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

Kees Cook
2018-06-20T20:36:51Z

On Wed, Jun 20, 2018 at 12:30 PM, Christophe Leroy
<christophe.leroy@c-s.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@chromium.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