On Thu, May 20, 2021, Jim Mattson wrote:
> 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@xxxxxxxxxx>
> ---
> arch/x86/kvm/vmx/nested.c | 15 ++++++++++++++-
> 1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 706c31821362..defd42201bb4 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3175,6 +3175,15 @@ 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_ERROR exit until
> + * someone tries to trigger posted interrupt
> + * processing on this vCPU, to avoid breaking
> + * existing kvm-unit-tests.
Run the lines out to 80 chars. Also, can we change the comment to tie it to
CPU behavior in someway? A few years down the road, "existing kvm-unit-tests"
may not have any relevant meaning, and it's not like kvm-unit-tests is bug free
either. E.g. something like
/*
* Defer the KVM_INTERNAL_ERROR exit until posted
* interrupt processing actually occurs on this vCPU.
* Until that happens, the descriptor is not accessed,
* and userspace can technically rely on that behavior.
*/
> + */
> + vmx->nested.pi_desc = NULL;
> + pin_controls_clearbit(vmx, PIN_BASED_POSTED_INTR);
> }
> }
> if (nested_vmx_prepare_msr_bitmap(vcpu, vmcs12))
> @@ -3689,10 +3698,14 @@ 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;
>
> --
> 2.31.1.818.g46aad6cb9e-goog
>
