ext4: fix timer use-after-free on failed mount

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guarantee the timer is no longer active before kfree(sbi).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 <[email protected]>
Reported-and-tested-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=59e0101c430934bc9a36
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>
Cc: [email protected]
Signed-off-by: Xiangyu Chen <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: xshen053

fs/ext4/super.c

@@ -5617,8 +5617,8 @@ failed_mount9: __maybe_unused
 failed_mount3:
 	/* flush s_error_work before sbi destroy */
 	flush_work(&sbi->s_error_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)