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Date:   Wed, 23 Feb 2022 17:53:07 -0800
From:   Eric Biggers <ebiggers@kernel.org>
To:     Gilad Ben-Yossef <gilad@benyossef.com>
Cc:     Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        Ofir Drang <ofir.drang@arm.com>,
        Corentin Labbe <clabbe.montjoie@gmail.com>,
        stable@vger.kernel.org, linux-crypto@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] crypto: drbg: fix crypto api abuse
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References: <20220223080400.139367-1-gilad@benyossef.com>
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On Wed, Feb 23, 2022 at 05:47:25PM -0800, Eric Biggers wrote:
> On Wed, Feb 23, 2022 at 10:04:00AM +0200, Gilad Ben-Yossef wrote:
> > the drbg code was binding the same buffer to two different
> > scatter gather lists and submitting those as source and
> > destination to a crypto api operation, thus potentially
> > causing HW crypto drivers to perform overlapping DMA
> > mappings which are not aware it is the same buffer.
> > 
> > This can have serious consequences of data corruption of
> > internal DRBG buffers and wrong RNG output.
> > 
> > Fix this by reusing the same scatter gatther list for both
> > src and dst.
> > 
> > Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
> > Reported-by: Corentin Labbe <clabbe.montjoie@gmail.com>
> > Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
> > Tested-on: r8a7795-salvator-x
> > Tested-on: xilinx-zc706
> > Fixes: 43490e8046b5d ("crypto: drbg - in-place cipher operation for CTR")
> > Cc: stable@vger.kernel.org
> 
> Where is it documented and tested that the API doesn't allow this?
> I wasn't aware of this case; it sounds perfectly allowed to me.
> There might be a lot of other users who do this, not just drbg.c.
> 

Just quickly looking through the code I maintain, there is another place that
uses scatterlists like this: in fscrypt_crypt_block() in fs/crypto/crypto.c, the
source and destination can be the same.  That's just the code I maintain; I'm
sure if you looked through the whole kernel you'd find a lot more.

This sounds more like a driver bug, and a case we need to add self-tests for.

- Eric