Commit b7e2b91f authored Feb 15, 2022 by ChenXiaoSong Committed by Greg Kroah-Hartman Mar 02, 2022

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



[ Upstream commit 84ec758fb2daa236026506868c8796b0500c047d ]

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: 7063fbf2 ("[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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent df14d2be

fs/configfs/dir.c

+14 −0

Original line number	Diff line number	Diff line
		@@ -36,6 +36,14 @@
		*/
		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)
		{
		@@ -1884,7 +1892,9 @@ 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);

		@@ -1909,7 +1919,9 @@ 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);
		@@ -1956,7 +1968,9 @@ 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();
		}