configfs: fix a race in configfs_{,un}register_subsystem()

When configfs_register_subsystem() or configfs_unregister_subsystem()
is executing link_group() or unlink_group(),
it is possible that two processes add or delete list concurrently.
Some unfortunate interleavings of them can cause kernel panic.

One of cases is:
A --> B --> C --> D
A <-- B <-- C <-- D

     delete list_head *B        |      delete list_head *C
--------------------------------|-----------------------------------
configfs_unregister_subsystem   |   configfs_unregister_subsystem
  unlink_group                  |     unlink_group
    unlink_obj                  |       unlink_obj
      list_del_init             |         list_del_init
        __list_del_entry        |           __list_del_entry
          __list_del            |             __list_del
            // next == C        |
            next->prev = prev   |
                                |               next->prev = prev
            prev->next = next   |
                                |                 // prev == B
                                |                 prev->next = next

Fix this by adding mutex when calling link_group() or unlink_group(),
but parent configfs_subsystem is NULL when config_item is root.
So I create a mutex configfs_subsystem_mutex.

Fixes: 7063fbf22611 ("[PATCH] configfs: User-driven configuration filesystem")
Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>
Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index d3cd2a9..d1f9d26 100644 (file)
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -34,6 +34,14 @@
  */
 DEFINE_SPINLOCK(configfs_dirent_lock);
 
+/*
+ * All of link_obj/unlink_obj/link_group/unlink_group require that
+ * subsys->su_mutex is held.
+ * But parent configfs_subsystem is NULL when config_item is root.
+ * Use this mutex when config_item is root.
+ */
+static DEFINE_MUTEX(configfs_subsystem_mutex);
+
 static void configfs_d_iput(struct dentry * dentry,
                            struct inode * inode)
 {
@@ -1859,7 +1867,9 @@ int configfs_register_subsystem(struct configfs_subsystem *subsys)
                group->cg_item.ci_name = group->cg_item.ci_namebuf;
 
        sd = root->d_fsdata;
+       mutex_lock(&configfs_subsystem_mutex);
        link_group(to_config_group(sd->s_element), group);
+       mutex_unlock(&configfs_subsystem_mutex);
 
        inode_lock_nested(d_inode(root), I_MUTEX_PARENT);
 
@@ -1884,7 +1894,9 @@ int configfs_register_subsystem(struct configfs_subsystem *subsys)
        inode_unlock(d_inode(root));
 
        if (err) {
+               mutex_lock(&configfs_subsystem_mutex);
                unlink_group(group);
+               mutex_unlock(&configfs_subsystem_mutex);
                configfs_release_fs();
        }
        put_fragment(frag);
@@ -1931,7 +1943,9 @@ void configfs_unregister_subsystem(struct configfs_subsystem *subsys)
 
        dput(dentry);
 
+       mutex_lock(&configfs_subsystem_mutex);
        unlink_group(group);
+       mutex_unlock(&configfs_subsystem_mutex);
        configfs_release_fs();
 }