Re: [PATCH v2] hex2bin: make the function hex_to_bin constant-time

[Mikulas Patocka]

On Wed, 4 May 2022, Stafford Horne wrote:

> On Mon, Apr 25, 2022 at 08:07:48AM -0400, Mikulas Patocka wrote:
>  
> > From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> > 
> > -int hex_to_bin(char ch)
> > +int hex_to_bin(unsigned char ch)
> >  {
> > -	if ((ch >= '0') && (ch <= '9'))
> > -		return ch - '0';
> > -	ch = tolower(ch);
> > -	if ((ch >= 'a') && (ch <= 'f'))
> > -		return ch - 'a' + 10;
> > -	return -1;
> > +	unsigned char cu = ch & 0xdf;
> > +	return -1 +
> > +		((ch - '0' +  1) & (unsigned)((ch - '9' - 1) & ('0' - 1 - ch)) >> 8) +
> > +		((cu - 'A' + 11) & (unsigned)((cu - 'F' - 1) & ('A' - 1 - cu)) >> 8);
> >  }
> >  EXPORT_SYMBOL(hex_to_bin);
> 
> Hello,
> 
> Just a heads up it seems this patch is causing some instability with crypto self
> tests on OpenRISC when using a PREEMPT kernel (no SMP).
> 
> This was reported by Jason A. Donenfeld as it came up in wireguard testing.
> 
> I am trying to figure out if this is an OpenRISC PREEMPT issue or something
> else.

Hi

That patch is so simple that I can't imagine how could it break the 
curve25519 test. Are you sure that you bisected it correctly?

Mikulas