Re: [PATCH 10/17] KVM: x86/mmu: Move private vs. shared check above slot validity checks

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 5/7/2024 11:58 PM, Paolo Bonzini wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> 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.
> 
> Fixes: 8dd2eee9d526 ("KVM: x86/mmu: Handle page fault for private memory")
> Cc: Yu Zhang <yu.c.zhang@linux.intel.com>
> Cc: Chao Peng <chao.p.peng@linux.intel.com>
> Cc: Fuad Tabba <tabba@google.com>
> Cc: Michael Roth <michael.roth@amd.com>
> Cc: Isaku Yamahata <isaku.yamahata@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Reviewed-by: Kai Huang <kai.huang@intel.com>
> Message-ID: <20240228024147.41573-10-seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

> ---
>   arch/x86/kvm/mmu/mmu.c | 20 +++++++++++++++-----
>   1 file changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 0d884d0b0f35..ba50b93e93ed 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4317,11 +4317,6 @@ 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);
>   
> @@ -4359,9 +4354,24 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>   {
>   	int ret;
>   
> +	/*
> +	 * Note that the mmu_invalidate_seq also serves to detect a concurrent
> +	 * change in attributes.  is_page_fault_stale() will detect an
> +	 * invalidation relate to fault->fn and resume the guest without

s/fault->fn/fault->gfn/

> +	 * installing a mapping in the page tables.
> +	 */
>   	fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
>   	smp_rmb();
>   
> +	/*
> +	 * Now that we have a snapshot of mmu_invalidate_seq we can check for a
> +	 * private vs. shared mismatch.
> +	 */
> +	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.