[PATCH 5.10 111/145] KVM: x86/mmu: Use -1 to flag an undefined spte in get_mmio_spte()

[Greg Kroah-Hartman]

From: Sean Christopherson <sea...@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 <bgar...@google.com>
Cc: sta...@vger.kernel.org
Signed-off-by: Sean Christopherson <sea...@google.com>
Message-Id: <20201218003139.2167891-2-sea...@google.com>
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gre...@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;