Should Linux set the new constant-time mode CPU flags?

[Catalin Marinas]

On Tue, Aug 30, 2022 at 07:25:29AM -0700, Dave Hansen wrote:
> On 8/29/22 09:39, Jason A. Donenfeld wrote:
> > On Thu, Aug 25, 2022 at 11:15:58PM +0000, Eric Biggers wrote:
> >> I'm wondering if people are aware of this issue, and whether anyone has any
> >> thoughts on whether/where the kernel should be setting these new CPU flags.
> >> There don't appear to have been any prior discussions about this.  (Thanks to
> > Maybe it should be set unconditionally now, until we figure out how to
> > make it more granular.
> 
> Personally, I'm in this camp as well.  Let's be safe and set it by
> default.  There's also this tidbit in the Intel docs (and chopping out a
> bunch of the noise):
> 
> 	(On) processors based on microarchitectures before Ice Lake ...
> 	the instructions listed here operate as if DOITM is enabled.
> 
> IOW, setting DOITM=0 isn't going back to the stone age.  At worst, I'd
> guess that you're giving up some optimization that only shows up in very
> recent CPUs in the first place.
> 
> If folks want DOITM=1 on their snazzy new CPUs, then they came come with
> performance data to demonstrate the gain they'll get from adding kernel
> code to get DOITM=1.  There are a range of ways we could handle it, all
> the way from adding a command-line parameter to per-task management.
> 
> Anybody disagree?

It's not my preferred option for arm64 but I admit the same reasoning
could equally apply to us. If some existing crypto libraries relied on
data independent timing for current CPUs but newer ones (with the DIT
feature) come up with more aggressive, data-dependent optimisations,
they may be caught off-guard. That said the ARM architecture spec never
promised any timing, that's a micro-architecture detail and not all
implementations are done by ARM Ltd. So I can't really tell what's out
there.

So I guess knobs for finer grained control would do, at least a sysctl
(or cmdline) to turn it on/off globally and maybe a prctl() for user. We
don't necessarily need this on arm64 but if x86 adds one, we might as
well wire it up.

-- 
Catalin