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

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Date:   Mon, 16 Jul 2018 09:24:06 +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>
Subject: Re: [PATCH v4 11/14] treewide: Prepare to remove VLA usage for AHASH_REQUEST_ON_STACK

On Mon, Jul 16, 2018 at 5:39 AM, Kees Cook <keescook@...omium.org> wrote:
> On Sun, Jul 15, 2018 at 5:01 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
>> On Sat, Jul 14, 2018 at 07:59:09PM -0700, Kees Cook wrote:
>>> On Sat, Jul 14, 2018 at 7:44 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
>>> > On Fri, Jul 13, 2018 at 08:07:10PM -0700, Kees Cook wrote:
>>> >>
>>> >> On a plane today I started converting all these to shash. IIUC, it
>>> >> just looks like this (apologies for whitespace damage):
>>> >
>>> > Yes if it doesn't actually make use of SGs then shash would be
>>> > the way to go.  However, for SG users ahash is the best interface.
>>>
>>> Nearly all of them artificially build an sg explicitly to use the
>>> ahash interface. :P
>>>
>>> So, I'll take that as a "yes, do these conversions." :) Thanks!
>>
>> Yeah anything that's doing a single-element SG list should just
>> be converted.
>
> There are a few that are multiple element SG list, but it's a locally
> allocated array of SGs, and filled with data. All easily replaced with
> just calls to ..._update() instead of sg helpers. For example
> net/wireless/lib80211_crypt_tkip.c:
>
> -       sg_init_table(sg, 2);
> -       sg_set_buf(&sg[0], hdr, 16);
> -       sg_set_buf(&sg[1], data, data_len);
> ...
> -       ahash_request_set_tfm(req, tfm_michael);
> -       ahash_request_set_callback(req, 0, NULL, NULL);
> -       ahash_request_set_crypt(req, sg, mic, data_len + 16);
> -       err = crypto_ahash_digest(req);
> -       ahash_request_zero(req);
> +       err = crypto_shash_init(desc);
> +       if (err)
> +               goto out;
> +       err = crypto_shash_update(desc, hdr, 16);
> +       if (err)
> +               goto out;
> +       err = crypto_shash_update(desc, data, data_len);
> +       if (err)
> +               goto out;
> +       err = crypto_shash_final(desc, mic);
> +
> +out:
> +       shash_desc_zero(desc);
>         return err;

There may be a little overhead in calling crypto_shash_update()/
crypto_shash_final() repeatedly compared to calling
crypto_ahash_digest() once. It's probably no worse (or maybe
better) in this case, since we call only three times and there
is less indirection, but if there are any cases with a long sglist,
it would be good to measure the performance difference.

       Arnd