Re: [PATCH 01/17] KVM: x86/mmu: Exit to userspace with -EFAULT if private fault hits emulation

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

On 5/7/2024 11:58 PM, Paolo Bonzini wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> Exit to userspace with -EFAULT / KVM_EXIT_MEMORY_FAULT if a private fault
> triggers emulation of any kind, as KVM doesn't currently support emulating
> access to guest private memory.  Practically speaking, private faults and
> emulation are already mutually exclusive, but there are many flow that
> can result in KVM returning RET_PF_EMULATE, and adding one last check
> to harden against weird, unexpected combinations and/or KVM bugs is
> inexpensive.
> 
> Suggested-by: Yan Zhao <yan.y.zhao@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Message-ID: <20240228024147.41573-2-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          |  8 --------
>   arch/x86/kvm/mmu/mmu_internal.h | 19 +++++++++++++++++++
>   2 files changed, 19 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 45b6d8f9e359..c72a2033ca96 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4257,14 +4257,6 @@ static inline u8 kvm_max_level_for_order(int order)
>   	return PG_LEVEL_4K;
>   }
>   
> -static void kvm_mmu_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
> -					      struct kvm_page_fault *fault)
> -{
> -	kvm_prepare_memory_fault_exit(vcpu, fault->gfn << PAGE_SHIFT,
> -				      PAGE_SIZE, fault->write, fault->exec,
> -				      fault->is_private);
> -}
> -
>   static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
>   				   struct kvm_page_fault *fault)
>   {
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index 5390a591a571..61f49967047a 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -279,6 +279,14 @@ enum {
>   	RET_PF_SPURIOUS,
>   };
>   
> +static inline void kvm_mmu_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
> +						     struct kvm_page_fault *fault)
> +{
> +	kvm_prepare_memory_fault_exit(vcpu, fault->gfn << PAGE_SHIFT,
> +				      PAGE_SIZE, fault->write, fault->exec,
> +				      fault->is_private);
> +}
> +
>   static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>   					u32 err, bool prefetch, int *emulation_type)
>   {
> @@ -320,6 +328,17 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>   	else
>   		r = vcpu->arch.mmu->page_fault(vcpu, &fault);
>   
> +	/*
> +	 * Not sure what's happening, but punt to userspace and hope that
> +	 * they can fix it by changing memory to shared, or they can
> +	 * provide a better error.
> +	 */
> +	if (r == RET_PF_EMULATE && fault.is_private) {
> +		pr_warn_ratelimited("kvm: unexpected emulation request on private memory\n");
> +		kvm_mmu_prepare_memory_fault_exit(vcpu, &fault);
> +		return -EFAULT;
> +	}
> +
>   	if (fault.write_fault_to_shadow_pgtable && emulation_type)
>   		*emulation_type |= EMULTYPE_WRITE_PF_TO_SP;
>