Re: [RFC/RFT PATCH 02/15] crypto: morus - fix handling chunked inputs

[Eric Biggers]

On Thu, Jan 31, 2019 at 10:05:17AM +0100, Ondrej Mosnacek wrote:
> Hi Eric,
> 
> On Wed, Jan 23, 2019 at 11:52 PM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
> > From: Eric Biggers <ebiggers@xxxxxxxxxx>
> >
> > The generic MORUS implementations all fail the improved AEAD tests
> > because they produce the wrong result with some data layouts.  Fix them.
> >
> > Fixes: 396be41f16fd ("crypto: morus - Add generic MORUS AEAD implementations")
> > Cc: <stable@xxxxxxxxxxxxxxx> # v4.18+
> > Cc: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
> > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> > ---
> >  crypto/morus1280.c | 13 +++++++------
> >  crypto/morus640.c  | 13 +++++++------
> >  2 files changed, 14 insertions(+), 12 deletions(-)
> >
> > diff --git a/crypto/morus1280.c b/crypto/morus1280.c
> > index 3889c188f266..b83576b4eb55 100644
> > --- a/crypto/morus1280.c
> > +++ b/crypto/morus1280.c
> > @@ -366,18 +366,19 @@ static void crypto_morus1280_process_crypt(struct morus1280_state *state,
> >                                            const struct morus1280_ops *ops)
> >  {
> >         struct skcipher_walk walk;
> > -       u8 *dst;
> > -       const u8 *src;
> >
> >         ops->skcipher_walk_init(&walk, req, false);
> >
> >         while (walk.nbytes) {
> > -               src = walk.src.virt.addr;
> > -               dst = walk.dst.virt.addr;
> > +               unsigned int nbytes = walk.nbytes;
> >
> > -               ops->crypt_chunk(state, dst, src, walk.nbytes);
> > +               if (nbytes < walk.total)
> > +                       nbytes = round_down(nbytes, walk.stride);
> >
> > -               skcipher_walk_done(&walk, 0);
> > +               ops->crypt_chunk(state, walk.dst.virt.addr, walk.src.virt.addr,
> > +                                nbytes);
> > +
> > +               skcipher_walk_done(&walk, walk.nbytes - nbytes);
> >         }
> 
> Hm... I assume the problem was that skcipher_walk may give you nbytes
> that is not rounded to the algorithm's walksize (aka walk.stride) even
> in the middle of the stream, right? I must have misread the code in
> skcipher.c and assumed that it automatically makes every step of a
> round size, with the exception of the very last one, which may include
> a final partial block. Thinking about it now it makes sense to me that
> this isn't the case, since for stream ciphers it would mean some
> unnecessary memcpy'ing in the skcipher_walk code...
> 
> Maybe you could explain the problem in one or two sentences in the
> commit message(s), since the contract of skcipher_walk doesn't seem to
> be documented anywhere and so it is not obvious why the change is
> necessary.
> 
> Thank you for all your work on this - the improved testmgr tests were
> long needed!
> 

Yes, that's correct.  I'll add an explanation to the commit messages.  I'm
actually not convinced that this behavior should be the default (vs. allowing
the caller to opt-in to it if needed), but this is how it works now...  

A while back I also started work on a documentation patch for the skcipher_walk
functions, but it's still pretty incomplete.  I'll see if I can get it in a
state to send out.

- Eric