[PATCH] ext4: Fix timer use-after-free on failed mount

[Xiaxi Shen]

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guarantee the timer is no longer active before kfree(sbi).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
---
 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 c682fb927b64..1b4b0d3d8889 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5626,8 +5626,8 @@ failed_mount8: __maybe_unused
 failed_mount3:
 	/* flush s_sb_upd_work before sbi destroy */
 	flush_work(&sbi->s_sb_upd_work);
-	del_timer_sync(&sbi->s_err_report);
 	ext4_stop_mmpd(sbi);
+	del_timer_sync(&sbi->s_err_report);
 	ext4_group_desc_free(sbi);
 failed_mount:
 	if (sbi->s_chksum_driver)
-- 
2.34.1