Re: [PATCH 2/2] crypto: skcipher: Remove VLA usage for SKCIPHER_REQUEST_ON_STACK

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 4 September 2018 at 20:16, 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. Looking at instrumented tcrypt output, the largest
> is for lrw:
>
>         crypt: testing lrw(aes)
>         crypto_skcipher_set_reqsize: 8
>         crypto_skcipher_set_reqsize: 88
>         crypto_skcipher_set_reqsize: 472
>

Are you sure this is a representative sampling? I haven't double
checked myself, but we have plenty of drivers for peripherals in
drivers/crypto that implement block ciphers, and they would not turn
up in tcrypt unless you are running on a platform that provides the
hardware in question.

> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  include/crypto/internal/skcipher.h | 3 +++
>  include/crypto/skcipher.h          | 4 +++-
>  2 files changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/include/crypto/internal/skcipher.h b/include/crypto/internal/skcipher.h
> index d2926ecae2ac..6da811c0747e 100644
> --- a/include/crypto/internal/skcipher.h
> +++ b/include/crypto/internal/skcipher.h
> @@ -130,6 +130,9 @@ static inline struct crypto_skcipher *crypto_spawn_skcipher(
>  static inline int crypto_skcipher_set_reqsize(
>         struct crypto_skcipher *skcipher, unsigned int reqsize)
>  {
> +       if (WARN_ON(reqsize > SKCIPHER_MAX_REQSIZE))
> +               return -EINVAL;
> +
>         skcipher->reqsize = reqsize;
>
>         return 0;
> diff --git a/include/crypto/skcipher.h b/include/crypto/skcipher.h
> index 2f327f090c3e..c48e194438cf 100644
> --- a/include/crypto/skcipher.h
> +++ b/include/crypto/skcipher.h
> @@ -139,9 +139,11 @@ struct skcipher_alg {
>         struct crypto_alg base;
>  };
>
> +#define SKCIPHER_MAX_REQSIZE   472
> +
>  #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
>
>  /**
> --
> 2.17.1
>