KVM: X86: Move PTE present check from loop body to __shadow_walk_next()

So far, the loop bodies already ensure the PTE is present before calling
__shadow_walk_next():  Some loop bodies simply exit with a !PRESENT
directly and some other loop bodies, i.e. FNAME(fetch) and __direct_map()
do not currently guard their walks with is_shadow_present_pte, but only
because they install present non-leaf SPTEs in the loop itself.

But checking pte present in __shadow_walk_next() (which is called from
shadow_walk_okay()) is more prudent; walking past a !PRESENT SPTE
would lead to attempting to read a the next level SPTE from a garbage
iter->shadow_addr.  It also allows to remove the is_shadow_present_pte()
checks from the loop bodies.
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20210906122547.263316-2-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -2220,7 +2220,7 @@ static bool shadow_walk_okay(struct kvm_shadow_walk_iterator *iterator)
 static void __shadow_walk_next(struct kvm_shadow_walk_iterator *iterator,
 			       u64 spte)
 {
-	if (is_last_spte(spte, iterator->level)) {
+	if (!is_shadow_present_pte(spte) || is_last_spte(spte, iterator->level)) {
 		iterator->level = 0;
 		return;
 	}
@@ -3189,9 +3189,6 @@ static u64 *fast_pf_get_last_sptep(struct kvm_vcpu *vcpu, gpa_t gpa, u64 *spte)
 	for_each_shadow_entry_lockless(vcpu, gpa, iterator, old_spte) {
 		sptep = iterator.sptep;
 		*spte = old_spte;
-
-		if (!is_shadow_present_pte(old_spte))
-			break;
 	}
 
 	return sptep;
@@ -3759,9 +3756,6 @@ static int get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes, int *root_level
 		spte = mmu_spte_get_lockless(iterator.sptep);
 
 		sptes[leaf] = spte;
-
-		if (!is_shadow_present_pte(spte))
-			break;
 	}
 
 	return leaf;
@@ -3877,11 +3871,8 @@ static void shadow_page_table_clear_flood(struct kvm_vcpu *vcpu, gva_t addr)
 	u64 spte;
 
 	walk_shadow_page_lockless_begin(vcpu);
-	for_each_shadow_entry_lockless(vcpu, addr, iterator, spte) {
+	for_each_shadow_entry_lockless(vcpu, addr, iterator, spte)
 		clear_sp_write_flooding_count(iterator.sptep);
-		if (!is_shadow_present_pte(spte))
-			break;
-	}
 	walk_shadow_page_lockless_end(vcpu);
 }
 

arch/x86/kvm/mmu/paging_tmpl.h

@@ -1002,7 +1002,7 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa)
 			FNAME(prefetch_gpte)(vcpu, sp, sptep, gpte, false);
 		}
 
-		if (!is_shadow_present_pte(*sptep) || !sp->unsync_children)
+		if (!sp->unsync_children)
 			break;
 	}
 	write_unlock(&vcpu->kvm->mmu_lock);