KVM: x86: MMU: Move initialization of parent_ptes out from kvm_mmu_alloc_page()

Make kvm_mmu_alloc_page() do just what its name tells to do, and remove
the extra allocation error check and zero-initialization of parent_ptes:
shadow page headers allocated by kmem_cache_zalloc() are always in the
per-VCPU pools.
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

CVE-2018-12207

(cherry picked from commit 47005792)
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/x86/kvm/mmu.c

@@ -1736,8 +1736,7 @@ static void drop_parent_pte(struct kvm_mmu_page *sp,
 	mmu_spte_clear_no_track(parent_pte);
 }
 
-static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu,
-					       u64 *parent_pte, int direct)
+static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu, int direct)
 {
 	struct kvm_mmu_page *sp;
 
@@ -1753,8 +1752,6 @@ static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu,
 	 * this feature. See the comments in kvm_zap_obsolete_pages().
 	 */
 	list_add(&sp->link, &vcpu->kvm->arch.active_mmu_pages);
-	sp->parent_ptes.val = 0;
-	mmu_page_add_parent_pte(vcpu, sp, parent_pte);
 	kvm_mod_used_mmu_pages(vcpu->kvm, +1);
 	return sp;
 }
@@ -2143,10 +2140,13 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
 		trace_kvm_mmu_get_page(sp, false);
 		return sp;
 	}
+
 	++vcpu->kvm->stat.mmu_cache_miss;
-	sp = kvm_mmu_alloc_page(vcpu, parent_pte, direct);
-	if (!sp)
-		return sp;
+
+	sp = kvm_mmu_alloc_page(vcpu, direct);
+
+	mmu_page_add_parent_pte(vcpu, sp, parent_pte);
+
 	sp->gfn = gfn;
 	sp->role = role;
 	hlist_add_head(&sp->hash_link,