Re: [PATCH v3 00/17] KVM: x86/pmu: Add support to enable Guest PEBS via DS

[Xu, Like]

On 2021/1/25 19:13, Peter Zijlstra wrote:
> On Mon, Jan 25, 2021 at 04:08:22PM +0800, Like Xu wrote:
> > Hi Peter,
> >
> > On 2021/1/22 17:56, Peter Zijlstra wrote:
> > > On Fri, Jan 15, 2021 at 10:51:38AM -0800, Sean Christopherson wrote:
> > > > On Fri, Jan 15, 2021, Andi Kleen wrote:
> > > > > > I'm asking about ucode/hardare.  Is the "guest pebs buffer write -> PEBS PMI"
> > > > > > guaranteed to be atomic?
> > > > > Of course not.
> > > > So there's still a window where the guest could observe the bad counter index,
> > > > correct?
> > > Guest could do a hypercall to fix up the DS area before it tries to read
> > > it I suppose. Or the HV could expose the index mapping and have the
> > > guest fix up it.
> > A weird (malicious) guest would read unmodified PEBS records in the
> > guest PEBS buffer from other vCPUs without the need for hypercall or
> > index mapping from HV.
> >
> > Do you see any security issues on this host index leak window?
> >
> > > Adding a little virt crud on top shouldn't be too hard.
> > The patches 13-17 in this version has modified the guest PEBS buffer
> > to correct the index mapping information in the guest PEBS records.
> Right, but given there is no atomicity between writing the DS area and
> triggering the PMI (as already established earlier in this thread), a
> malicious guest can already access this information, no?

So under the premise that counter cross-mapping is allowed,
how can hypercall help fix it ?

Personally,  I think it is acceptable at the moment, and
no security issues based on this have been defined and found.