Re: [PATCH] crypto/ecc: Remove stack VLA usage

[Tudor Ambarus]

Hi, Kees,

On 03/07/2018 11:56 PM, Kees Cook wrote:
> On the quest to remove all VLAs from the kernel[1], this switches to
> a pair of kmalloc regions instead of using the stack. This also moves
> the get_random_bytes() after all allocations (and drops the needless
> "nbytes" variable).
>
> [1] https://lkml.org/lkml/2018/3/7/621
>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>   crypto/ecc.c | 23 +++++++++++++++++------
>   1 file changed, 17 insertions(+), 6 deletions(-)
>
> diff --git a/crypto/ecc.c b/crypto/ecc.c
> index 18f32f2a5e1c..5bfa63603da0 100644
> --- a/crypto/ecc.c
> +++ b/crypto/ecc.c
> @@ -1025,9 +1025,7 @@ int crypto_ecdh_shared_secret(unsigned int curve_id, unsigned int ndigits,
>   {
>   	int ret = 0;
>   	struct ecc_point *product, *pk;
> -	u64 priv[ndigits];
> -	u64 rand_z[ndigits];
> -	unsigned int nbytes;
> +	u64 *priv, *rand_z;
>   	const struct ecc_curve *curve = ecc_get_curve(curve_id);
>   
>   	if (!private_key || !public_key || !curve) {
> @@ -1035,14 +1033,22 @@ int crypto_ecdh_shared_secret(unsigned int curve_id, unsigned int ndigits,
>   		goto out;
>   	}
>   
> -	nbytes = ndigits << ECC_DIGITS_TO_BYTES_SHIFT;
> +	priv = kmalloc_array(ndigits, sizeof(*priv), GFP_KERNEL);
> +	if (!priv) {
> +		ret = -ENOMEM;
> +		goto out;
> +	}
>   
> -	get_random_bytes(rand_z, nbytes);
> +	rand_z = kmalloc_array(ndigits, sizeof(*rand_z), GFP_KERNEL);
> +	if (!rand_z) {
> +		ret = -ENOMEM;
> +		goto kfree_out;
> +	}
>   
>   	pk = ecc_alloc_point(ndigits);
>   	if (!pk) {
>   		ret = -ENOMEM;
> -		goto out;
> +		goto kfree_out;
>   	}
>   
>   	product = ecc_alloc_point(ndigits);
> @@ -1051,6 +1057,8 @@ int crypto_ecdh_shared_secret(unsigned int curve_id, unsigned int ndigits,
>   		goto err_alloc_product;
>   	}
>   
> +	get_random_bytes(rand_z, ndigits << ECC_DIGITS_TO_BYTES_SHIFT);
> +
>   	ecc_swap_digits(public_key, pk->x, ndigits);
>   	ecc_swap_digits(&public_key[ndigits], pk->y, ndigits);
>   	ecc_swap_digits(private_key, priv, ndigits);
> @@ -1065,6 +1073,9 @@ int crypto_ecdh_shared_secret(unsigned int curve_id, unsigned int ndigits,
>   	ecc_free_point(product);
>   err_alloc_product:
>   	ecc_free_point(pk);
> +kfree_out:
> +	kfree(priv);

I think we should use kzfree here.

> +	kfree(rand_z);

Probably here too. Looks like there are few intermediate buffers in ecc
that should be zeroized as well.

Best,
ta
>   out:
>   	return ret;
>   }