[PATCH 11/28] KVM: x86/mmu: Check for !leaf=>leaf, not PFN change, in TDP MMU SP removal

Sean Christopherson
2021-11-20T04:52:56Z

Look for a !leaf=>leaf conversion instead of a PFN change when checking
if a SPTE change removed a TDP MMU shadow page. Convert the PFN check
into a WARN, as KVM should never change the PFN of a shadow page (except
when its being zapped or replaced).

From a purely theoretical perspective, it's not illegal to replace a SP
with a hugepage pointing at the same PFN. In practice, it's impossible
as that would require mapping guest memory overtop a kernel-allocated SP.
Either way, the check is odd.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/tdp_mmu.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 138c7dc41d2c..8e446ef03022 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -489,9 +489,12 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,

/*
* Recursively handle child PTs if the change removed a subtree from
- * the paging structure.
+ * the paging structure. Note the WARN on the PFN changing without the
+ * SPTE being converted to a hugepage (leaf) or being zapped. Shadow
+ * pages are kernel allocations and should never be migrated.
*/
- if (was_present && !was_leaf && (pfn_changed || !is_present))
+ if (was_present && !was_leaf &&
+ (is_leaf || !is_present || WARN_ON_ONCE(pfn_changed)))
handle_removed_tdp_mmu_page(kvm,
spte_to_child_pt(old_spte, level), shared);
}
--
2.34.0.rc2.393.gf8c9666880-goog