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

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