linux-kernel - Re: [PATCH 2/2] crypto: skcipher: Remove VLA usage for SKCIPHER_REQUEST_ON

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Message-ID: <CAKv+Gu_0ehX9sLCtUYR00D4iQw+ytwmsLKmG6dgq9EnLjidKFw@mail.gmail.com>
Date:   Thu, 6 Sep 2018 11:29:41 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Gilad Ben-Yossef <gilad@...yossef.com>,
        Kees Cook <keescook@...omium.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Antoine Tenart <antoine.tenart@...tlin.com>,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        Arnaud Ebalard <arno@...isbad.org>,
        Corentin Labbe <clabbe.montjoie@...il.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Christian Lamparter <chunkeey@...il.com>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/2] crypto: skcipher: Remove VLA usage for SKCIPHER_REQUEST_ON_STACK

On 6 September 2018 at 10:51, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> On Thu, Sep 06, 2018 at 10:11:59AM +0200, Ard Biesheuvel wrote:
>>
>> That way, we will almost certainly oops on a NULL pointer dereference
>> right after, but we at least the stack corruption.
>
> A crash is just as bad as a BUG_ON.
>
> Is this even a real problem? Do we have any users of this construct
> that is using it on async algorithms?
>

Perhaps not, but it is not enforced atm.

In any case, limiting the reqsize is going to break things, so that
needs to occur based on the sync/async nature of the algo. That also
means we'll corrupt the stack if we ever end up using
SKCIPHER_REQUEST_ON_STACK() with an async algo whose reqsize is
greater than the sync reqsize limit, so I do think some additional
sanity check is appropriate.