linux-kernel - Re: [PATCH v4 11/14] treewide: Prepare to remove VLA usage for AHASH_REQUEST_ON

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Message-ID: <CAKv+Gu-NBKxzV5PYAuzeP5xXfsk=Kw41677ASzRkD6=xO23zeQ@mail.gmail.com>
Date:   Thu, 19 Jul 2018 00:33:54 +0900
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Kees Cook <keescook@...omium.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Alasdair Kergon <agk@...hat.com>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        Lars Persson <larper@...s.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Rabin Vincent <rabinv@...s.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>, qat-linux@...el.com,
        dm-devel@...hat.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 11/14] treewide: Prepare to remove VLA usage for AHASH_REQUEST_ON_STACK

On 19 July 2018 at 00:33, Arnd Bergmann <arnd@...db.de> wrote:
> On Wed, Jul 18, 2018 at 5:19 PM, Ard Biesheuvel
> <ard.biesheuvel@...aro.org> wrote:
>> On 18 July 2018 at 23:50, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>>> On 18 July 2018 at 05:59, Arnd Bergmann <arnd@...db.de> wrote:
>>>> On Sun, Jul 15, 2018 at 6:28 AM, Kees Cook <keescook@...omium.org> wrote:
>>>>>
>>>>> After my ahash to shash conversions, only ccm is left as an ahash
>>>>> user, since it actually uses sg. But with the hard-coded value reduced
>>>>> to 376, this doesn't trip the frame warnings any more. :)
>>>>>
>>>>> I'll send an updated series soon.
>>>>
>>>> Maybe we should get rid of that one as well then and remove
>>>> AHASH_REQUEST_ON_STACK()?
>>>>
>>>> I see that Ard (now on Cc) added this usage only recently. Looking
>>>> at the code some more, I also find that the descsize is probably
>>>> much smaller than 376 for all possible cases   of "cbcmac(*)",
>>>> either alg->cra_blocksize plus a few bytes or sizeof(mac_desc_ctx)
>>>> (i.e. 20) for arch/arm64/crypto/aes-glue.c.
>>>>
>>>> Walking the sglist here means open-coding a shash_ahash_update()
>>>> implementation in crypto_ccm_auth(), that that doesn't seem to
>>>> add much complexity over what it already has to do to chain
>>>> the sglist today.
>>>>
>>>
>>> It would be better to add a variably sized ahash request member to
>>> struct crypto_ccm_req_priv_ctx, the only problem is that the last
>>> member of that struct (skreq) is variably sized already, so it would
>>> involve having a struct ahash_request pointer pointing into the same
>>> struct, after the skreq member.
>>
>> Actually, I think the below should already do the trick: ahreq and
>> skreq are not used at the same time, so we can stick them in a union,
>> and take the max() of the reqsize to ensure there's enough empty space
>> after it.
>
> This looks very nice indeed.
>
>> --------8<----------
>> diff --git a/crypto/ccm.c b/crypto/ccm.c
>> index 0a083342ec8c..b242fd0d3262 100644
>> --- a/crypto/ccm.c
>> +++ b/crypto/ccm.c
>> @@ -50,7 +50,10 @@ struct crypto_ccm_req_priv_ctx {
>>         u32 flags;
>>         struct scatterlist src[3];
>>         struct scatterlist dst[3];
>> -       struct skcipher_request skreq;
>> +       union {
>> +               struct ahash_request ahreq;
>> +               struct skcipher_request skreq;
>> +       };
>>  };
>>
>
> And this structure is never put on the stack anywhere but
> always dynamically allocated anyway, right?
>

Yes.