Re: [PATCH 16/16] KVM: x86/mmu: Sanity check that __kvm_faultin_pfn() doesn't create noslot pfns

[Huang, Kai]

On 28/02/2024 3:41 pm, Sean Christopherson wrote:
> 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@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   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 43f24a74571a..cedacb1b89c5 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4468,7 +4468,7 @@ 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);
>   
>   	/*

Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>