Re: [PATCH 07/12] KVM: nVMX: Disable vmcs02 posted interrupts if vmcs12 PID isn't mappable

From: Sean Christopherson <seanjc@google.com>
To: Jim Mattson <jmattson@google.com>
Cc: kvm@vger.kernel.org, pbonzini@redhat.com
Subject: Re: [PATCH 07/12] KVM: nVMX: Disable vmcs02 posted interrupts if vmcs12 PID isn't mappable
Date: Mon, 24 May 2021 23:21:56 +0000	[thread overview]
Message-ID: <YKw1FGuq5YzSiael@google.com> (raw)
In-Reply-To: <20210520230339.267445-8-jmattson@google.com>

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@google.com>
> ---
>  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
> 