linux-kernel - Re: [PATCH] crypto: caam - properly set IV after {en,de}crypt

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Message-ID: <VI1PR0402MB3342A3AB9EEC3B36A79F253698960@VI1PR0402MB3342.eurprd04.prod.outlook.com>
Date:   Tue, 5 Sep 2017 15:33:01 +0000
From:   Horia Geantă <horia.geanta@....com>
To:     Gilad Ben-Yossef <gilad@...yossef.com>
CC:     David Gstir <david@...ma-star.at>,
        Dan Douglass <dan.douglass@....com>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "richard@...ma-star.at" <richard@...ma-star.at>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] crypto: caam - properly set IV after {en,de}crypt

On 8/14/2017 10:59 AM, Gilad Ben-Yossef wrote:
> Hi,
> 
> On Thu, Jun 29, 2017 at 1:19 PM, Horia Geantă <horia.geanta@....com> wrote:
>> On 6/28/2017 4:42 PM, Horia Geantă wrote:
>>> On 6/28/2017 4:27 PM, David Gstir wrote:
>>>> Certain cipher modes like CTS expect the IV (req->info) of
>>>> ablkcipher_request (or equivalently req->iv of skcipher_request) to
>>>> contain the last ciphertext block when the {en,de}crypt operation is done.
>>>> This is currently not the case for the CAAM driver which in turn breaks
>>>> e.g. cts(cbc(aes)) when the CAAM driver is enabled.
>>>>
>>>> This patch fixes the CAAM driver to properly set the IV after the
>>>> {en,de}crypt operation of ablkcipher finishes.
>>>>
>>>> This issue was revealed by the changes in the SW CTS mode in commit
>>>> 0605c41cc53ca ("crypto: cts - Convert to skcipher")
>>>>
>>>> Cc: <stable@...r.kernel.org> # 4.8+
>>>> Signed-off-by: David Gstir <david@...ma-star.at>
>>> Reviewed-by: Horia Geantă <horia.geanta@....com>
>>>
>> Btw, instead of updating the IV in SW, CAAM engine could be programmed
>> to do it - by saving the Context Register of the AES accelerator.
>>
>> Unfortunately this would require changes in quite a few places: shared
>> descriptor, HW S/G generation logic, IV dma (un)mapping and maybe others.
>>
>> So it's better to have this fix now (which, considering size, is
>> appropriate for -stable) and later, if needed, offload IV updating in HW.
>>
> 
> My apologies for reviving this thread from the dead, but doesn't the patch fail
> for in-place decryption since we are copying from req->dst after
> the operation is done, and therefore it no longer contains the ciphertext?
> 
You are right, thanks! Will follow up with a fix.
Though cts(cbc(aes)) in particular is working, see below.

> I'm asking since I ran into a similar issue in the ccree driver and thought
> to deploy a similar fix but could not convince myself why this works.
> 
IIUC cts(cbc(aes)) in-place decryption (with cbc(aes) offloaded to CAAM
engine) works since SW implementation of cts, when performing the
ciphertext stealing phase in cts_cbc_decrypt() does not use req->iv, but
a previously value, saved before staring decryption in crypto_cts_decrypt():

if (cbc_blocks <= 1)
	memcpy(space, req->iv, bsize);
else
	scatterwalk_map_and_copy(space, req->src, offset - 2 * bsize,
                                 bsize, 0);

Horia