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: 6beb7bd52e48 ("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>

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 4e54599..98b5f5f 100644 (file)
--- a/arch/x86/kvm/vmx/nested.c
+++ b/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;