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    Subject[PATCH 5.10 111/145] KVM: x86/mmu: Use -1 to flag an undefined spte in get_mmio_spte()
    Date
    From: Sean Christopherson <seanjc@google.com>

    commit 2aa078932ff6c66bf10cc5b3144440dbfa7d813d upstream.

    Return -1 from the get_walk() helpers if the shadow walk doesn't fill at
    least one spte, which can theoretically happen if the walk hits a
    not-present PDPTR. Returning the root level in such a case will cause
    get_mmio_spte() to return garbage (uninitialized stack data). In
    practice, such a scenario should be impossible as KVM shouldn't get a
    reserved-bit page fault with a not-present PDPTR.

    Note, using mmu->root_level in get_walk() is wrong for other reasons,
    too, but that's now a moot point.

    Fixes: 95fb5b0258b7 ("kvm: x86/mmu: Support MMIO in the TDP MMU")
    Cc: Ben Gardon <bgardon@google.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: Sean Christopherson <seanjc@google.com>
    Message-Id: <20201218003139.2167891-2-seanjc@google.com>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/kvm/mmu/mmu.c | 7 ++++++-
    arch/x86/kvm/mmu/tdp_mmu.c | 2 +-
    2 files changed, 7 insertions(+), 2 deletions(-)

    --- a/arch/x86/kvm/mmu/mmu.c
    +++ b/arch/x86/kvm/mmu/mmu.c
    @@ -3488,7 +3488,7 @@ static bool mmio_info_in_cache(struct kv
    static int get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes)
    {
    struct kvm_shadow_walk_iterator iterator;
    - int leaf = vcpu->arch.mmu->root_level;
    + int leaf = -1;
    u64 spte;


    @@ -3532,6 +3532,11 @@ static bool get_mmio_spte(struct kvm_vcp
    else
    leaf = get_walk(vcpu, addr, sptes);

    + if (unlikely(leaf < 0)) {
    + *sptep = 0ull;
    + return reserved;
    + }
    +
    rsvd_check = &vcpu->arch.mmu->shadow_zero_check;

    for (level = root; level >= leaf; level--) {
    --- a/arch/x86/kvm/mmu/tdp_mmu.c
    +++ b/arch/x86/kvm/mmu/tdp_mmu.c
    @@ -1152,8 +1152,8 @@ int kvm_tdp_mmu_get_walk(struct kvm_vcpu
    {
    struct tdp_iter iter;
    struct kvm_mmu *mmu = vcpu->arch.mmu;
    - int leaf = vcpu->arch.mmu->shadow_root_level;
    gfn_t gfn = addr >> PAGE_SHIFT;
    + int leaf = -1;

    tdp_mmu_for_each_pte(iter, mmu, gfn, gfn + 1) {
    leaf = iter.level;

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