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

[Arnd Bergmann]

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.

        Arnd