Re: [PATCH v3 15/28] KVM: x86/mmu: Add dedicated helper to zap TDP MMU root shadow page

[Ben Gardon <bgardon@google.com>]

On Fri, Feb 25, 2022 at 4:16 PM Sean Christopherson <seanjc@google.com> wrote:
>
> Add a dedicated helper for zapping a TDP MMU root, and use it in the three
> flows that do "zap_all" and intentionally do not do a TLB flush if SPTEs
> are zapped (zapping an entire root is safe if and only if it cannot be in
> use by any vCPU).  Because a TLB flush is never required, unconditionally
> pass "false" to tdp_mmu_iter_cond_resched() when potentially yielding.
>
> Opportunistically document why KVM must not yield when zapping roots that
> are being zapped by kvm_tdp_mmu_put_root(), i.e. roots whose refcount has
> reached zero, and further harden the flow to detect improper KVM behavior
> with respect to roots that are supposed to be unreachable.
>
> In addition to hardening zapping of roots, isolating zapping of roots
> will allow future simplification of zap_gfn_range() by having it zap only
> leaf SPTEs, and by removing its tricky "zap all" heuristic.  By having
> all paths that truly need to free _all_ SPs flow through the dedicated
> root zapper, the generic zapper can be freed of those concerns.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Ben Gardon <bgardon@google.com>

Nice!

> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 98 +++++++++++++++++++++++++++++++-------
>  1 file changed, 82 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 87706e9cc6f3..c5df9a552470 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -56,10 +56,6 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
>         rcu_barrier();
>  }
>
> -static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> -                         gfn_t start, gfn_t end, bool can_yield, bool flush,
> -                         bool shared);
> -
>  static void tdp_mmu_free_sp(struct kvm_mmu_page *sp)
>  {
>         free_page((unsigned long)sp->spt);
> @@ -82,6 +78,9 @@ static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head)
>         tdp_mmu_free_sp(sp);
>  }
>
> +static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
> +                            bool shared);
> +
>  void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
>                           bool shared)
>  {
> @@ -104,7 +103,7 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
>          * intermediate paging structures, that may be zapped, as such entries
>          * are associated with the ASID on both VMX and SVM.
>          */
> -       (void)zap_gfn_range(kvm, root, 0, -1ull, false, false, shared);
> +       tdp_mmu_zap_root(kvm, root, shared);
>
>         call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
>  }
> @@ -751,6 +750,76 @@ static inline bool __must_check tdp_mmu_iter_cond_resched(struct kvm *kvm,
>         return iter->yielded;
>  }
>
> +static inline gfn_t tdp_mmu_max_gfn_host(void)
> +{
> +       /*
> +        * Bound TDP MMU walks at host.MAXPHYADDR, guest accesses beyond that
> +        * will hit a #PF(RSVD) and never hit an EPT Violation/Misconfig / #NPF,
> +        * and so KVM will never install a SPTE for such addresses.
> +        */
> +       return 1ULL << (shadow_phys_bits - PAGE_SHIFT);
> +}
> +
> +static void tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
> +                            bool shared)
> +{
> +       bool root_is_unreachable = !refcount_read(&root->tdp_mmu_root_count);
> +       struct tdp_iter iter;
> +
> +       gfn_t end = tdp_mmu_max_gfn_host();
> +       gfn_t start = 0;
> +
> +       kvm_lockdep_assert_mmu_lock_held(kvm, shared);
> +
> +       rcu_read_lock();
> +
> +       /*
> +        * No need to try to step down in the iterator when zapping an entire
> +        * root, zapping an upper-level SPTE will recurse on its children.
> +        */
> +       for_each_tdp_pte_min_level(iter, root, root->role.level, start, end) {
> +retry:
> +               /*
> +                * Yielding isn't allowed when zapping an unreachable root as
> +                * the root won't be processed by mmu_notifier callbacks.  When
> +                * handling an unmap/release mmu_notifier command, KVM must
> +                * drop all references to relevant pages prior to completing
> +                * the callback.  Dropping mmu_lock can result in zapping SPTEs
> +                * for an unreachable root after a relevant callback completes,
> +                * which leads to use-after-free as zapping a SPTE triggers
> +                * "writeback" of dirty/accessed bits to the SPTE's associated
> +                * struct page.
> +                */
> +               if (!root_is_unreachable &&
> +                   tdp_mmu_iter_cond_resched(kvm, &iter, false, shared))
> +                       continue;
> +
> +               if (!is_shadow_present_pte(iter.old_spte))
> +                       continue;
> +
> +               if (!shared) {
> +                       tdp_mmu_set_spte(kvm, &iter, 0);
> +               } else if (tdp_mmu_set_spte_atomic(kvm, &iter, 0)) {
> +                       /*
> +                        * cmpxchg() shouldn't fail if the root is unreachable.
> +                        * Retry so as not to leak the page and its children.
> +                        */
> +                       WARN_ONCE(root_is_unreachable,
> +                                 "Contended TDP MMU SPTE in unreachable root.");
> +                       goto retry;
> +               }
> +
> +               /*
> +                * WARN if the root is invalid and is unreachable, all SPTEs
> +                * should've been zapped by kvm_tdp_mmu_zap_invalidated_roots(),
> +                * and inserting new SPTEs under an invalid root is a KVM bug.
> +                */
> +               WARN_ON_ONCE(root_is_unreachable && root->role.invalid);
> +       }
> +
> +       rcu_read_unlock();
> +}
> +
>  bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
>  {
>         u64 old_spte;
> @@ -799,8 +868,7 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>                           gfn_t start, gfn_t end, bool can_yield, bool flush,
>                           bool shared)
>  {
> -       gfn_t max_gfn_host = 1ULL << (shadow_phys_bits - PAGE_SHIFT);
> -       bool zap_all = (start == 0 && end >= max_gfn_host);
> +       bool zap_all = (start == 0 && end >= tdp_mmu_max_gfn_host());
>         struct tdp_iter iter;
>
>         /*
> @@ -809,12 +877,7 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>          */
>         int min_level = zap_all ? root->role.level : PG_LEVEL_4K;
>
> -       /*
> -        * Bound the walk at host.MAXPHYADDR, guest accesses beyond that will
> -        * hit a #PF(RSVD) and never get to an EPT Violation/Misconfig / #NPF,
> -        * and so KVM will never install a SPTE for such addresses.
> -        */
> -       end = min(end, max_gfn_host);
> +       end = min(end, tdp_mmu_max_gfn_host());
>
>         kvm_lockdep_assert_mmu_lock_held(kvm, shared);
>
> @@ -874,6 +937,7 @@ bool __kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id, gfn_t start,
>
>  void kvm_tdp_mmu_zap_all(struct kvm *kvm)
>  {
> +       struct kvm_mmu_page *root;
>         int i;
>
>         /*
> @@ -881,8 +945,10 @@ void kvm_tdp_mmu_zap_all(struct kvm *kvm)
>          * is being destroyed or the userspace VMM has exited.  In both cases,
>          * KVM_RUN is unreachable, i.e. no vCPUs will ever service the request.
>          */
> -       for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
> -               (void)kvm_tdp_mmu_zap_gfn_range(kvm, i, 0, -1ull, false);
> +       for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> +               for_each_tdp_mmu_root_yield_safe(kvm, root, i, false)
> +                       tdp_mmu_zap_root(kvm, root, false);
> +       }
>  }
>
>  /*
> @@ -908,7 +974,7 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
>                  * will still flush on yield, but that's a minor performance
>                  * blip and not a functional issue.
>                  */
> -               (void)zap_gfn_range(kvm, root, 0, -1ull, true, false, true);
> +               tdp_mmu_zap_root(kvm, root, true);
>
>                 /*
>                  * Put the reference acquired in kvm_tdp_mmu_invalidate_roots().
> --
> 2.35.1.574.g5d30c73bfb-goog
>