[PATCH 2/2] KVM: x86/pmu: Protect kvm->arch.pmu_event_filter with SRCU

[Like Xu]

From: Like Xu <likexu@xxxxxxxxxxx>

Fix the following positive warning:

 =============================
 WARNING: suspicious RCU usage
 arch/x86/kvm/pmu.c:190 suspicious rcu_dereference_check() usage!
 other info that might help us debug this:
 rcu_scheduler_active = 2, debug_locks = 1
 1 lock held by CPU 28/KVM/370841:
 #0: ff11004089f280b8 (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0x87/0x730 [kvm]
 Call Trace:
  <TASK>
  dump_stack_lvl+0x59/0x73
  reprogram_fixed_counter+0x15d/0x1a0 [kvm]
  kvm_pmu_trigger_event+0x1a3/0x260 [kvm]
  ? free_moved_vector+0x1b4/0x1e0
  complete_fast_pio_in+0x8a/0xd0 [kvm]
  [...]

It's possible to call KVM_SET_PMU_EVENT_FILTER ioctl with the vCPU running.
Similar to "kvm->arch.msr_filter", KVM should guarantee that vCPUs will
see either the previous filter or the new filter so that guest pmu events
with identical settings in both the old and new filter have deterministic
behavior.

Fixes: 66bb8a065f5a ("KVM: x86: PMU Event Filter")
Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
Reviewed-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
---
 arch/x86/kvm/pmu.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index af2a3dd22dd9..94319f627f64 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -185,8 +185,9 @@ static bool check_pmu_event_filter(struct kvm_pmc *pmc)
 	struct kvm *kvm = pmc->vcpu->kvm;
 	bool allow_event = true;
 	__u64 key;
-	int idx;
+	int idx, srcu_idx;
 
+	srcu_idx = srcu_read_lock(&kvm->srcu);
 	filter = srcu_dereference(kvm->arch.pmu_event_filter, &kvm->srcu);
 	if (!filter)
 		goto out;
@@ -209,6 +210,7 @@ static bool check_pmu_event_filter(struct kvm_pmc *pmc)
 	}
 
 out:
+	srcu_read_unlock(&kvm->srcu, srcu_idx);
 	return allow_event;
 }
 
-- 
2.35.0