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

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: f8e144971c ("kvm: x86/mmu: Add access tracking for tdp_mmu")
Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2021-02-25 12:47:27 -08:00 committed by Paolo Bonzini
parent 2032195713
commit 64bb2769d7
1 changed files with 3 additions and 4 deletions
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7
arch/x86/kvm/mmu/tdp_mmu.c
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@ -197,13 +197,12 @@ 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))
return;
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)))
kvm_set_pfn_accessed(spte_to_pfn(old_spte));
}
@ -442,7 +441,7 @@ 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))
kvm_set_pfn_dirty(spte_to_pfn(old_spte));
/*