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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 19 May 2020 19:53:57 +0200
Message-ID: <CAMj1kXEiijbgq=P53+qKjEzuneiinDbwyBacpMWtcN2YwEKWKA@mail.gmail.com>
Subject: Re: ARM CE: CTS IV handling
To:     Stephan Mueller <smueller@chronox.de>
Cc:     Eric Biggers <ebiggers@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>
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On Tue, 19 May 2020 at 19:50, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Tue, 19 May 2020 at 19:35, Stephan Mueller <smueller@chronox.de> wrote:
> >
> > Am Dienstag, 19. Mai 2020, 18:21:01 CEST schrieb Ard Biesheuvel:
> >
> > Hi Ard,
> >
> > >
> > > To be honest, this looks like the API is being used incorrectly. Is
> > > this a similar issue to the one Herbert spotted recently with the CTR
> > > code?
> > >
> > > When you say 'leaving the TFM untouched' do you mean the skcipher
> > > request? The TFM should not retain any per-request state in the first
> > > place.
> > >
> > > The skcipher request struct is not meant to retain any state either -
> > > the API simply does not support incremental encryption if the input is
> > > not a multiple of the chunksize.
> > >
> > > Could you give some sample code on how you are using the API in this case?
> >
> > What I am doing technically is to allocate a new tfm and request at the
> > beginning and then reuse the TFM and request. In that sense, I think I violate
> > that constraint.
> >
> > But in order to implement such repetition, I can surely clear / allocate a new
> > TFM. But in order to get that right, I need the resulting IV after the cipher
> > operation.
> >
> > This IV that I get after the cipher operation completes is different between C
> > and CE.
> >
>
> So is the expected output IV simply the last block of ciphertext that
> was generated (as usual), but located before the truncated block in
> the output?

If so, does the below fix the encrypt case?

index cf618d8f6cec..22f190a44689 100644
--- a/arch/arm64/crypto/aes-modes.S
+++ b/arch/arm64/crypto/aes-modes.S
@@ -275,6 +275,7 @@ AES_FUNC_START(aes_cbc_cts_encrypt)
        add             x4, x0, x4
        st1             {v0.16b}, [x4]                  /* overlapping stores */
        st1             {v1.16b}, [x0]
+       st1             {v1.16b}, [x5]
        ret
 AES_FUNC_END(aes_cbc_cts_encrypt)