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

[Alexander Stein <alexander.stein@systec-electronic.com>]

On Tuesday, September 4, 2018, 8:16:29 PM CEST Kees Cook 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
> 
> [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

Now tfm could be removed from the macro arguments, no?

Best regards,
Alexander