Re: [PATCH 4/4] KVM: x86/mmu: Add sanity check to detect stale page faults in "map private PFN"

[Yan Zhao]

On Thu, Aug 06, 2026 at 02:40:50PM -0700, Sean Christopherson wrote:
> Harden the "map private PFN" flow against potentially-fatal bugs or future
> KVM changes by checking for a stale "fault" prior to actually mapping the
> PFN into the guest.  While it should be impossible for the "page fault" to
> become stale, the sanity check is cheap, whereas a broken assumption would
> have a high probability of leading to a guest-expoitable use-after-free.
> 
> Snapshot the invalidation sequence after acquiring mmu_lock to avoid false
> positives, even though doing so completely voids anys and all protection
> against unexpected invalidations.  Pretty much the entire point of
> kvm_tdp_mmu_map_private_pfn() is that it allows mapping a PFN that was
> gifted by the caller, i.e. the caller would have to mess up its one and
> only responsibility.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 379f570ef04f..76e3cd717324 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5210,6 +5210,16 @@ int kvm_tdp_mmu_map_private_pfn(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
>  		 */
>  		WARN_ON_ONCE(kvm_test_request(KVM_REQ_MMU_FREE_OBSOLETE_ROOTS, vcpu));
>  
> +		/*
> +		 * Snapshot the invalidation sequence counter after acquiring
> +		 * mmu_lock, as guest_memfd guarantees the validity of the pfn,
> +		 * i.e. any concurrent invalidations are guaranteed to be
> +		 * irrelevant.
> +		 */
> +		fault.mmu_seq = vcpu->kvm->mmu_invalidate_seq;
Could you explain more about the conditions under which a fault is stale while
guest_memfd guarantees the validity of the pfn?

Given that kvm_tdp_mmu_map_private_pfn() already asserts holding slots_lock and
invalidate_lock, I can't think of one. If we add the sanity check because it's
cheap, why don't we save fault.mmu_seq before getting the pfn to guard against
stale pfn as well?

> +		if (is_page_fault_stale(vcpu, &fault))
> +			continue;
> +
>  		r = kvm_tdp_mmu_map(vcpu, &fault);
>  	} while (r == RET_PF_RETRY);
>