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

Segher Boessenkool
2024-05-16T20:10:35Z

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