[PATCH v2 4/4] KVM: x86/mmu: Add sanity check to detect stale page faults in "map private PFN"

[Sean Christopherson <seanjc@google.com>]

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@google.com>
---
 arch/x86/kvm/mmu/mmu.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 19a501029f08..79c450d677b4 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5236,6 +5236,18 @@ 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;
+		if (is_page_fault_stale(vcpu, &fault)) {
+			r = RET_PF_RETRY;
+			continue;
+		}
+
 		r = kvm_tdp_mmu_map(vcpu, &fault);
 	} while (r == RET_PF_RETRY);
 
-- 
2.55.0.860.g4b6b3295ed-goog