linux-kernel - Re: [PATCH 09/11] crypto: shash: Remove VLA usage in unaligned hashing

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Date:   Wed, 20 Jun 2018 17:15:05 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Eric Biggers <ebiggers@...gle.com>,
        Mike Snitzer <snitzer@...hat.com>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        qat-linux@...el.com, LKML <linux-kernel@...r.kernel.org>,
        dm-devel@...hat.com, linux-crypto <linux-crypto@...r.kernel.org>,
        Lars Persson <larper@...s.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alasdair Kergon <agk@...hat.com>,
        Rabin Vincent <rabinv@...s.com>
Subject: Re: [PATCH 09/11] crypto: shash: Remove VLA usage in unaligned hashing

On Wed, Jun 20, 2018 at 4:57 PM, Eric Biggers <ebiggers3@...il.com> wrote:
> On Wed, Jun 20, 2018 at 12:04:06PM -0700, Kees Cook wrote:
>> In the quest to remove all stack VLA usage from the kernel[1], this uses
>> the newly defined max alignment to perform unaligned hashing to avoid
>> VLAs, and drops the helper function while adding sanity checks on the
>> resulting buffer sizes.
>>
>> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
>>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>>  crypto/shash.c | 21 ++++++++++-----------
>>  1 file changed, 10 insertions(+), 11 deletions(-)
>>
>> diff --git a/crypto/shash.c b/crypto/shash.c
>> index ab6902c6dae7..1bb58209330a 100644
>> --- a/crypto/shash.c
>> +++ b/crypto/shash.c
>> @@ -73,13 +73,6 @@ int crypto_shash_setkey(struct crypto_shash *tfm, const u8 *key,
>>  }
>>  EXPORT_SYMBOL_GPL(crypto_shash_setkey);
>>
>> -static inline unsigned int shash_align_buffer_size(unsigned len,
>> -                                                unsigned long mask)
>> -{
>> -     typedef u8 __aligned_largest u8_aligned;
>> -     return len + (mask & ~(__alignof__(u8_aligned) - 1));
>> -}
>> -
>>  static int shash_update_unaligned(struct shash_desc *desc, const u8 *data,
>>                                 unsigned int len)
>>  {
>> @@ -88,11 +81,14 @@ static int shash_update_unaligned(struct shash_desc *desc, const u8 *data,
>>       unsigned long alignmask = crypto_shash_alignmask(tfm);
>>       unsigned int unaligned_len = alignmask + 1 -
>>                                    ((unsigned long)data & alignmask);
>> -     u8 ubuf[shash_align_buffer_size(unaligned_len, alignmask)]
>> -             __aligned_largest;
>> +     u8 ubuf[CRYPTO_ALG_MAX_ALIGNMASK]
>> +             __aligned(CRYPTO_ALG_MAX_ALIGNMASK + 1);
>>       u8 *buf = PTR_ALIGN(&ubuf[0], alignmask + 1);
>>       int err;
>
> Are you sure that __attribute__((aligned(64))) works correctly on stack
> variables on all architectures?
>
> And if it is expected to work, then why is the buffer still aligned by hand on
> the very next line?

I really don't know -- the existing code was doing both the __align
bit and the manual alignment, so I was trying to simplify it while
removing the VLA. I'm totally open to suggestions here.

BTW, these are also the only users of __aligned_largest() in the
kernel, and the only use of unsized __attribute__((aligned))

-Kees

-- 
Kees Cook
Pixel Security