Received-SPF: pass (google.com: domain of linux-kernel+bounces-177211-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Message-ID: <90795694-93ce-4668-9059-efbaafd7f4f9@intel.com>
Date: Mon, 13 May 2024 14:40:13 +0800
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH 17/17] KVM: x86/mmu: Sanity check that __kvm_faultin_pfn()
 doesn't create noslot pfns
To: Paolo Bonzini <pbonzini@redhat.com>, linux-kernel@vger.kernel.org,
 kvm@vger.kernel.org
Cc: Sean Christopherson <seanjc@google.com>,
 David Matlack <dmatlack@google.com>, Kai Huang <kai.huang@intel.com>
References: <20240507155817.3951344-1-pbonzini@redhat.com>
 <20240507155817.3951344-18-pbonzini@redhat.com>
Content-Language: en-US
From: Xiaoyao Li <xiaoyao.li@intel.com>
In-Reply-To: <20240507155817.3951344-18-pbonzini@redhat.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

On 5/7/2024 11:58 PM, Paolo Bonzini wrote:
> From: Sean Christopherson <seanjc@google.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@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Reviewed-by: Kai Huang <kai.huang@intel.com>
> Message-ID: <20240228024147.41573-17-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 | 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);
>   
>   	/*