[PATCH] ext4: Unregister sysfs path before destroying jbd2 journal

[Ritesh Harjani <riteshh@linux.ibm.com>]

Call ext4_unregister_sysfs(), before destroying jbd2 journal,
since below might cause, NULL pointer dereference issue.
This got reported with LTP tests.

ext4_put_super() 		cat /sys/fs/ext4/loop2/journal_task
	| 				ext4_attr_show();
ext4_jbd2_journal_destroy();  			|
    	|				 journal_task_show()
	| 					|
	| 				task_pid_vnr(NULL);
sbi->s_journal = NULL;

Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
---
 fs/ext4/super.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 5dc65b7583cb..27ab130a40d1 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1024,6 +1024,13 @@ static void ext4_put_super(struct super_block *sb)
 
 	destroy_workqueue(sbi->rsv_conversion_wq);
 
+	/*
+	 * Unregister sysfs before destroying jbd2 journal.
+	 * Since we could still access attr_journal_task attribute via sysfs
+	 * path which could have sbi->s_journal->j_task as NULL
+	 */
+	ext4_unregister_sysfs(sb);
+
 	if (sbi->s_journal) {
 		aborted = is_journal_aborted(sbi->s_journal);
 		err = jbd2_journal_destroy(sbi->s_journal);
@@ -1034,7 +1041,6 @@ static void ext4_put_super(struct super_block *sb)
 		}
 	}
 
-	ext4_unregister_sysfs(sb);
 	ext4_es_unregister_shrinker(sbi);
 	del_timer_sync(&sbi->s_err_report);
 	ext4_release_system_zone(sb);
-- 
2.21.0