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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAK8P3a3C1ihC=X=kZ0S836xeMJSGSy6OLNAcBEkdZ35N04bAMw@mail.gmail.com>
Date:   Wed, 18 Jul 2018 17:33:08 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
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 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?

      Arnd