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

[Ard Biesheuvel]

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.