Re: [PATCH 09/16] KVM: x86/mmu: Move private vs. shared check above slot validity checks

[Huang, Kai]

On 28/02/2024 3:41 pm, Sean Christopherson wrote:
> Prioritize private vs. shared gfn attribute checks above slot validity
> checks to ensure a consistent userspace ABI.  E.g. as is, KVM will exit to
> userspace if there is no memslot, but emulate accesses to the APIC access
> page even if the attributes mismatch.

IMHO, it would be helpful to explicitly say that, in the later case 
(emulate APIC access page) we still want to report MEMORY_FAULT error 
first (so that userspace can have chance to fixup, IIUC) instead of 
emulating directly, which will unlikely work.

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

> Fixes: 8dd2eee9d526 ("KVM: x86/mmu: Handle page fault for private memory")
> Cc: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> Cc: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> Cc: Fuad Tabba <tabba@xxxxxxxxxx>
> Cc: Michael Roth <michael.roth@xxxxxxx>
> Cc: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/mmu/mmu.c | 15 ++++++++++-----
>   1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 9206cfa58feb..58c5ae8be66c 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4365,11 +4365,6 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>   			return RET_PF_EMULATE;
>   	}
>   
> -	if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> -		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> -		return -EFAULT;
> -	}
> -
>   	if (fault->is_private)
>   		return kvm_faultin_pfn_private(vcpu, fault);
>   
> @@ -4410,6 +4405,16 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>   	fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
>   	smp_rmb();
>   
> +	/*
> +	 * Check for a private vs. shared mismatch *after* taking a snapshot of
> +	 * mmu_invalidate_seq, as changes to gfn attributes are guarded by the
> +	 * invalidation notifier.
> +	 */
> +	if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> +		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> +		return -EFAULT;
> +	}
> +
>   	/*
>   	 * Check for a relevant mmu_notifier invalidation event before getting
>   	 * the pfn from the primary MMU, and before acquiring mmu_lock.