Do a remote TLB flush if installing a leaf SPTE overwrites an existing
leaf SPTE (with the same target pfn, which is enforced by a BUG() in
handle_changed_spte()) and clears the MMU-Writable bit.  Since the TDP MMU
passes ACC_ALL to make_spte(), i.e. always requests a Writable SPTE, the
only scenario in which make_spte() should create a !MMU-Writable SPTE is
if the gfn is write-tracked or if KVM is prefetching a SPTE.

When write-protecting for write-tracking, KVM must hold mmu_lock for write,
i.e. can't race with a vCPU faulting in the SPTE.  And when prefetching a
SPTE, the TDP MMU takes care to avoid clobbering a shadow-present SPTE,
i.e. it should be impossible to replace a MMU-writable SPTE with a
!MMU-writable SPTE when handling a TDP MMU fault.

Cc: David Matlack <dmatlack@google.com>
Cc: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 9c66be7fb002..bc9e2f50dc80 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1033,7 +1033,9 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
 	else if (tdp_mmu_set_spte_atomic(vcpu->kvm, iter, new_spte))
 		return RET_PF_RETRY;
 	else if (is_shadow_present_pte(iter->old_spte) &&
-		 !is_last_spte(iter->old_spte, iter->level))
+		 (!is_last_spte(iter->old_spte, iter->level) ||
+		  WARN_ON_ONCE(is_mmu_writable_spte(iter->old_spte) &&
+			       !is_mmu_writable_spte(new_spte))))
 		kvm_flush_remote_tlbs_gfn(vcpu->kvm, iter->gfn, iter->level);
 
 	/*
-- 
2.47.0.rc1.288.g06298d1525-goog