[PATCH 4.9 078/111] configfs: fix registered group removal

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mike Christie <mchristi@xxxxxxxxxx>

[ Upstream commit cc57c07343bd071cdf1915a91a24ab7d40c9b590 ]

This patch fixes a bug where configfs_register_group had added
a group in a tree, and userspace has done a rmdir on a dir somewhere
above that group and we hit a kernel crash. The problem is configfs_rmdir
will detach everything under it and unlink groups on the default_groups
list. It will not unlink groups added with configfs_register_group so when
configfs_unregister_group is called to drop its references to the group/items
we crash when we try to access the freed dentrys.

The patch just adds a check for if a rmdir has been done above
us and if so just does the unlink part of unregistration.

Sorry if you are getting this multiple times. I thouhgt I sent
this to some of you and lkml, but I do not see it.

Signed-off-by: Mike Christie <mchristi@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Joel Becker <jlbec@xxxxxxxxxxxx>
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/configfs/dir.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -1777,6 +1777,16 @@ void configfs_unregister_group(struct co
 	struct dentry *dentry = group->cg_item.ci_dentry;
 	struct dentry *parent = group->cg_item.ci_parent->ci_dentry;
 
+	mutex_lock(&subsys->su_mutex);
+	if (!group->cg_item.ci_parent->ci_group) {
+		/*
+		 * The parent has already been unlinked and detached
+		 * due to a rmdir.
+		 */
+		goto unlink_group;
+	}
+	mutex_unlock(&subsys->su_mutex);
+
 	inode_lock_nested(d_inode(parent), I_MUTEX_PARENT);
 	spin_lock(&configfs_dirent_lock);
 	configfs_detach_prep(dentry, NULL);
@@ -1791,6 +1801,7 @@ void configfs_unregister_group(struct co
 	dput(dentry);
 
 	mutex_lock(&subsys->su_mutex);
+unlink_group:
 	unlink_group(group);
 	mutex_unlock(&subsys->su_mutex);
 }