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    Subject[PATCH 5.16 0001/1039] KVM: x86/mmu: Fix write-protection of PTs mapped by the TDP MMU
    From: David Matlack <>

    commit 7c8a4742c4abe205ec9daf416c9d42fd6b406e8e upstream.

    When the TDP MMU is write-protection GFNs for page table protection (as
    opposed to for dirty logging, or due to the HVA not being writable), it
    checks if the SPTE is already write-protected and if so skips modifying
    the SPTE and the TLB flush.

    This behavior is incorrect because it fails to check if the SPTE
    is write-protected for page table protection, i.e. fails to check
    that MMU-writable is '0'. If the SPTE was write-protected for dirty
    logging but not page table protection, the SPTE could locklessly be made
    writable, and vCPUs could still be running with writable mappings cached
    in their TLB.

    Fix this by only skipping setting the SPTE if the SPTE is already
    write-protected *and* MMU-writable is already clear. Technically,
    checking only MMU-writable would suffice; a SPTE cannot be writable
    without MMU-writable being set. But check both to be paranoid and
    because it arguably yields more readable code.

    Fixes: 46044f72c382 ("kvm: x86/mmu: Support write protection for nesting in tdp MMU")
    Signed-off-by: David Matlack <>
    Message-Id: <>
    Reviewed-by: Sean Christopherson <>
    Signed-off-by: Paolo Bonzini <>
    Signed-off-by: Greg Kroah-Hartman <>
    arch/x86/kvm/mmu/tdp_mmu.c | 6 +++---
    1 file changed, 3 insertions(+), 3 deletions(-)

    --- a/arch/x86/kvm/mmu/tdp_mmu.c
    +++ b/arch/x86/kvm/mmu/tdp_mmu.c
    @@ -1442,12 +1442,12 @@ static bool write_protect_gfn(struct kvm
    !is_last_spte(iter.old_spte, iter.level))

    - if (!is_writable_pte(iter.old_spte))
    - break;
    new_spte = iter.old_spte &
    ~(PT_WRITABLE_MASK | shadow_mmu_writable_mask);

    + if (new_spte == iter.old_spte)
    + break;
    tdp_mmu_set_spte(kvm, &iter, new_spte);
    spte_set = true;

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