Re: [PATCH v4 11/14] treewide: Prepare to remove VLA usage for AHASH_REQUEST_ON_STACK

[Ard Biesheuvel]

On 19 July 2018 at 00:33, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Wed, Jul 18, 2018 at 5:19 PM, Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
>> On 18 July 2018 at 23:50, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
>>> On 18 July 2018 at 05:59, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>>>> On Sun, Jul 15, 2018 at 6:28 AM, Kees Cook <keescook@xxxxxxxxxxxx> 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.