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

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Date:   Wed, 20 Jun 2018 13:42:41 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christophe LEROY <christophe.leroy@....fr>,
        Arnd Bergmann <arnd@...db.de>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Alasdair Kergon <agk@...hat.com>,
        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 1:39 PM, Christophe LEROY
<christophe.leroy@....fr> wrote:
>
>
> Le 20/06/2018 à 22:36, Kees Cook a écrit :
>>
>> 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.
>
>
> Maybe the issue is because I have selected 16k pages ?

That would do it! And that's exactly the problem Arnd mentioned. For
v2, I will switch all the PAGE_SIZE-related limits to an explicit 512.
(And I'll do some 32-bit builds too to see if any other cases pop up
that I need to mask out.)

-Kees

-- 
Kees Cook
Pixel Security