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From:   Kees Cook <keescook@chromium.org>
Date:   Wed, 20 Jun 2018 17:04:08 -0700
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Subject: Re: [dm-devel] [PATCH 05/11] crypto alg: Introduce max blocksize and alignmask
To:     Eric Biggers <ebiggers3@gmail.com>
Cc:     Herbert Xu <herbert@gondor.apana.org.au>,
        Giovanni Cabiddu <giovanni.cabiddu@intel.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Eric Biggers <ebiggers@google.com>,
        Mike Snitzer <snitzer@redhat.com>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        qat-linux@intel.com, LKML <linux-kernel@vger.kernel.org>,
        dm-devel@redhat.com, linux-crypto <linux-crypto@vger.kernel.org>,
        Lars Persson <larper@axis.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        Alasdair Kergon <agk@redhat.com>,
        Rabin Vincent <rabinv@axis.com>
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On Wed, Jun 20, 2018 at 4:40 PM, Eric Biggers <ebiggers3@gmail.com> wrote:
> On Wed, Jun 20, 2018 at 12:04:02PM -0700, Kees Cook wrote:
>> In the quest to remove all stack VLA usage from the kernel[1], this
>> exposes the existing upper bound on crypto block sizes for VLA removal,
>> and introduces a new check for alignmask (current maximum in the kernel
>> is 63 from manual inspection of all cra_alignmask settings).
>>
>> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
>>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>>  crypto/algapi.c        | 5 ++++-
>>  include/linux/crypto.h | 4 ++++
>>  2 files changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/crypto/algapi.c b/crypto/algapi.c
>> index c0755cf4f53f..760a412b059c 100644
>> --- a/crypto/algapi.c
>> +++ b/crypto/algapi.c
>> @@ -57,7 +57,10 @@ static int crypto_check_alg(struct crypto_alg *alg)
>>       if (alg->cra_alignmask & (alg->cra_alignmask + 1))
>>               return -EINVAL;
>>
>> -     if (alg->cra_blocksize > PAGE_SIZE / 8)
>> +     if (alg->cra_blocksize > CRYPTO_ALG_MAX_BLOCKSIZE)
>> +             return -EINVAL;
>> +
>> +     if (alg->cra_alignmask > CRYPTO_ALG_MAX_ALIGNMASK)
>>               return -EINVAL;
>>
>>       if (!alg->cra_type && (alg->cra_flags & CRYPTO_ALG_TYPE_MASK) ==
>> diff --git a/include/linux/crypto.h b/include/linux/crypto.h
>> index 6eb06101089f..e76ffcbd5aa6 100644
>> --- a/include/linux/crypto.h
>> +++ b/include/linux/crypto.h
>> @@ -134,6 +134,10 @@
>>   */
>>  #define CRYPTO_MAX_ALG_NAME          128
>>
>> +/* Maximum values for registered algorithms. */
>> +#define CRYPTO_ALG_MAX_BLOCKSIZE    (PAGE_SIZE / 8)
>> +#define CRYPTO_ALG_MAX_ALIGNMASK    63
>> +
>
> How do these differ from MAX_CIPHER_BLOCKSIZE and MAX_CIPHER_ALIGNMASK, and why
> are they declared in different places?

This is what I get for staring at crypto code for so long. I entirely
missed these checks... even though they're 8 line away:

        if (!alg->cra_type && (alg->cra_flags & CRYPTO_ALG_TYPE_MASK) ==
                               CRYPTO_ALG_TYPE_CIPHER) {
                if (alg->cra_alignmask > MAX_CIPHER_ALIGNMASK)
                        return -EINVAL;

                if (alg->cra_blocksize > MAX_CIPHER_BLOCKSIZE)
                        return -EINVAL;
        }

However, this is only checking CRYPTO_ALG_TYPE_CIPHER, and
cra_blocksize can be used for all kinds of things.

include/crypto/algapi.h:#define MAX_CIPHER_ALIGNMASK            15
...
drivers/crypto/mxs-dcp.c:                       .cra_flags
 = CRYPTO_ALG_ASYNC,
drivers/crypto/mxs-dcp.c:                       .cra_alignmask          = 63,

Is this one broken? It has no CRYPTO_ALG_TYPE_... ?

For my CRYPTO_ALG_MAX_BLOCKSIZE, there is:

crypto/xcbc.c:  u8 key1[CRYPTO_ALG_MAX_BLOCKSIZE];
drivers/crypto/qat/qat_common/qat_algs.c:       char
ipad[CRYPTO_ALG_MAX_BLOCKSIZE];
drivers/crypto/qat/qat_common/qat_algs.c:       char
opad[CRYPTO_ALG_MAX_BLOCKSIZE];

It looks like both xcbc and qat are used with shash, so that needs a
separate max blocksize.

For my CRYPTO_ALG_MAX_ALIGNMASK, there is:

crypto/shash.c: u8 ubuf[CRYPTO_ALG_MAX_ALIGNMASK]
crypto/shash.c:         __aligned(CRYPTO_ALG_MAX_ALIGNMASK + 1);
crypto/shash.c:         __aligned(CRYPTO_ALG_MAX_ALIGNMASK + 1);

which is also shash.

How should I rename these and best apply the registration-time sanity checks?

-Kees

-- 
Kees Cook
Pixel Security