linux-kernel - [PATCH] KVM: x86: avoid out of bounds indices for fixed performance counters

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Message-Id: <20211209191101.288041-1-pbonzini@redhat.com>
Date:   Thu,  9 Dec 2021 14:11:01 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     jmattson@...gle.com, like.xu.linux@...il.com, wanpengli@...cent.com
Subject: [PATCH] KVM: x86: avoid out of bounds indices for fixed performance counters

Because IceLake has 4 fixed performance counters but KVM only supports 3,
it is possible for reprogram_fixed_counters to pass to
reprogram_fixed_counter an index that is out of bounds for
the fixed_pmc_events array.

Ultimately intel_find_fixed_event, which is the only place that uses
fixed_pmc_events, handles this correctly because it checks against the
size of fixed_pmc_events anyway.  Every other place
operates on the fixed_counters[] array which is sized
according to INTEL_PMC_MAX_FIXED.  However, it is cleaner if
the unsupported performance counters are culled early on
in reprogram_fixed_counters.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/vmx/pmu_intel.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index 1b7456b2177b..d33e9799276e 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -91,7 +91,7 @@ static unsigned intel_find_fixed_event(int idx)
 	u32 event;
 	size_t size = ARRAY_SIZE(fixed_pmc_events);
 
-	if (idx >= size)
+	if (WARN_ON_ONCE(idx >= size))
 		return PERF_COUNT_HW_MAX;
 
 	event = fixed_pmc_events[array_index_nospec(idx, size)];
@@ -500,8 +500,9 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
 		pmu->nr_arch_fixed_counters = 0;
 	} else {
 		pmu->nr_arch_fixed_counters =
-			min_t(int, edx.split.num_counters_fixed,
-			      x86_pmu.num_counters_fixed);
+			min3(ARRAY_SIZE(fixed_pmc_events),
+			     (size_t) edx.split.num_counters_fixed,
+			     (size_t) x86_pmu.num_counters_fixed);
 		edx.split.bit_width_fixed = min_t(int,
 			edx.split.bit_width_fixed, x86_pmu.bit_width_fixed);
 		pmu->counter_bitmask[KVM_PMC_FIXED] =
-- 
2.31.1