linux-kernel - Re: [PATCH] crypto: ecc - Protect ecc_digits_from

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Date: Mon, 29 Apr 2024 13:14:15 +0300
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Lukas Wunner" <lukas@...ner.de>
Cc: "Stefan Berger" <stefanb@...ux.ibm.com>, <keyrings@...r.kernel.org>,
 <linux-crypto@...r.kernel.org>, <herbert@...dor.apana.org.au>,
 <davem@...emloft.net>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] crypto: ecc - Protect ecc_digits_from_bytes from
 reading too many bytes

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@...ux.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()?

BR, Jarkko