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

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guarantee the timer is no longer active before kfree(sbi).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@gmail.com>
Reported-and-tested-by: syzbot+59e0101c430934bc9a36@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=59e0101c430934bc9a36
Link: https://patch.msgid.link/20240715043336.98097-1-shenxiaxi26@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 7806044ed8162cd267f37eccaeb1f209b52e70f5..99ecd4ac7be00aba2fce975813c7dcd582df3e9e 100644 (file)
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5621,8 +5621,8 @@ failed_mount3a:
 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)