KVM: nVMX: Disable vmcs02 posted interrupts if vmcs12 PID isn't mappable

Don't allow posted interrupts to modify a stale posted interrupt
descriptor (including the initial value of 0).

Empirical tests on real hardware reveal that a posted interrupt
descriptor referencing an unbacked address has PCI bus error semantics
(reads as all 1's; writes are ignored). However, kvm can't distinguish
unbacked addresses from device-backed (MMIO) addresses, so it should
really ask userspace for an MMIO completion. That's overly
complicated, so just punt with KVM_INTERNAL_ERROR.

Don't return the error until the posted interrupt descriptor is
actually accessed. We don't want to break the existing kvm-unit-tests
that assume they can launch an L2 VM with a posted interrupt
descriptor that references MMIO space in L1.

Fixes: 6beb7bd52e ("kvm: nVMX: Refactor nested_get_vmcs12_pages()")
Signed-off-by: Jim Mattson <jmattson@google.com>
Message-Id: <20210604172611.281819-8-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -3184,6 +3184,15 @@ static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
 				offset_in_page(vmcs12->posted_intr_desc_addr));
 			vmcs_write64(POSTED_INTR_DESC_ADDR,
 				     pfn_to_hpa(map->pfn) + offset_in_page(vmcs12->posted_intr_desc_addr));
+		} else {
+			/*
+			 * Defer the KVM_INTERNAL_EXIT until KVM tries to
+			 * access the contents of the VMCS12 posted interrupt
+			 * descriptor. (Note that KVM may do this when it
+			 * should not, per the architectural specification.)
+			 */
+			vmx->nested.pi_desc = NULL;
+			pin_controls_clearbit(vmx, PIN_BASED_POSTED_INTR);
 		}
 	}
 	if (nested_vmx_prepare_msr_bitmap(vcpu, vmcs12))
@@ -3696,10 +3705,14 @@ static int vmx_complete_nested_posted_interrupt(struct kvm_vcpu *vcpu)
 	void *vapic_page;
 	u16 status;
 
-	if (!vmx->nested.pi_desc || !vmx->nested.pi_pending)
+	if (!vmx->nested.pi_pending)
 		return 0;
 
+	if (!vmx->nested.pi_desc)
+		goto mmio_needed;
+
 	vmx->nested.pi_pending = false;
+
 	if (!pi_test_and_clear_on(vmx->nested.pi_desc))
 		return 0;