[MODERATED] Re: [PATCH 9/9] TAA 9

Missed these questions the first time around...

On Thu, Oct 24, 2019 at 06:58:28PM +0200, speck for Borislav Petkov wrote:
> On Thu, Oct 24, 2019 at 11:10:16AM -0500, speck for Josh Poimboeuf wrote:
> > I think this is misleading.  tsx=on doesn't make you vulnerable to TAA,
> > because we still the TAA mitigation.
> 
> Changed to:
> 
>           Therefore TSX is not enabled by default (aka tsx=off). An admin
>           might override this decision by tsx=on the command line parameter.
>           Even with TSX enabled, the kernel will attempt to enable the best 
>           possible TAA mitigation setting depending on the microcode available 
>           for the particular machine.
> 
> > tsx=on vs tsx=auto is not a security consideration, but rather a
> > performance one.  With tsx=auto you disable TSX on some TAA-affected
> > CPUs so you don't have to pay the performance penalty of the MDS
> > mitigations.
> 
> By performance penalty you mean, when you have TSX disabled on those
> parts, you'll save yourself the VERW which should be taking care of TAA
> too?

Right.

> > > +config X86_INTEL_TSX_MODE_OFF
> > > +	bool "off"
> > > +	help
> > > +	  TSX is always disabled - equals tsx=off command line parameter.
> > 
> > Define "always" :-)
> 
> Changed to:
> 
> "TSX is disabled if possible - equals to tsx=off command line parameter."
> 
> > Not exactly :-)  This also leaves TSX enabled on MDS vulnerable parts.
> 
> Your point being, the MD_CLEAR which takes care of TAA too?

You overtrimmed :-) Going back, I believe this comment was about

config X86_INTEL_TSX_MODE_AUTO
	bool "auto"
	help
	  TSX is enabled on TSX capable HW that is believed to be safe against
	  side channel attacks- equals tsx=auto command line parameter.

My point is that this option makes it sounds like TSX is *only* enabled
on non-TAA parts, when in fact it's also enabled on those TAA parts
which are also vulnerable to MDS (this of course assumes the
"overlapping" model described by Andrew).

-- 
Josh