debugfs: Fix filesystem reference counting on debugfs_remove() failure

When __debugfs_remove() fails (because simple_rmdir() fails e.g. when a
directory is not empty), we must not decrement use count of the filesystem
as nothing was in fact deleted.

This fixes use after free caused by debugfs in some cases.
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

fs/debugfs/inode.c

@@ -307,7 +307,7 @@ struct dentry *debugfs_create_symlink(const char *name, struct dentry *parent,
 }
 EXPORT_SYMBOL_GPL(debugfs_create_symlink);
 
-static void __debugfs_remove(struct dentry *dentry, struct dentry *parent)
+static int __debugfs_remove(struct dentry *dentry, struct dentry *parent)
 {
 	int ret = 0;
 
@@ -330,6 +330,7 @@ static void __debugfs_remove(struct dentry *dentry, struct dentry *parent)
 			dput(dentry);
 		}
 	}
+	return ret;
 }
 
 /**
@@ -348,6 +349,7 @@ static void __debugfs_remove(struct dentry *dentry, struct dentry *parent)
 void debugfs_remove(struct dentry *dentry)
 {
 	struct dentry *parent;
+	int ret;
 
 	if (!dentry)
 		return;
@@ -357,8 +359,9 @@ void debugfs_remove(struct dentry *dentry)
 		return;
 
 	mutex_lock(&parent->d_inode->i_mutex);
-	__debugfs_remove(dentry, parent);
+	ret = __debugfs_remove(dentry, parent);
 	mutex_unlock(&parent->d_inode->i_mutex);
+	if (!ret)
 		simple_release_fs(&debugfs_mount, &debugfs_mount_count);
 }
 EXPORT_SYMBOL_GPL(debugfs_remove);