linux-kernel - Re: [dm-devel] [PATCH v5 05/11] crypto: ahash: Remove VLA usage

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Date:   Tue, 17 Jul 2018 13:07:50 -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>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Eric Biggers <ebiggers@...gle.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>,
        Alasdair Kergon <agk@...hat.com>,
        Rabin Vincent <rabinv@...s.com>
Subject: Re: [dm-devel] [PATCH v5 05/11] crypto: ahash: Remove VLA usage

On Tue, Jul 17, 2018 at 9:39 AM, Eric Biggers <ebiggers3@...il.com> wrote:
> On Mon, Jul 16, 2018 at 09:21:44PM -0700, Kees Cook wrote:
>> In the quest to remove all stack VLA usage from the kernel[1], this
>> introduces max size macros for ahash, as already done for shash, and
>> adjust the crypto user to max state size.
>>
>> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
>>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>>  crypto/ahash.c        | 4 ++--
>>  crypto/algif_hash.c   | 2 +-
>>  include/crypto/hash.h | 3 +++
>>  3 files changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/crypto/ahash.c b/crypto/ahash.c
>> index a64c143165b1..6435bdbe42fd 100644
>> --- a/crypto/ahash.c
>> +++ b/crypto/ahash.c
>> @@ -550,8 +550,8 @@ static int ahash_prepare_alg(struct ahash_alg *alg)
>>  {
>>       struct crypto_alg *base = &alg->halg.base;
>>
>> -     if (alg->halg.digestsize > PAGE_SIZE / 8 ||
>> -         alg->halg.statesize > PAGE_SIZE / 8 ||
>> +     if (alg->halg.digestsize > AHASH_MAX_DIGESTSIZE ||
>> +         alg->halg.statesize > AHASH_MAX_STATESIZE ||
>>           alg->halg.statesize == 0)
>>               return -EINVAL;
>>
>> diff --git a/crypto/algif_hash.c b/crypto/algif_hash.c
>> index bfcf595fd8f9..8974ee8ebead 100644
>> --- a/crypto/algif_hash.c
>> +++ b/crypto/algif_hash.c
>> @@ -239,7 +239,7 @@ static int hash_accept(struct socket *sock, struct socket *newsock, int flags,
>>       struct alg_sock *ask = alg_sk(sk);
>>       struct hash_ctx *ctx = ask->private;
>>       struct ahash_request *req = &ctx->req;
>> -     char state[crypto_ahash_statesize(crypto_ahash_reqtfm(req)) ? : 1];
>> +     char state[AHASH_MAX_STATESIZE];
>>       struct sock *sk2;
>>       struct alg_sock *ask2;
>>       struct hash_ctx *ctx2;
>> diff --git a/include/crypto/hash.h b/include/crypto/hash.h
>> index ae14cc0e0cdb..4fcd0e2368cd 100644
>> --- a/include/crypto/hash.h
>> +++ b/include/crypto/hash.h
>> @@ -64,6 +64,9 @@ struct ahash_request {
>>       void *__ctx[] CRYPTO_MINALIGN_ATTR;
>>  };
>>
>> +#define AHASH_MAX_DIGESTSIZE 512
>> +#define AHASH_MAX_STATESIZE  512
>> +
>
> Why is AHASH_MAX_DIGESTSIZE (512) so much larger than SHASH_MAX_DIGESTSIZE (64)?
> I would have expected them to be the same.

This was a direct replacement of the PAGE_SIZE / 8 original limit.
This this didn't trip any frame size warnings, it seemed like we
didn't need to do the more narrow limitations done elsewhere.

I am, of course, happy to do a manual review and find a lower
alternative if that's desired.

-Kees

-- 
Kees Cook
Pixel Security