Re: [PATCH] crypto: xts - Add support for Cipher Text Stealing

["Ondrej Mosnáček" <omosnacek@gmail.com>]

st 7. 8. 2019 o 19:44 Milan Broz <gmazyland@gmail.com> napísal(a):
> On 07/08/2019 17:13, Pascal Van Leeuwen wrote:
> >>>> Seems there is no mistake in your code, it is some bug in aesni_intel implementation.
> >>>> If I disable this module, it works as expected (with aes generic and aes_i586).
> >>>>
> >>> That's odd though, considering there is a dedicated xts-aes-ni implementation,
> >>> i.e. I would not expect that to end up at the generic xts wrapper at all?
> >>
> >> Note it is 32bit system, AESNI XTS is under #ifdef CONFIG_X86_64 so it is not used.
> >>
> > Ok, so I guess no one bothered to make an optimized XTS version for i386.
> > I quickly browsed through the code - took me a while to realise the assembly is
> > "backwards" compared to the original Intel definition :-) - but I did not spot
> > anything obvious :-(
> >
> >> I guess it only ECB part ...
>
> Mystery solved, the skcipher subreq must be te last member in the struct.
> (Some comments in Adiantum code mentions it too, so I do not think it
> just hides the corruption after the struct. Seems like another magic requirement
> in crypto API :-)

Oh, yes, this makes sense! I would have noticed this immediately if I
had looked carefully at the struct definition :) The reason is that
the skcipher_request struct is followed by a variable-length request
context. So when you want to nest requests, you need to make the
subrequest the last member and declare your request context size as:
size of your request context struct + size of the sub-algorithm's
request context.

It is a bit confusing, but it is the only reasonable way to support
variably sized context and at the same time keep the whole request in
a single allocation.

>
> This chunk is enough to fix it for me:
>
> --- a/crypto/xts.c
> +++ b/crypto/xts.c
> @@ -33,8 +33,8 @@ struct xts_instance_ctx {
>
>  struct rctx {
>         le128 t, tcur;
> -       struct skcipher_request subreq;
>         int rem_bytes, is_encrypt;
> +       struct skcipher_request subreq;
>  };
>
> While at it, shouldn't be is_encrypt bool?
>
> Thanks,
> Milan