Re: [PATCH] crypto: ecc - Protect ecc_digits_from_bytes from reading too many bytes

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Sat Apr 27, 2024 at 1:55 AM EEST, Stefan Berger wrote:
> Protect ecc_digits_from_bytes from reading too many bytes from the input
> byte array in case an insufficient number of bytes is provided to fill the
> output digit array of ndigits. Therefore, initialize the most significant
> digits with 0 to avoid trying to read too many bytes later on.
>
> If too many bytes are provided on the input byte array the extra bytes
> are ignored since the input variable 'ndigits' limits the number of digits
> that will be filled.
>
> Fixes: d67c96fb97b5 ("crypto: ecdsa - Convert byte arrays with key coordinates to digits")
> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
> ---
>  include/crypto/internal/ecc.h | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/include/crypto/internal/ecc.h b/include/crypto/internal/ecc.h
> index 7ca1f463d1ec..56215f14ff96 100644
> --- a/include/crypto/internal/ecc.h
> +++ b/include/crypto/internal/ecc.h
> @@ -67,9 +67,16 @@ static inline void ecc_swap_digits(const void *in, u64 *out, unsigned int ndigit
>  static inline void ecc_digits_from_bytes(const u8 *in, unsigned int nbytes,
>  					 u64 *out, unsigned int ndigits)
>  {
> +	int diff = ndigits - DIV_ROUND_UP(nbytes, sizeof(u64));
>  	unsigned int o = nbytes & 7;
>  	__be64 msd = 0;
>  
> +	/* diff > 0: not enough input bytes: set most significant digits to 0 */
> +	while (diff > 0) {
> +		out[--ndigits] = 0;
> +		diff--;
> +	}

Could be just trivial for-loop:

for (i = 0; i < diff; i++)
	out[--ndigits] = 0;

Or also simpler while-loop could work:

while (diff-- > 0)
	out[--ndigits] = 0;

BR, Jarkko