Re: [patch 4/8] x86/tsc: Verify TSC_ADJUST from idle

[Peter Zijlstra]

On Mon, Nov 21, 2016 at 09:16:44AM +0100, Thomas Gleixner wrote:
> On Sun, 20 Nov 2016, Peter Zijlstra wrote:
> > On Sat, Nov 19, 2016 at 01:47:37PM -0000, Thomas Gleixner wrote:
> > > When entering idle, it's a good oportunity to verify that the TSC_ADJUST
> > > MSR has not been tampered with (BIOS hiding SMM cycles). If tampering is
> > > detected, emit a warning and restore it to the previous value.
> > 
> > > +++ b/arch/x86/kernel/process.c
> > > @@ -277,6 +277,7 @@ void exit_idle(void)
> > >  
> > >  void arch_cpu_idle_enter(void)
> > >  {
> > > +	tsc_verify_tsc_adjust();
> > >  	local_touch_nmi();
> > >  	enter_idle();
> > >  }
> > 
> > Doing a RDMSR on the idle path isn't going to be popular. That path is
> > already way too slow.
> 
> Of course we can ratelimit that MSR read with jiffies, but do you have any
> better suggestion aside of doing it timer based?

Not really :/