Re: [PATCH 07/16] KVM: x86: Move synthetic PFERR_* sanity checks to SVM's #NPF handler

[Huang, Kai]

On 28/02/2024 3:41 pm, Sean Christopherson wrote:
> Move the sanity check that hardware never sets bits that collide with KVM-
> define synthetic bits from kvm_mmu_page_fault() to npf_interception(),
> i.e. make the sanity check #NPF specific.  The legacy #PF path already
> WARNs if _any_ of bits 63:32 are set, and the error code that comes from
> VMX's EPT Violatation and Misconfig is 100% synthesized (KVM morphs VMX's
> EXIT_QUALIFICATION into error code flags).
>
> Add a compile-time assert in the legacy #PF handler to make sure that KVM-
> define flags are covered by its existing sanity check on the upper bits.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/mmu/mmu.c | 12 +++---------
>   arch/x86/kvm/svm/svm.c |  9 +++++++++
>   2 files changed, 12 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 5d892bd59c97..bd342ebd0809 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4561,6 +4561,9 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
>   	if (WARN_ON_ONCE(error_code >> 32))
>   		error_code = lower_32_bits(error_code);
>   
> +	/* Ensure the above sanity check also covers KVM-defined flags. */
> +	BUILD_BUG_ON(lower_32_bits(PFERR_SYNTHETIC_MASK));
> +

Could you explain why adding this BUILD_BUG_ON() here, but not ...

>   	vcpu->arch.l1tf_flush_l1d = true;
>   	if (!flags) {
>   		trace_kvm_page_fault(vcpu, fault_address, error_code);
> @@ -5845,15 +5848,6 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
>   	int r, emulation_type = EMULTYPE_PF;
>   	bool direct = vcpu->arch.mmu->root_role.direct;
>   
> -	/*
> -	 * WARN if hardware generates a fault with an error code that collides
> -	 * with KVM-defined sythentic flags.  Clear the flags and continue on,
> -	 * i.e. don't terminate the VM, as KVM can't possibly be relying on a
> -	 * flag that KVM doesn't know about.
> -	 */
> -	if (WARN_ON_ONCE(error_code & PFERR_SYNTHETIC_MASK))
> -		error_code &= ~PFERR_SYNTHETIC_MASK;
> -
>   	if (WARN_ON_ONCE(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
>   		return RET_PF_RETRY;
>   
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index e90b429c84f1..199c4dd8d214 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2055,6 +2055,15 @@ static int npf_interception(struct kvm_vcpu *vcpu)
>   	u64 fault_address = svm->vmcb->control.exit_info_2;
>   	u64 error_code = svm->vmcb->control.exit_info_1;
>   
> +	/*
> +	 * WARN if hardware generates a fault with an error code that collides
> +	 * with KVM-defined sythentic flags.  Clear the flags and continue on,
> +	 * i.e. don't terminate the VM, as KVM can't possibly be relying on a
> +	 * flag that KVM doesn't know about.
> +	 */
> +	if (WARN_ON_ONCE(error_code & PFERR_SYNTHETIC_MASK))
> +		error_code &= ~PFERR_SYNTHETIC_MASK;
> +
>   	trace_kvm_page_fault(vcpu, fault_address, error_code);
>   	return kvm_mmu_page_fault(vcpu, fault_address, error_code,
>   			static_cpu_has(X86_FEATURE_DECODEASSISTS) ?

..  in npf_interception() or some common place like in 
kvm_mmu_page_fault()?

Otherwise,

Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>