KVM: x86: Ensure vcpu->mode is loaded from memory in kvm_vcpu_exit_request()

Wrap kvm_vcpu_exit_request()'s load of vcpu->mode with READ_ONCE() to
ensure the variable is re-loaded from memory, as there is no guarantee the
caller provides the necessary annotations to ensure KVM sees a fresh value,
e.g. the VM-Exit fastpath could theoretically reuse the pre-VM-Enter value.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20240828232013.768446-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -2144,8 +2144,9 @@ EXPORT_SYMBOL_GPL(kvm_emulate_monitor);
 static inline bool kvm_vcpu_exit_request(struct kvm_vcpu *vcpu)
 {
 	xfer_to_guest_mode_prepare();
-	return vcpu->mode == EXITING_GUEST_MODE || kvm_request_pending(vcpu) ||
-		xfer_to_guest_mode_work_pending();
+
+	return READ_ONCE(vcpu->mode) == EXITING_GUEST_MODE ||
+	       kvm_request_pending(vcpu) || xfer_to_guest_mode_work_pending();
 }
 
 /*