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Date:   Thu, 12 Jul 2018 23:15:51 +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>,
        David Howells <dhowells@...hat.com>
Subject: Re: [PATCH v4 13/14] rxrpc: Prepare to remove VLA usage for SKCIPHER_REQUEST_ON_STACK

On Thu, Jul 12, 2018 at 10:30 PM, Kees Cook <keescook@...omium.org> wrote:
> On Thu, Jul 12, 2018 at 1:23 PM, Kees Cook <keescook@...omium.org> wrote:
>> On Thu, Jul 12, 2018 at 8:11 AM, Arnd Bergmann <arnd@...db.de> wrote:
>>> On Wed, Jul 11, 2018 at 10:36 PM, Kees Cook <keescook@...omium.org> wrote:
>>>> Two uses of SKCIPHER_REQUEST_ON_STACK() will trigger FRAME_WARN warnings
>>>> (when less than 2048) once the VLA is no longer hidden from the check:
>>>>
>>>> net/rxrpc/rxkad.c:398:1: warning: the frame size of 1152 bytes is larger than 1024 bytes [-Wframe-larger-than=]
>>>> net/rxrpc/rxkad.c:242:1: warning: the frame size of 1152 bytes is larger than 1024 bytes [-Wframe-larger-than=]
>>>>
>>>> This bumps the affected objects by 20% to silence the warnings while
>>>> still providing coverage is anything grows even more.
>>>>
>>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>>
>>> (adding David Howells to cc)
>>>
>>> I don't think these are in a fast path, it should be possible to just use
>>> skcipher_alloc_req() instead of SKCIPHER_REQUEST_ON_STACK() here.
>>> From what I can tell, neither of the two are called in atomic context, so
>>> you should be able to use a GFP_KERNEL allocation.
>>
>> Sure, I can do that instead.
>
> Actually, I think this can actually be adjusted to just re-use the
> stack allocation, since rxkad_verify_packet() finishes one before
> doing another in rxkad_verify_packet_1():

That looks very nice, yes. The same thing is needed in
rxkad_secure_packet(), right?

       Arnd

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