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

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Message-ID: <CAK8P3a3Dh+g=v++BMCtmmiOSnVu4CEUgss0U7BZS_K45aTddqg@mail.gmail.com>
Date:   Tue, 17 Jul 2018 22:59:00 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     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>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH v4 11/14] treewide: Prepare to remove VLA usage for AHASH_REQUEST_ON_STACK

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.

      Arnd