[PATCH 5.16 108/164] configfs: fix a race in configfs_{,un}register_subsystem()

Greg Kroah-Hartman
2022-02-28T18:13:08Z

From: ChenXiaoSong <chenxiaosong2@huawei.com>

[ 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: 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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/configfs/dir.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index d3cd2a94d1e8c..d1f9d26322027 100644
--- 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();
}

--
2.34.1