[PATCH] configfs: Prevent userspace from creating new entries under attaching directories

process 1: 					process 2:
configfs_mkdir("A")
  attach_group("A")
    attach_item("A")
      d_instantiate("A")
    populate_groups("A")
      mutex_lock("A")
      attach_group("A/B")
        attach_item("A")
          d_instantiate("A/B")
						mkdir("A/B/C")
						  do_path_lookup("A/B/C", LOOKUP_PARENT)
						    ok
						  lookup_create("A/B/C")
						    mutex_lock("A/B")
						    ok
						  configfs_mkdir("A/B/C")
						    ok
      attach_group("A/C")
        attach_item("A/C")
          d_instantiate("A/C")
        populate_groups("A/C")
          mutex_lock("A/C")
          attach_group("A/C/D")
            attach_item("A/C/D")
              failure
          mutex_unlock("A/C")
          detach_groups("A/C")
            nothing to do
						mkdir("A/C/E")
						  do_path_lookup("A/C/E", LOOKUP_PARENT)
						    ok
						  lookup_create("A/C/E")
						    mutex_lock("A/C")
						    ok
						  configfs_mkdir("A/C/E")
						    ok
        detach_item("A/C")
        d_delete("A/C")
      mutex_unlock("A")
      detach_groups("A")
        mutex_lock("A/B")
        detach_group("A/B")
	  detach_groups("A/B")
	    nothing since no _default_ group
          detach_item("A/B")
        mutex_unlock("A/B")
        d_delete("A/B")
    detach_item("A")
    d_delete("A")

Two bugs:

1/ "A/B/C" and "A/C/E" are created, but never removed while their parent are
removed in the end. The same could happen with symlink() instead of mkdir().

2/ "A" and "A/C" inodes are not locked while detach_item() is called on them,
   which may probably confuse VFS.

This commit fixes 1/, tagging new directories with CONFIGFS_USET_CREATING before
building the inode and instantiating the dentry, and validating the whole
group+default groups hierarchy in a second pass by clearing
CONFIGFS_USET_CREATING.
	mkdir(), symlink(), lookup(), and dir_open() simply return -ENOENT if
called in (or linking to) a directory tagged with CONFIGFS_USET_CREATING. This
does not prevent userspace from calling stat() successfuly on such directories,
but this prevents userspace from adding (children to | symlinking from/to |
read/write attributes of | listing the contents of) not validated items. In
other words, userspace will not interact with the subsystem on a new item until
the new item creation completes correctly.
	It was first proposed to re-use CONFIGFS_USET_IN_MKDIR instead of a new
flag CONFIGFS_USET_CREATING, but this generated conflicts when checking the
target of a new symlink: a valid target directory in the middle of attaching
a new user-created child item could be wrongly detected as being attached.

2/ is fixed by next commit.

Signed-off-by: Louis Rilling <louis.rilling@kerlabs.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/fs/configfs/symlink.c b/fs/configfs/symlink.c
index 61a886dbd601..bf74973b0492 100644
--- a/fs/configfs/symlink.c
+++ b/fs/configfs/symlink.c
@@ -76,6 +76,9 @@ static int create_link(struct config_item *parent_item,
         struct configfs_symlink *sl;
         int ret;
 
+        ret = -ENOENT;
+        if (!configfs_dirent_is_ready(target_sd))
+                goto out;
         ret = -ENOMEM;
         sl = kmalloc(sizeof(struct configfs_symlink), GFP_KERNEL);
         if (sl) {
@@ -100,6 +103,7 @@ static int create_link(struct config_item *parent_item,
                 }
         }
 
+out:
         return ret;
 }
 
@@ -129,6 +133,7 @@ int configfs_symlink(struct inode *dir, struct dentry *dentry, const char *symna
 {
         int ret;
         struct nameidata nd;
+        struct configfs_dirent *sd;
         struct config_item *parent_item;
         struct config_item *target_item;
         struct config_item_type *type;
@@ -137,9 +142,19 @@ int configfs_symlink(struct inode *dir, struct dentry *dentry, const char *symna
         if (dentry->d_parent == configfs_sb->s_root)
                 goto out;
 
+        sd = dentry->d_parent->d_fsdata;
+        /*
+         * Fake invisibility if dir belongs to a group/default groups hierarchy
+         * being attached
+         */
+        ret = -ENOENT;
+        if (!configfs_dirent_is_ready(sd))
+                goto out;
+
         parent_item = configfs_get_config_item(dentry->d_parent);
         type = parent_item->ci_type;
 
+        ret = -EPERM;
         if (!type || !type->ct_item_ops ||
             !type->ct_item_ops->allow_link)
                 goto out_put;