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

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Message-ID: <ZeqZ+BDTN5bIx0rm@yilunxu-OptiPlex-7050>
Date: Fri, 8 Mar 2024 12:54:16 +0800
From: Xu Yilun <yilun.xu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Yan Zhao <yan.y.zhao@...el.com>,
	Isaku Yamahata <isaku.yamahata@...el.com>,
	Michael Roth <michael.roth@....com>,
	Yu Zhang <yu.c.zhang@...ux.intel.com>,
	Chao Peng <chao.p.peng@...ux.intel.com>,
	Fuad Tabba <tabba@...gle.com>, David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH 09/16] KVM: x86/mmu: Move private vs. shared check above
 slot validity checks

On Tue, Feb 27, 2024 at 06:41:40PM -0800, 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.
> 
> Fixes: 8dd2eee9d526 ("KVM: x86/mmu: Handle page fault for private memory")
> Cc: Yu Zhang <yu.c.zhang@...ux.intel.com>
> Cc: Chao Peng <chao.p.peng@...ux.intel.com>
> Cc: Fuad Tabba <tabba@...gle.com>
> Cc: Michael Roth <michael.roth@....com>
> Cc: Isaku Yamahata <isaku.yamahata@...el.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  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.

I didn't see how mmu_invalidate_seq influences gfn attribute judgement.
And there is no synchronization between the below check and
kvm_vm_set_mem_attributes(), the gfn attribute could still be changing
after the snapshot.  So why this comment?

Thanks,
Yilun

> +	 */
> +	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.
> -- 
> 2.44.0.278.ge034bb2e1d-goog
> 
>