[dm-devel] [PATCH v9 3/7] md: dm-crypt: switch to ESSIV crypto API template

[Ard Biesheuvel]

On Mon, 12 Aug 2019 at 16:51, Milan Broz <gmazyland at gmail.com> wrote:
>
> On 12/08/2019 09:50, Ard Biesheuvel wrote:
> > On Mon, 12 Aug 2019 at 10:44, Milan Broz <gmazyland at gmail.com> wrote:
> >>
> >> On 12/08/2019 08:54, Ard Biesheuvel wrote:
> >>> On Mon, 12 Aug 2019 at 09:33, Milan Broz <gmazyland at gmail.com> wrote:
> >>>> Try for example
> >>>> # cryptsetup luksFormat /dev/sdc -c aes-cbc-essiv:sha256 --integrity hmac-sha256 -q -i1
> >>>>
> >>>> It should produce Crypto API string
> >>>>   authenc(hmac(sha256),essiv(cbc(aes),sha256))
> >>>> while it produces
> >>>>   essiv(authenc(hmac(sha256),cbc(aes)),sha256)
> >>>> (and fails).
> >>>>
> >>>
> >>> No. I don't know why it fails, but the latter is actually the correct
> >>> string. The essiv template is instantiated either as a skcipher or as
> >>> an aead, and it encapsulates the entire transformation. (This is
> >>> necessary considering that the IV is passed via the AAD and so the
> >>> ESSIV handling needs to touch that as well)
> >>
> >> Hm. Constructing these strings seems to be more confusing than dmcrypt mode combinations :-)
> >>
> >> But you are right, I actually tried the former string (authenc(hmac(sha256),essiv(cbc(aes),sha256)))
> >> and it worked, but I guess the authenticated IV (AAD) was actually the input to IV (plain sector number)
> >> not the output of ESSIV? Do I understand it correctly now?
> >>
> >
> > Indeed. The former string instantiates the skcipher version of the
> > ESSIV template, and so the AAD handling is omitted, and we end up
> > using the plain IV in the authentication rather than the encrypted IV.
> >
> > So when using the latter string, does it produce any error messages
> > when it fails?
>
> The error is
> table: 253:1: crypt: Error decoding and setting key
>
> and it is failing in crypt_setkey() int this  crypto_aead_setkey();
>
> And it is because it now wrongly calculates MAC key length.
> (We have two keys here - one for length-preserving CBC-ESSIV encryption
> and one for HMAC.)
>
> This super-ugly hotfix helps here... I guess it can be done better :-)
>

Weird. It did work fine before, but now that I have dropped the 'md:
dm-crypt: infer ESSIV block cipher from cipher string directly' patch,
we are probably taking a different code path and hitting this error.

I'll try to fix this cleanly. Thanks for doing the diagnosis.


> diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
> index e9a0093c88ee..7b06d975a2e1 100644
> --- a/drivers/md/dm-crypt.c
> +++ b/drivers/md/dm-crypt.c
> @@ -2342,6 +2342,9 @@ static int crypt_ctr_auth_cipher(struct crypt_config *cc, char *cipher_api)
>         char *start, *end, *mac_alg = NULL;
>         struct crypto_ahash *mac;
>
> +       if (strstarts(cipher_api, "essiv(authenc("))
> +               cipher_api += strlen("essiv(");
> +
>         if (!strstarts(cipher_api, "authenc("))
>                 return 0;
>
> Milan