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

[Stefan Berger <stefanb@linux.ibm.com>]

On 4/29/24 06:14, Jarkko Sakkinen wrote:
> On Mon Apr 29, 2024 at 6:30 AM EEST, Lukas Wunner wrote:
>> On Mon, Apr 29, 2024 at 01:12:00AM +0300, Jarkko Sakkinen wrote:
>>> 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;
>>
>> Or just use memset(), which uses optimized instructions on many arches.
> 
> Yeah, sure, that would be even better, or even memzero_explicit()?

Thanks. The function isn't getting too big for an inline?

> 
> BR, Jarkko