[PATCH 2/3] KVM: x86: Fix a semi theoretical bug in kvm_arch_async_page_present_queued

[Maxim Levitsky <mlevitsk@redhat.com>]

Fix a semi theoretical race condition in reading of page_ready_pending
in kvm_arch_async_page_present_queued.

Only trust the value of page_ready_pending if the guest is about to
enter guest mode (vcpu->mode).

To achieve this, read the vcpu->mode using smp_load_acquire which is
paired with smp_store_release in vcpu_enter_guest.

Then only if vcpu_mode is IN_GUEST_MODE, trust the value of the
page_ready_pending because it was written before and therefore its correct
value is visible.

Also if the above mentioned check is true, avoid raising the request
on the target vCPU.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 arch/x86/kvm/x86.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9018d56b4b0a..3d45a4cd08a4 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -13459,9 +13459,14 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
 
 void kvm_arch_async_page_present_queued(struct kvm_vcpu *vcpu)
 {
-	kvm_make_request(KVM_REQ_APF_READY, vcpu);
-	if (!vcpu->arch.apf.pageready_pending)
+	/* Pairs with smp_store_release in vcpu_enter_guest. */
+	bool in_guest_mode = (smp_load_acquire(&vcpu->mode) == IN_GUEST_MODE);
+	bool page_ready_pending = READ_ONCE(vcpu->arch.apf.pageready_pending);
+
+	if (!in_guest_mode || !page_ready_pending) {
+		kvm_make_request(KVM_REQ_APF_READY, vcpu);
 		kvm_vcpu_kick(vcpu);
+	}
 }
 
 bool kvm_arch_can_dequeue_async_page_present(struct kvm_vcpu *vcpu)
-- 
2.49.0