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

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Message-ID: <CAGXu5jLR0zJ6b9axJuG_uvttNShNcN3rhQKGVjW37qPd9saY-Q@mail.gmail.com>
Date:   Thu, 12 Jul 2018 22:17:29 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Arnd Bergmann <arnd@...db.de>,
        "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 Thu, Jul 12, 2018 at 8:44 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> On Thu, Jul 12, 2018 at 08:33:24PM -0700, Kees Cook wrote:
>> On Thu, Jul 12, 2018 at 5:40 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
>> > On Thu, Jul 12, 2018 at 06:02:26PM +0200, Arnd Bergmann wrote:
>> >>
>> >> Looking through some of the drivers, I found this interesting one:
>> >
>> > As I said before these patches are fundamentally broken.  Users
>> > of AHASH_REQUEST_ON_STACK can only use sync algorithm providers
>> > and therefore drivers are irrelevant.
>>
>> I don't understand what this means. Can you give an example of what
>> you want to see happen that will accomplish the VLA removals?
>
> Any algorithm that is async must be ignored when you're calculating
> the maximum on-stack size of the request.  For example, sha512-mb
> is marked as async and therefore must not be used in conjunction
> with AHASH_REQUEST_ON_STACK.

Then why does the instrumented tcrypt output show the huge size? Is
tcrypt doing something incorrectly?

What is the correct value to use for AHASH_REQUEST_ON_STACK?

-Kees

-- 
Kees Cook
Pixel Security