On Wed, 2021-05-05 at 10:01 -0700, Ben Gardon wrote:
> On Wed, May 5, 2021 at 9:28 AM Ben Gardon <bgardon@xxxxxxxxxx> wrote:
> >
> > On Wed, May 5, 2021 at 2:38 AM Kai Huang <kai.huang@xxxxxxxxx> 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.
Oh correct. I missed this. I'll add this in next version. Thanks.
>
> >
> > >
> > > Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
> > > ---
> > > 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
> > >
