linux-kernel - Re: [PATCH 17/17] KVM: x86/mmu: Sanity check that __kvm_faultin

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Date: Mon, 13 May 2024 14:40:13 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>,
 David Matlack <dmatlack@...gle.com>, Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCH 17/17] KVM: x86/mmu: Sanity check that __kvm_faultin_pfn()
 doesn't create noslot pfns

On 5/7/2024 11:58 PM, Paolo Bonzini wrote:
> From: Sean Christopherson <seanjc@...gle.com>
> 
> WARN if __kvm_faultin_pfn() generates a "no slot" pfn, and gracefully
> handle the unexpected behavior instead of continuing on with dangerous
> state, e.g. tdp_mmu_map_handle_target_level() _only_ checks fault->slot,
> and so could install a bogus PFN into the guest.
> 
> The existing code is functionally ok, because kvm_faultin_pfn() pre-checks
> all of the cases that result in KVM_PFN_NOSLOT, but it is unnecessarily
> unsafe as it relies on __gfn_to_pfn_memslot() getting the _exact_ same
> memslot, i.e. not a re-retrieved pointer with KVM_MEMSLOT_INVALID set.
> And checking only fault->slot would fall apart if KVM ever added a flag or
> condition that forced emulation, similar to how KVM handles writes to
> read-only memslots.
> 
> Cc: David Matlack <dmatlack@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Reviewed-by: Kai Huang <kai.huang@...el.com>
> Message-ID: <20240228024147.41573-17-seanjc@...gle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

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

> ---
>   arch/x86/kvm/mmu/mmu.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index d717d60c6f19..510eb1117012 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4425,7 +4425,7 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>   	if (unlikely(is_error_pfn(fault->pfn)))
>   		return kvm_handle_error_pfn(vcpu, fault);
>   
> -	if (WARN_ON_ONCE(!fault->slot))
> +	if (WARN_ON_ONCE(!fault->slot || is_noslot_pfn(fault->pfn)))
>   		return kvm_handle_noslot_fault(vcpu, fault, access);
>   
>   	/*