Re: [PATCH] x86/power/64: Remove VLA usage

[Eric Biggers <ebiggers@google.com>]

On Sun, Jul 15, 2018 at 08:56:57PM -0700, Kees Cook wrote:
> In the quest to remove all stack VLA usage from the kernel[1], this
> removes the discouraged use of AHASH_REQUEST_ON_STACK by switching to
> shash directly and allocating the descriptor in heap memory (which should
> be fine: the tfm has already been allocated there too).
> 
> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/x86/power/hibernate_64.c | 35 +++++++++++++++++++----------------
>  1 file changed, 19 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/power/hibernate_64.c b/arch/x86/power/hibernate_64.c
> index 67ccf64c8bd8..0ed01bb935a6 100644
> --- a/arch/x86/power/hibernate_64.c
> +++ b/arch/x86/power/hibernate_64.c
> @@ -233,28 +233,31 @@ struct restore_data_record {
>   */
>  static int get_e820_md5(struct e820_table *table, void *buf)
>  {
> -	struct scatterlist sg;
> -	struct crypto_ahash *tfm;
> +	struct crypto_shash *tfm;
> +	struct shash_desc *desc;
>  	int size;
>  	int ret = 0;
>  
> -	tfm = crypto_alloc_ahash("md5", 0, CRYPTO_ALG_ASYNC);
> +	tfm = crypto_alloc_shash("md5", 0, 0);
>  	if (IS_ERR(tfm))
>  		return -ENOMEM;
>  
> -	{
> -		AHASH_REQUEST_ON_STACK(req, tfm);
> -		size = offsetof(struct e820_table, entries) + sizeof(struct e820_entry) * table->nr_entries;
> -		ahash_request_set_tfm(req, tfm);
> -		sg_init_one(&sg, (u8 *)table, size);
> -		ahash_request_set_callback(req, 0, NULL, NULL);
> -		ahash_request_set_crypt(req, &sg, buf, size);
> -
> -		if (crypto_ahash_digest(req))
> -			ret = -EINVAL;
> -		ahash_request_zero(req);
> -	}
> -	crypto_free_ahash(tfm);
> +	desc = kmalloc(sizeof(struct shash_desc) + crypto_shash_descsize(tfm),
> +		       GFP_KERNEL);
> +	if (!desc)
> +		return -ENOMEM;

Need crypto_free_shash(tfm) if the kmalloc() here fails.

> +
> +	desc->tfm = tfm;
> +	desc->flags = 0;
> +
> +	size = offsetof(struct e820_table, entries) +
> +		sizeof(struct e820_entry) * table->nr_entries;
> +
> +	if (crypto_shash_digest(desc, (u8 *)table, size, buf))
> +		ret = -EINVAL;
> +
> +	kzfree(desc);
> +	crypto_free_shash(tfm);
>  
>  	return ret;
>  }
> -- 
> 2.17.1
> 
> 
> -- 
> Kees Cook
> Pixel Security