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

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Date:   Fri, 13 Jul 2018 12:14:16 +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 Fri, Jul 13, 2018 at 8:00 AM, Kees Cook <keescook@...omium.org> wrote:
> On Thu, Jul 12, 2018 at 10:20 PM, Herbert Xu
> <herbert@...dor.apana.org.au> wrote:
>> On Thu, Jul 12, 2018 at 10:17:29PM -0700, Kees Cook wrote:
>>> What is the correct value to use for AHASH_REQUEST_ON_STACK?
>>
>> As I said to arrive at a fixed value you should examine all sync
>> ahash algorithms (e.g., all shash ones plus ahash ones marked as
>> sync if there are any).
>
> The "value" for the ahash I understand: it has a request size
> (tfm->reqsize, as set by crypto_ahash_set_reqsize()) what should be
> used to measure the shash value? (And how does this relate to the
> value returned by crypto_ahash_reqsize()?) The closest clue I can find
> is this:
>
> crypto_init_shash_ops_async() does:
>         crt->reqsize = sizeof(struct shash_desc) + crypto_shash_descsize(shash);
>
> and that gets called from crypto_ahash_init_tfm(), so if it starts
> with the above reqsize and adds to it with a call to
> crypto_ahash_set_reqsize() later, we'll have that maximum?
>
> So, do I want to calculate this answer as:
>
> sizeof(struct shash_desc) + max(shash descsize) + max(ahash-sync reqsize) ?
> 16 + 360 + 0

I arrived at the same number, looking at all the sizes in shash,
The largest I found are sha3_state (360 bytes) and s390_sha_ctx
(336 bytes), everything else is way smaller.

> It's 0 above because if I look at all the callers of
> crypto_ahash_set_reqsize() that do wrapping, all are ASYNC.
>
> So, should this really just be 376? Where is best to validate this
> size, as it seems checking in crypto_ahash_set_reqsize() is
> inappropriate?

How about crypto_init_shash_ops_async()?

      Arnd