linux-kernel - Re: [PATCH 1/2] crypto: talitos - Work around SEC6 ERRATA (AES-CTR mode data size error)

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Message-ID: <80ea6d2d-c29e-0c4c-9402-76579fe1eef9@csgroup.eu>
Date:   Thu, 21 Jan 2021 11:12:08 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 1/2] crypto: talitos - Work around SEC6 ERRATA (AES-CTR
 mode data size error)



Le 21/01/2021 à 11:02, Ard Biesheuvel a écrit :
> On Thu, 21 Jan 2021 at 10:54, Christophe Leroy
> <christophe.leroy@...roup.eu> wrote:
>>
>>
>>
>> Le 21/01/2021 à 08:31, Ard Biesheuvel a écrit :
>>> On Thu, 21 Jan 2021 at 06:35, Christophe Leroy
>>> <christophe.leroy@...roup.eu> wrote:
>>>>
>>>>
>>>>
>>>> Le 20/01/2021 à 23:23, Ard Biesheuvel a écrit :
>>>>> On Wed, 20 Jan 2021 at 19:59, Christophe Leroy
>>>>> <christophe.leroy@...roup.eu> wrote:
>>>>>>
>>>>>> Talitos Security Engine AESU considers any input
>>>>>> data size that is not a multiple of 16 bytes to be an error.
>>>>>> This is not a problem in general, except for Counter mode
>>>>>> that is a stream cipher and can have an input of any size.
>>>>>>
>>>>>> Test Manager for ctr(aes) fails on 4th test vector which has
>>>>>> a length of 499 while all previous vectors which have a 16 bytes
>>>>>> multiple length succeed.
>>>>>>
>>>>>> As suggested by Freescale, round up the input data length to the
>>>>>> nearest 16 bytes.
>>>>>>
>>>>>> Fixes: 5e75ae1b3cef ("crypto: talitos - add new crypto modes")
>>>>>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>>>>>
>>>>> Doesn't this cause the hardware to write outside the given buffer?
>>>>
>>>>
>>>> Only the input length is modified. Not the output length.
>>>>
>>>> The ERRATA says:
>>>>
>>>> The input data length (in the descriptor) can be rounded up to the nearest 16B. Set the
>>>> data-in length (in the descriptor) to include X bytes of data beyond the payload. Set the
>>>> data-out length to only output the relevant payload (don't need to output the padding).
>>>> SEC reads from memory are not destructive, so the extra bytes included in the AES-CTR
>>>> operation can be whatever bytes are contiguously trailing the payload.
>>>
>>> So what happens if the input is not 16 byte aligned, and rounding it
>>> up causes it to extend across a page boundary into a page that is not
>>> mapped by the IOMMU/SMMU?
>>>
>>
>> What is the IOMMU/SMMU ?
>>
>> The mpc8xx, mpc82xx and mpc83xx which embed the Talitos Security Engine don't have such thing, the
>> security engine uses DMA and has direct access to the memory bus for reading and writing.
>>
> 
> OK, good. So the only case where this could break is when the DMA
> access spills over into a page that does not exist, and I suppose this
> could only happen if the transfer involves a buffer located at the
> very top of DRAM, right?
> 

Right.

Christophe