[MODERATED] Re: [PATCH 4/9] TAA 4

On Thu, Oct 24, 2019 at 08:13:44PM +0100, speck for Andrew Cooper wrote:
> On 24/10/2019 19:56, speck for Josh Poimboeuf wrote:
> > On Thu, Oct 24, 2019 at 07:23:57PM +0100, speck for Andrew Cooper wrote:
> >> On 24/10/2019 17:43, speck for Borislav Petkov wrote:
> >>> On Thu, Oct 24, 2019 at 10:32:40AM -0500, speck for Josh Poimboeuf wrote:
> >>>> As I said before this would be a lot nicer if we could just add NO_TAA
> >>>> to the cpu_vuln_whitelist.
> >>> We're waiting for a list of CPUs from Intel here, right?
> >>>
> >> There is no model list required.  Vulnerability to TAA is calculable
> >> directly from existing architectural sources.
> > Can you elaborate?  Earlier I suggested relying on NO_MDS in
> > cpu_vuln_whitelist, but I believe you said that's not sufficient,
> > because some of the non-MDS models don't have TSX, in which case we
> > shouldn't set TAA_BUG.
> >
> > Which models are those?
> 
> Ok.  First things first.  Do you (and by this, I really mean Linux) want
> to consider TAA an overlapping set with MDS, or a disjoint set?
> 
> After considering this for ages, and particularly, how to explain it
> clearly to non-experts in Xen's security advisory, I chose to go with this:
> 
> ---8<---
> Vulnerability to TAA is a little complicated to quantify.
> 
> In the pipeline, it is just another way to get speculative access to
> stale load port, store buffer or fill buffer data, and therefore can be
> considered a superset of MDS.  On parts which predate MDS_NO, the
> existing VERW flushing will mitigate this sidechannel as well.
> 
> On parts which contain MDS_NO, the lack of VERW flushing means that an
> attacker can still target microarchitectural buffers to leak secrets.
> Therefore, we consider TAA to be the set of parts which have MDS_NO but
> lack TAA_NO.
> ---8<---
> 
> The simplifying fact is that vulnerability to TAA doesn't matter on CPUs
> which don't advertise MDS_NO, because you're already doing VERW and
> disabling hyperthreading, *and* can't turn TSX off if it actually available.
> 
> People who were not taking MDS mitigations in the first place won't
> change their minds because of TAA, either.

Good question.

The current Linux patches consider them overlapping.  But it _might_
possibly be easier to communicate if we considered them disjoint.  I
don't know if there's a good answer, but at this point it might be
easiest to stick with our current overlapping approach.

-- 
Josh