Re: [PATCH 2/3] crypto: X25519 core functions for ppc64le

[Segher Boessenkool <segher@kernel.crashing.org>]

On Wed, May 15, 2024 at 10:29:56AM +0200, Andy Polyakov wrote:
> >+static void cswap(fe51 p, fe51 q, unsigned int bit)
> 
> The "c" in cswap stands for "constant-time," and the problem is that 
> contemporary compilers have exhibited the ability to produce 
> non-constant-time machine code as result of compilation of the above 
> kind of technique.

This can happen with *any* comnpiler, on *any* platform.  In general,
you have to write machine code if you want to be sure what machine code
will eventually be executed.

>  The outcome is platform-specific and ironically some 
> of PPC code generators were observed to generate "most" 
> non-constant-time code. "Most" in sense that execution time variations 
> would be most easy to catch. One way to work around the problem, at 
> least for the time being, is to add 'asm volatile("" : "+r"(c))' after 
> you calculate 'c'. But there is no guarantee that the next compiler 
> version won't see through it, hence the permanent solution is to do it 
> in assembly. I can put together something...

Such tricks can help ameliorate the problem, sure.  But it is not a
solution ever.


Segher