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

[Lukas Wunner <lukas@wunner.de>]

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.