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

Mingwei Zhang
2022-03-03T21:24:43Z

On Thu, Mar 03, 2022, Mingwei Zhang wrote:
> On Sat, Feb 26, 2022, Sean Christopherson 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>
> > ---
> > 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.
> > + */
>
> I have a quick question here: when the roots are unreachable, we can't
> yield, understand that after reading the comments. However, what if
> there are too many SPTEs that need to be zapped that requires yielding.
> In this case, I guess we will have a RCU warning, which is unavoidable,
> right?

I will take that back. I think the subsequent patches solve the problem
using two passes.

> > + 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
> >