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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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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.

~Andrew