Re: [PATCH 11/11] crypto: skcipher: Remove VLA usage for SKCIPHER_REQUEST_ON_STACK

[Kees Cook]

On Wed, Jun 20, 2018 at 12:44 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Wed, Jun 20, 2018 at 9:04 PM, Kees Cook <keescook@xxxxxxxxxxxx> 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