linux-kernel - Re: [RFC v3] crypto: ccree - protect against short scatterlists

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Date:   Mon, 27 Jan 2020 18:55:51 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Gilad Ben-Yossef <gilad@...yossef.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Ofir Drang <ofir.drang@....com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC v3] crypto: ccree - protect against short scatterlists

On Mon, Jan 27, 2020 at 05:08:21PM +0200, Gilad Ben-Yossef wrote:
> Deal gracefully with the event of being handed a scatterlist
> which is shorter than expected.
> 
> This mitigates a crash in some cases due to
> attempt to map empty (but not NULL) scatterlists with none
> zero lengths.
> 
> Signed-off-by: Gilad Ben-Yossef <gilad@...yossef.com>
> Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>

It's definitely wrong use of the crypto API to pass a scatterlist that's too
short.  Note that this is *not* what the test code is doing.

So I don't think you should be hacking around it here.

It is possible the bug is actually in cc_aead_chain_data()?  It looks like it's
adding the authentication tag size to the source data size for encryption, which
is not correct.  The authentication tag is part of the destination only.

	size_for_map += (direct == DRV_CRYPTO_DIRECTION_ENCRYPT) ?
			authsize : 0;
	src_mapped_nents = cc_get_sgl_nents(dev, req->src, size_for_map,
					    &src_last_bytes);

- Eric