Re: [PATCH 3/7] KVM: x86/pmu: Avoid setting BIT_ULL(-1) to pmu->host_cross_mapped_mask

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 21, 2022, Like Xu wrote:
> On 21/7/2022 8:45 am, Sean Christopherson wrote:
> > On Wed, Jul 13, 2022, Like Xu wrote:
> > > From: Like Xu <likexu@tencent.com>
> > > 
> > > In the extreme case of host counters multiplexing and contention, the
> > > perf_event requested by the guest's pebs counter is not allocated to any
> > > actual physical counter, in which case hw.idx is bookkept as -1,
> > > resulting in an out-of-bounds access to host_cross_mapped_mask.
> > > 
> > > Fixes: 854250329c02 ("KVM: x86/pmu: Disable guest PEBS temporarily in two rare situations")
> > > Signed-off-by: Like Xu <likexu@tencent.com>
> > > ---
> > >   arch/x86/kvm/vmx/pmu_intel.c | 11 +++++------
> > >   1 file changed, 5 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> > > index 53ccba896e77..1588627974fa 100644
> > > --- a/arch/x86/kvm/vmx/pmu_intel.c
> > > +++ b/arch/x86/kvm/vmx/pmu_intel.c
> > > @@ -783,20 +783,19 @@ static void intel_pmu_cleanup(struct kvm_vcpu *vcpu)
> > >   void intel_pmu_cross_mapped_check(struct kvm_pmu *pmu)
> > >   {
> > >   	struct kvm_pmc *pmc = NULL;
> > > -	int bit;
> > > +	int bit, hw_idx;
> > >   	for_each_set_bit(bit, (unsigned long *)&pmu->global_ctrl,
> > >   			 X86_PMC_IDX_MAX) {
> > >   		pmc = intel_pmc_idx_to_pmc(pmu, bit);
> > >   		if (!pmc || !pmc_speculative_in_use(pmc) ||
> > > -		    !intel_pmc_is_enabled(pmc))
> > > +		    !intel_pmc_is_enabled(pmc) || !pmc->perf_event)
> > >   			continue;
> > > -		if (pmc->perf_event && pmc->idx != pmc->perf_event->hw.idx) {
> > > -			pmu->host_cross_mapped_mask |=
> > > -				BIT_ULL(pmc->perf_event->hw.idx);
> > > -		}
> > > +		hw_idx = pmc->perf_event->hw.idx;
> > > +		if (hw_idx != pmc->idx && hw_idx != -1)
> > 
> > How about "hw_idx > 0" so that KVM is a little less dependent on perf's exact
> > behavior?  A comment here would be nice too.
> 
> The "hw->idx = 0" means that it occupies counter 0, so this part will look
> like this:

Doh, typo on my part, meant "hw_idx >= 0".  "> -1" is ok, though it's definitely
less idiomatic:

  $ git grep ">= 0" | wc -l
  5679
  $ git grep "> -1" | wc -l
  66