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[02/24] KVM: x86/mmu: Check for shadow-present SPTE before querying A/D status

Message ID 20210225204749.1512652-3-seanjc@google.com (mailing list archive)
State New, archived
Headers show
Series KVM: x86/mmu: Introduce MMU_PRESENT and fix bugs | expand

Commit Message

Sean Christopherson Feb. 25, 2021, 8:47 p.m. UTC
When updating accessed and dirty bits, check that the new SPTE is present
before attempting to query its A/D bits.  Failure to confirm the SPTE is
present can theoretically cause a false negative, e.g. if a MMIO SPTE
replaces a "real" SPTE and somehow the PFNs magically match.

Realistically, this is all but guaranteed to be a benign bug.  Fix it up
primarily so that a future patch can tweak the MMU_WARN_ON checking A/D
status to fire if the SPTE is not-present.

Fixes: f8e144971c68 ("kvm: x86/mmu: Add access tracking for tdp_mmu")
Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
 arch/x86/kvm/mmu/tdp_mmu.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)
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diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index c926c6b899a1..f46972892a2d 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -210,13 +210,12 @@  static int kvm_mmu_page_as_id(struct kvm_mmu_page *sp)
 static void handle_changed_spte_acc_track(u64 old_spte, u64 new_spte, int level)
-	bool pfn_changed = spte_to_pfn(old_spte) != spte_to_pfn(new_spte);
 	if (!is_shadow_present_pte(old_spte) || !is_last_spte(old_spte, level))
 	if (is_accessed_spte(old_spte) &&
-	    (!is_accessed_spte(new_spte) || pfn_changed))
+	    (!is_shadow_present_pte(new_spte) || !is_accessed_spte(new_spte) ||
+	     spte_to_pfn(old_spte) != spte_to_pfn(new_spte)))
@@ -444,7 +443,7 @@  static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
 	if (was_leaf && is_dirty_spte(old_spte) &&
-	    (!is_dirty_spte(new_spte) || pfn_changed))
+	    (!is_present || !is_dirty_spte(new_spte) || pfn_changed))