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From:   Stephan Mueller <stephan.mueller@atsec.com>
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     Ondrej =?utf-8?B?TW9zbsOhxI1law==?= <omosnacek@gmail.com>,
        linux-crypto@vger.kernel.org, Sahana Prasad <saprasad@redhat.com>,
        Tomas Mraz <tmraz@redhat.com>, Ard Biesheuvel <ardb@kernel.org>
Subject: Re: libkcapi tests are failing on kernels 5.5+
Date:   Tue, 28 Apr 2020 10:19:19 +0200
Message-ID: <2055037.HuOjPaZDMi@tauon.chronox.de>
Organization: atsec information security GmbH
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Am Dienstag, 21. April 2020, 11:19:36 CEST schrieb Stephan Mueller:

Hi Herbert,

could you please help us with the answer to the question below?

> Am Dienstag, 21. April 2020, 10:08:14 CEST schrieb Ondrej Mosnáček:
> 
> Hi Ondrej,
> 
> > Hi all,
> > 
> > the libkcapi [1] tests are failing on kernels 5.5-rc1 and above [2].
> > All encryption/decryption tests that use 'ctr(aes)' and a message size
> > that is not a multiple of 16 fail due to kcapi-enc returning different
> > output than expected.
> 
> Confirmed.
> 
> On the recent kernels, the data generated by kcapi-enc contains trailing
> zero bytes for data that is a fraction of the block size.
> 
> I think the issue is in the following kernel code in _skcipher_recvmsg:
> 
>         unsigned int bs = crypto_skcipher_chunksize(tfm);
> 
>         /*
>          * If more buffers are to be expected to be processed, process only
>          * full block size buffers.
>          */
>         if (ctx->more || len < ctx->used)
>                 len -= len % bs;
> 
> 
> The kernel truncates the size to be processed to the chunk size. As the
> chunksize returns the block size of the underlying cipher (e.g. AES -> 16),
> the kernel code will not process non-aligned data.
> 
> Herbert, could you help me identifying what exactly was the root cause for
> the patch 5b0fe9552336338acb52756daf65dd7a4eeca73f ? I.e. it seems that
> stream ciphers made out of a block cipher would not generate the data part
> that is a fraction of the block size (e.g. CTR, CTS).
> 
> Ciao
> Stephan


Ciao
Stephan
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