[PATCH 3/6] KVM: SVM: fix AVIC race of host->guest IPI delivery vs AVIC inhibition

Maxim Levitsky
2021-12-09T11:55:39Z

If svm_deliver_avic_intr is called just after the target vcpu's AVIC got
inhibited, it might read a stale value of vcpu->arch.apicv_active
which can lead to the target vCPU not noticing the interrupt.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
arch/x86/kvm/svm/avic.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 859ad2dc50f1..8c1b934bfa9b 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -691,6 +691,15 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
* automatically process AVIC interrupts at VMRUN.
*/
if (vcpu->mode == IN_GUEST_MODE) {
+
+ /*
+ * At this point we had read the vcpu->arch.apicv_active == true
+ * and the vcpu->mode == IN_GUEST_MODE.
+ * Since we have a memory barrier after setting IN_GUEST_MODE,
+ * it ensures that AVIC inhibition is complete and thus
+ * the target is really running with AVIC enabled.
+ */
+
int cpu = READ_ONCE(vcpu->cpu);

/*
@@ -706,10 +715,11 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
put_cpu();
} else {
/*
- * Wake the vCPU if it was blocking. KVM will then detect the
- * pending IRQ when checking if the vCPU has a wake event.
+ * Kick the target vCPU otherwise, to make sure
+ * it processes the interrupt even if its AVIC is inhibited.
*/
- kvm_vcpu_wake_up(vcpu);
+ kvm_make_request(KVM_REQ_EVENT, vcpu);
+ kvm_vcpu_kick(vcpu);
}

return 0;
--
2.26.3