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

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Date: Thu, 16 May 2024 14:28:06 -0500
From: Segher Boessenkool <segher@...nel.crashing.org>
To: Andy Polyakov <appro@...ptogams.org>
Cc: Danny Tsen <dtsen@...ux.ibm.com>, linux-crypto@...r.kernel.org,
        herbert@...dor.apana.org.au, dtsen@...ibm.com, nayna@...ux.ibm.com,
        linux-kernel@...r.kernel.org, ltcgcw@...ux.vnet.ibm.com,
        leitao@...ian.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/3] crypto: X25519 core functions for ppc64le

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