linux-kernel - Re: [PATCH v7 2/9] crypto: cbc: Remove VLA usage

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Date:   Tue, 7 Aug 2018 13:52:20 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        device-mapper development <dm-devel@...hat.com>,
        Tudor-Dan Ambarus <tudor.ambarus@...rochip.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>,
        Will Deacon <will.deacon@....com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-crypto <linux-crypto@...r.kernel.org>, qat-linux@...el.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 2/9] crypto: cbc: Remove VLA usage

On Tue, Aug 7, 2018 at 2:47 AM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> On Thu, Aug 02, 2018 at 03:51:45PM -0700, Kees Cook wrote:
>> In the quest to remove all stack VLA usage from the kernel[1], this
>> uses the upper bounds on blocksize. Since this is always a cipher
>> blocksize, use the existing cipher max blocksize.
>>
>> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
>>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>>  include/crypto/cbc.h | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/crypto/cbc.h b/include/crypto/cbc.h
>> index f5b8bfc22e6d..47db0aac2ab9 100644
>> --- a/include/crypto/cbc.h
>> +++ b/include/crypto/cbc.h
>> @@ -113,7 +113,9 @@ static inline int crypto_cbc_decrypt_inplace(
>>       unsigned int bsize = crypto_skcipher_blocksize(tfm);
>>       unsigned int nbytes = walk->nbytes;
>>       u8 *src = walk->src.virt.addr;
>> -     u8 last_iv[bsize];
>> +     u8 last_iv[MAX_CIPHER_BLOCKSIZE];
>> +
>> +     BUG_ON(bsize > sizeof(last_iv));
>
> Ugh, please don't add these BUG_ONs.  Add them to the places where
> the algorithm is created (if they aren't checking that already).

It's already being checked (cra_blocksize vs MAX_CIPHER_BLOCKSIZE) so
I was just adding the BUG_ON to catch "impossible" behavior. I'll
leave it out in the next revision.

-Kees

-- 
Kees Cook
Pixel Security