[PATCH] KVM: x86/pmu: SRCU protect the PMU event filter in the fast path

[Aaron Lewis <aaronlewis@xxxxxxxxxx>]

When running KVM's fast path it is possible to get into a situation
where the PMU event filter is dereferenced without grabbing KVM's SRCU
read lock.

The following callstack demonstrates how that is possible.

Call Trace:
  dump_stack+0x85/0xdf
  lockdep_rcu_suspicious+0x109/0x120
  pmc_event_is_allowed+0x165/0x170
  kvm_pmu_trigger_event+0xa5/0x190
  handle_fastpath_set_msr_irqoff+0xca/0x1e0
  svm_vcpu_run+0x5c3/0x7b0 [kvm_amd]
  vcpu_enter_guest+0x2108/0x2580

Fix that by explicitly grabbing the read lock before dereferencing the
PMU event filter.

Fixes: dfdeda67ea2d ("KVM: x86/pmu: Prevent the PMU from counting disallowed events")

Signed-off-by: Aaron Lewis <aaronlewis@xxxxxxxxxx>
---
 arch/x86/kvm/pmu.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index bf653df86112..2b2247f74ab7 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -381,18 +381,29 @@ static bool check_pmu_event_filter(struct kvm_pmc *pmc)
 {
 	struct kvm_x86_pmu_event_filter *filter;
 	struct kvm *kvm = pmc->vcpu->kvm;
+	bool allowed;
+	int idx;
 
 	if (!static_call(kvm_x86_pmu_hw_event_available)(pmc))
 		return false;
 
+	idx = srcu_read_lock(&kvm->srcu);
+
 	filter = srcu_dereference(kvm->arch.pmu_event_filter, &kvm->srcu);
-	if (!filter)
-		return true;
+	if (!filter) {
+		allowed = true;
+		goto out;
+	}
 
 	if (pmc_is_gp(pmc))
-		return is_gp_event_allowed(filter, pmc->eventsel);
+		allowed = is_gp_event_allowed(filter, pmc->eventsel);
+	else
+		allowed = is_fixed_event_allowed(filter, pmc->idx);
+
+out:
+	srcu_read_unlock(&kvm->srcu, idx);
 
-	return is_fixed_event_allowed(filter, pmc->idx);
+	return allowed;
 }
 
 static bool pmc_event_is_allowed(struct kvm_pmc *pmc)
-- 
2.41.0.178.g377b9f9a00-goog