[PATCH] ocfs2: Fix null-ptr-deref in jbd2_journal_update_sb_log_tail

[Edward Adam Davis]

Journal too short will cause ocfs2_check_volume failed, and will set
journal->j_sb_buffer to NULL in journal_fail_superblock before running
journal shutdown.

Reported-and-tested-by: syzbot+05b9b39d8bdfe1a0861f@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=05b9b39d8bdfe1a0861f
Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
---
 fs/ocfs2/journal.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
index 530fba34f6d3..25821077b855 100644
--- a/fs/ocfs2/journal.c
+++ b/fs/ocfs2/journal.c
@@ -1077,9 +1077,11 @@ void ocfs2_journal_shutdown(struct ocfs2_super *osb)
 	BUG_ON(atomic_read(&(osb->journal->j_num_trans)) != 0);
 
 	if (ocfs2_mount_local(osb)) {
-		jbd2_journal_lock_updates(journal->j_journal);
-		status = jbd2_journal_flush(journal->j_journal, 0);
-		jbd2_journal_unlock_updates(journal->j_journal);
+		if (journal->j_journal->j_sb_buffer) {
+			jbd2_journal_lock_updates(journal->j_journal);
+			status = jbd2_journal_flush(journal->j_journal, 0);
+			jbd2_journal_unlock_updates(journal->j_journal);
+		}
 		if (status < 0)
 			mlog_errno(status);
 	}
-- 
2.43.0