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

[Pascal Van Leeuwen <pvanleeuwen@verimatrix.com>]

> -----Original Message-----
> From: Ondrej Mosnáček <omosnacek@gmail.com>
> Sent: Wednesday, August 7, 2019 10:33 PM
> To: Milan Broz <gmazyland@gmail.com>
> Cc: Pascal Van Leeuwen <pvanleeuwen@verimatrix.com>; Pascal van Leeuwen
> <pascalvanl@gmail.com>; linux-crypto@vger.kernel.org; rsnel@cube.dyndns.org;
> herbert@gondor.apana.org.au; davem@davemloft.net
> Subject: Re: [PATCH] crypto: xts - Add support for Cipher Text Stealing
> 
> 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.
> 
Ah, ok, I did not know anything about that ... so there's really no way
I could've done this correctly or to have found the problem myself really.
Good that it's resolved now, though.

I fixed a couple of other minor things already, is it OK if I roll this
into an update to my original patch?

Regards,
Pascal van Leeuwen
Silicon IP Architect, Multi-Protocol Engines @ Verimatrix
www.insidesecure.com