linux-kernel - Re: [PATCH 01/11] crypto: shash: Remove VLA usage

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Date:   Wed, 20 Jun 2018 13:36:45 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Alasdair Kergon <agk@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Eric Biggers <ebiggers@...gle.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>,
        linux-crypto <linux-crypto@...r.kernel.org>, qat-linux@...el.com,
        dm-devel@...hat.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/11] crypto: shash: Remove VLA usage

On Wed, Jun 20, 2018 at 12:30 PM, Christophe Leroy
<christophe.leroy@....fr> wrote:
>
>
> On 06/20/2018 07:03 PM, Kees Cook wrote:
>>
>> In the quest to remove all stack VLA usage from the kernel[1], this
>> removes the VLAs in SHASH_DESC_ON_STACK (via crypto_shash_descsize())
>> by using the maximum allowable size (which is now more clearly captured
>> in a macro). Similar limits are turned into macros as well.
>>
>> [1]
>> https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
>>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>
>
> Got the following warnings:
>
> crypto/testmgr.c: In function ‘alg_test_crc32c.part.4’:
> crypto/testmgr.c:1896:1: warning: the frame size of 2088 bytes is larger
> than 1024 bytes [-Wframe-larger-than=]
> crypto/hmac.c: In function ‘hmac_setkey’:
> crypto/hmac.c:88:1: warning: the frame size of 2088 bytes is larger than
> 1024 bytes [-Wframe-larger-than=]

Ah yes. I didn't do 32-bit builds. So, here's the issue: this uncovers
the frame size problems that were hidden by in being a VLA before. It
was always possible for the frame to get this big, it's just that the
compiler couldn't see it.

For qat, I raised the -Wframe-larger-than flag. It seems we'll need to
do this in some other places too.

-Kees

-- 
Kees Cook
Pixel Security