm-crypt doesn't use that flag.

We could certainly try to fix that in dm-crypt, though I expect that some
dm-crypt users have started relying on such keys.  It is a common misconception
that XTS is secure when the two halves of the key are the same.

- Eric