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From:   Kees Cook <keescook@chromium.org>
Date:   Wed, 20 Jun 2018 13:38:20 -0700
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Subject: Re: [PATCH 11/11] crypto: skcipher: Remove VLA usage for SKCIPHER_REQUEST_ON_STACK
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Herbert Xu <herbert@gondor.apana.org.au>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        Alasdair Kergon <agk@redhat.com>,
        Eric Biggers <ebiggers@google.com>,
        Giovanni Cabiddu <giovanni.cabiddu@intel.com>,
        Lars Persson <larper@axis.com>,
        Mike Snitzer <snitzer@redhat.com>,
        Rabin Vincent <rabinv@axis.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@vger.kernel.org>, qat-linux@intel.com,
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On Wed, Jun 20, 2018 at 12:44 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Wed, Jun 20, 2018 at 9:04 PM, Kees Cook <keescook@chromium.org> wrote:
>> In the quest to remove all stack VLA usage from the kernel[1], this
>> caps the skcipher request size similar to other limits and adds a sanity
>> check at registration.
>>
>>
>> +#define SKCIPHER_MAX_REQSIZE   (PAGE_SIZE / 8)
>> +
>>  #define SKCIPHER_REQUEST_ON_STACK(name, tfm) \
>>         char __##name##_desc[sizeof(struct skcipher_request) + \
>> -               crypto_skcipher_reqsize(tfm)] CRYPTO_MINALIGN_ATTR; \
>> +               SKCIPHER_MAX_REQSIZE] CRYPTO_MINALIGN_ATTR; \
>>         struct skcipher_request *name = (void *)__##name##_desc
>>
>
> This is probably a bad idea on kernels with large values of PAGE_SIZE.
> Some users on ppc64 and arm64 use 64KB here, but still limit
> the per-function stack size to 2KB.

We could make all of these PAGE_SIZE-related limits explicitly 512? I
think that was the intent originally.

-Kees

-- 
Kees Cook
Pixel Security