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

[Mingwei Zhang <mizhang@google.com>]

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
> >