Re: [PATCH 3/3] KVM: x86/mmu: Fix TDP MMU page table level

[Ben Gardon <bgardon@google.com>]

On Wed, May 5, 2021 at 9:28 AM Ben Gardon <bgardon@google.com> wrote:
>
> On Wed, May 5, 2021 at 2:38 AM Kai Huang <kai.huang@intel.com> wrote:
> >
> > TDP MMU iterator's level is identical to page table's actual level.  For
> > instance, for the last level page table (whose entry points to one 4K
> > page), iter->level is 1 (PG_LEVEL_4K), and in case of 5 level paging,
> > the iter->level is mmu->shadow_root_level, which is 5.  However, struct
> > kvm_mmu_page's level currently is not set correctly when it is allocated
> > in kvm_tdp_mmu_map().  When iterator hits non-present SPTE and needs to
> > allocate a new child page table, currently iter->level, which is the
> > level of the page table where the non-present SPTE belongs to, is used.
> > This results in struct kvm_mmu_page's level always having its parent's
> > level (excpet root table's level, which is initialized explicitly using
> > mmu->shadow_root_level).  This is kinda wrong, and not consistent with
> > existing non TDP MMU code.  Fortuantely the sp->role.level is only used
> > in handle_removed_tdp_mmu_page(), which apparently is already aware of
> > this, and handles correctly.  However to make it consistent with non TDP
> > MMU code (and fix the issue that both root page table and any child of
> > it having shadow_root_level), fix this by using iter->level - 1 in
> > kvm_tdp_mmu_map().  Also modify handle_removed_tdp_mmu_page() to handle
> > such change.
>
> Ugh. Thank you for catching this. This is going to take me a bit to
> review as I should audit the code more broadly for this problem in the
> TDP MMU.
> It would probably also be a good idea to add a comment on the level
> field to say that it represents the level of the SPTEs in the
> associated page, not the level of the SPTE that links to the
> associated page.
> Hopefully that will prevent similar future misunderstandings.

I went through and manually audited the code. I think the only case
that needs to be added to this is for nx recovery:

--- a/arch/x86/kvm/mmu/tdp_mmu.h
+++ b/arch/x86/kvm/mmu/tdp_mmu.h
@@ -31,7 +31,7 @@ static inline bool kvm_tdp_mmu_zap_gfn_range(struct
kvm *kvm, int as_id,
 }
 static inline bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
 {
-       gfn_t end = sp->gfn + KVM_PAGES_PER_HPAGE(sp->role.level);
+       gfn_t end = sp->gfn + KVM_PAGES_PER_HPAGE(sp->role.level + 1);

        /*
         * Don't allow yielding, as the caller may have a flush pending.  Note,

Otherwise we won't zap the full page with this change, resulting in
ineffective or less reliable NX recovery.

>
> >
> > Signed-off-by: Kai Huang <kai.huang@intel.com>
> > ---
> >  arch/x86/kvm/mmu/tdp_mmu.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index debe8c3ec844..bcfb87e1c06e 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -335,7 +335,7 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, tdp_ptep_t pt,
> >
> >         for (i = 0; i < PT64_ENT_PER_PAGE; i++) {
> >                 sptep = rcu_dereference(pt) + i;
> > -               gfn = base_gfn + (i * KVM_PAGES_PER_HPAGE(level - 1));
> > +               gfn = base_gfn + i * KVM_PAGES_PER_HPAGE(level);
> >
> >                 if (shared) {
> >                         /*
> > @@ -377,12 +377,12 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, tdp_ptep_t pt,
> >                         WRITE_ONCE(*sptep, REMOVED_SPTE);
> >                 }
> >                 handle_changed_spte(kvm, kvm_mmu_page_as_id(sp), gfn,
> > -                                   old_child_spte, REMOVED_SPTE, level - 1,
> > +                                   old_child_spte, REMOVED_SPTE, level,
> >                                     shared);
> >         }
> >
> >         kvm_flush_remote_tlbs_with_address(kvm, gfn,
> > -                                          KVM_PAGES_PER_HPAGE(level));
> > +                                          KVM_PAGES_PER_HPAGE(level + 1));
> >
> >         call_rcu(&sp->rcu_head, tdp_mmu_free_sp_rcu_callback);
> >  }
> > @@ -1009,7 +1009,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> >                 }
> >
> >                 if (!is_shadow_present_pte(iter.old_spte)) {
> > -                       sp = alloc_tdp_mmu_page(vcpu, iter.gfn, iter.level);
> > +                       sp = alloc_tdp_mmu_page(vcpu, iter.gfn, iter.level - 1);
> >                         child_pt = sp->spt;
> >
> >                         new_spte = make_nonleaf_spte(child_pt,
> > --
> > 2.31.1
> >