[PATCH v2 3/7] KVM: x86/pmu: Avoid setting BIT_ULL(-1) to pmu->host_cross_mapped_mask

[Like Xu <like.xu.linux@gmail.com>]

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 | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index 4bc098fbec31..22793348aa14 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -783,20 +783,20 @@ 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;
+		/* make it a little less dependent on perf's exact behavior */
+		if (hw_idx != pmc->idx && hw_idx > -1)
+			pmu->host_cross_mapped_mask |= BIT_ULL(hw_idx);
 	}
 }
 
-- 
2.37.1