fix cgroup_do_mount() handling of failure exits

same story as with last May fixes in sysfs (7b745a4e
"unfuck sysfs_mount()"); new_sb is left uninitialized
in case of early errors in kernfs_mount_ns() and papering
over it by treating any error from kernfs_mount_ns() as
equivalent to !new_ns ends up conflating the cases when
objects had never been transferred to a superblock with
ones when that has happened and resulting new superblock
had been dropped.  Easily fixed (same way as in sysfs
case).  Additionally, there's a superblock leak on
kernfs_node_dentry() failure *and* a dentry leak inside
kernfs_node_dentry() itself - the latter on probably
impossible errors, but the former not impossible to trigger
(as the matter of fact, injecting allocation failures
at that point *does* trigger it).

Cc: stable@kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/kernfs/mount.c

@@ -196,8 +196,10 @@ struct dentry *kernfs_node_dentry(struct kernfs_node *kn,
 		return dentry;
 
 	knparent = find_next_ancestor(kn, NULL);
-	if (WARN_ON(!knparent))
+	if (WARN_ON(!knparent)) {
+		dput(dentry);
 		return ERR_PTR(-EINVAL);
+	}
 
 	do {
 		struct dentry *dtmp;
@@ -206,8 +208,10 @@ struct dentry *kernfs_node_dentry(struct kernfs_node *kn,
 		if (kn == knparent)
 			return dentry;
 		kntmp = find_next_ancestor(kn, knparent);
-		if (WARN_ON(!kntmp))
+		if (WARN_ON(!kntmp)) {
+			dput(dentry);
 			return ERR_PTR(-EINVAL);
+		}
 		dtmp = lookup_one_len_unlocked(kntmp->name, dentry,
 					       strlen(kntmp->name));
 		dput(dentry);

kernel/cgroup/cgroup.c

@@ -2033,7 +2033,7 @@ struct dentry *cgroup_do_mount(struct file_system_type *fs_type, int flags,
 			       struct cgroup_namespace *ns)
 {
 	struct dentry *dentry;
-	bool new_sb;
+	bool new_sb = false;
 
 	dentry = kernfs_mount(fs_type, flags, root->kf_root, magic, &new_sb);
 
@@ -2043,6 +2043,7 @@ struct dentry *cgroup_do_mount(struct file_system_type *fs_type, int flags,
 	 */
 	if (!IS_ERR(dentry) && ns != &init_cgroup_ns) {
 		struct dentry *nsdentry;
+		struct super_block *sb = dentry->d_sb;
 		struct cgroup *cgrp;
 
 		mutex_lock(&cgroup_mutex);
@@ -2053,12 +2054,14 @@ struct dentry *cgroup_do_mount(struct file_system_type *fs_type, int flags,
 		spin_unlock_irq(&css_set_lock);
 		mutex_unlock(&cgroup_mutex);
 
-		nsdentry = kernfs_node_dentry(cgrp->kn, dentry->d_sb);
+		nsdentry = kernfs_node_dentry(cgrp->kn, sb);
 		dput(dentry);
+		if (IS_ERR(nsdentry))
+			deactivate_locked_super(sb);
 		dentry = nsdentry;
 	}
 
-	if (IS_ERR(dentry) || !new_sb)
+	if (!new_sb)
 		cgroup_put(&root->cgrp);
 
 	return dentry;