[PATCH 5.10/5.15 1/1] ext4: fix timer use-after-free on failed mount

From: d . privalov
Date: Thu Dec 26 2024 - 04:33:35 EST


From: Xiaxi Shen <shenxiaxi26@xxxxxxxxx>

commit 0ce160c5bdb67081a62293028dc85758a8efb22a upstream.

Syzbot has found an ODEBUG bug in ext4_fill_super

The del_timer_sync function cancels the s_err_report timer,
which reminds about filesystem errors daily. We should
guarantee the timer is no longer active before kfree(sbi).

When filesystem mounting fails, the flow goes to failed_mount3,
where an error occurs when ext4_stop_mmpd is called, causing
a read I/O failure. This triggers the ext4_handle_error function
that ultimately re-arms the timer,
leaving the s_err_report timer active before kfree(sbi) is called.

Fix the issue by canceling
the s_err_report timer after calling ext4_stop_mmpd.

Signed-off-by: Xiaxi Shen <shenxiaxi26@xxxxxxxxx>
Reported-and-tested-by: syzbot+59e0101c430934bc9a36@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=59e0101c430934bc9a36
Link: https://patch.msgid.link/20240715043336.98097-1-shenxiaxi26@xxxxxxxxx
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Cc: stable@xxxxxxxxxx
Signed-off-by: Dmitriy Privalov <d.privalov@xxxxxx>
---
fs/ext4/super.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 9d7800d66200..522a5d85fbf5 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5185,8 +5185,8 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
failed_mount3a:
ext4_es_unregister_shrinker(sbi);
failed_mount3:
- del_timer_sync(&sbi->s_err_report);
ext4_stop_mmpd(sbi);
+ del_timer_sync(&sbi->s_err_report);
failed_mount2:
rcu_read_lock();
group_desc = rcu_dereference(sbi->s_group_desc);
--
2.34.1