[PATCH] ext4: fix use-after-free race in ext4_remount()'s error path

[Theodore Ts'o]

It's possible for ext4_show_quota_options() to try reading
s_qf_names[i] while it is being modified by ext4_remount() --- most
notably, in ext4_remount's error path when the original values of the
quota file name gets restored.

Reported-by: syzbot+a2872d6feea6918008a9@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Cc: stable@xxxxxxxxxx
---
 fs/ext4/super.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index faf293ed8060..11a1bfae0937 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2048,11 +2048,13 @@ static inline void ext4_show_quota_options(struct seq_file *seq,
 		seq_printf(seq, ",jqfmt=%s", fmtname);
 	}
 
+	down_read(&sb->s_umount);
 	if (sbi->s_qf_names[USRQUOTA])
 		seq_show_option(seq, "usrjquota", sbi->s_qf_names[USRQUOTA]);
 
 	if (sbi->s_qf_names[GRPQUOTA])
 		seq_show_option(seq, "grpjquota", sbi->s_qf_names[GRPQUOTA]);
+	up_read(&sb->s_umount);
 #endif
 }
 
-- 
2.18.0.rc0