[PATCH] configfs: Pin configfs subsystems separately from new config_items.

configfs_mkdir() creates a new item by calling its parent's
->make_item/group() functions. Once that object is created,
configfs_mkdir() calls try_module_get() on the new item's module. If it
succeeds, the module owning the new item cannot be unloaded, and
configfs is safe to reference the item.

If the item and the subsystem it belongs to are part of the same module,
the subsystem is also pinned. This is the common case.

However, if the subsystem is made up of multiple modules, this may not
pin the subsystem. Thus, it would be possible to unload the toplevel
subsystem module while there is still a child item. Thus, we now
try_module_get() the subsystem's module. This only really affects
children of the toplevel subsystem group. Deeper children already have
their parents pinned.

Signed-off-by: Joel Becker <[e-mail]>
Signed-off-by: Mark Fasheh <[e-mail]>

fs/configfs/dir.c

···
 	struct configfs_subsystem *subsys;
 	struct configfs_dirent *sd;
 	struct config_item_type *type;
-	struct module *owner = NULL;
+	struct module *subsys_owner = NULL, *new_item_owner = NULL;
 	char *name;
 
 	if (dentry->d_parent == configfs_sb->s_root) {
···
 		goto out_put;
 	}
 
+	/*
+	 * The subsystem may belong to a different module than the item
+	 * being created.  We don't want to safely pin the new item but
+	 * fail to pin the subsystem it sits under.
+	 */
+	if (!subsys->su_group.cg_item.ci_type) {
+		ret = -EINVAL;
+		goto out_put;
+	}
+	subsys_owner = subsys->su_group.cg_item.ci_type->ct_owner;
+	if (!try_module_get(subsys_owner)) {
+		ret = -EINVAL;
+		goto out_put;
+	}
+
 	name = kmalloc(dentry->d_name.len + 1, GFP_KERNEL);
 	if (!name) {
 		ret = -ENOMEM;
-		goto out_put;
+		goto out_subsys_put;
 	}
 
 	snprintf(name, dentry->d_name.len + 1, "%s", dentry->d_name.name);
···
 		 * If ret != 0, then link_obj() was never called.
 		 * There are no extra references to clean up.
 		 */
-		goto out_put;
+		goto out_subsys_put;
 	}
 
 	/*
···
 		goto out_unlink;
 	}
 
-	owner = type->ct_owner;
-	if (!try_module_get(owner)) {
+	new_item_owner = type->ct_owner;
+	if (!try_module_get(new_item_owner)) {
 		ret = -EINVAL;
 		goto out_unlink;
 	}
···
 		mutex_unlock(&subsys->su_mutex);
 
 		if (module_got)
-			module_put(owner);
+			module_put(new_item_owner);
 	}
+
+out_subsys_put:
+	if (ret)
+		module_put(subsys_owner);
 
 out_put:
 	/*
···
 	struct config_item *item;
 	struct configfs_subsystem *subsys;
 	struct configfs_dirent *sd;
-	struct module *owner = NULL;
+	struct module *subsys_owner = NULL, *dead_item_owner = NULL;
 	int ret;
 
 	if (dentry->d_parent == configfs_sb->s_root)
···
 		config_item_put(parent_item);
 		return -EINVAL;
 	}
+
+	/* configfs_mkdir() shouldn't have allowed this */
+	BUG_ON(!subsys->su_group.cg_item.ci_type);
+	subsys_owner = subsys->su_group.cg_item.ci_type->ct_owner;
 
 	/*
 	 * Ensure that no racing symlink() will make detach_prep() fail while
···
 	config_item_put(parent_item);
 
 	if (item->ci_type)
-		owner = item->ci_type->ct_owner;
+		dead_item_owner = item->ci_type->ct_owner;
 
 	if (sd->s_type & CONFIGFS_USET_DIR) {
 		configfs_detach_group(item);
···
 	/* Drop our reference from above */
 	config_item_put(item);
 
-	module_put(owner);
+	module_put(dead_item_owner);
+	module_put(subsys_owner);
 
 	return 0;
 }