On Tue 06-07-21 13:12:08, Theodore Ts'o wrote:
> After commit 618f003199c6 ("ext4: fix memory leak in
> ext4_fill_super"), after the file system is remounted read-only, there
> is a race where the kmmpd thread can exit, causing sbi->s_mmp_tsk to
> point at freed memory, which the call to ext4_stop_mmpd() can trip
> over.
>
> Fix this by only allowing kmmpd() to exit when it is stopped via
> ext4_stop_mmpd().
>
> Link: https://lore.kernel.org/r/YONtEGojq7LcXnuC@xxxxxxx
> Reported-by: Ye Bin <yebin10@xxxxxxxxxx>
> Bug-Report-Link: <20210629143603.2166962-1-yebin10@xxxxxxxxxx>
> Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
The patch looks mostly fine. Two comments below.
> @@ -242,9 +237,13 @@ static int kmmpd(void *data)
> mmp->mmp_seq = cpu_to_le32(EXT4_MMP_SEQ_CLEAN);
> mmp->mmp_time = cpu_to_le64(ktime_get_real_seconds());
>
> - retval = write_mmp_block(sb, bh);
> + return write_mmp_block(sb, bh);
I think we need to keep retval = write_mmp_block() here. Otherwise we could
exit early in sb_rdonly() case and still have potential use-after-free.
>
> -exit_thread:
> +wait_to_exit:
> + set_current_state(TASK_INTERRUPTIBLE);
> + while (!kthread_should_stop())
> + schedule();
> + set_current_state(TASK_RUNNING);
> return retval;
> }
This is more or less fine but if we get a spurious wakeup for whatever
reason (which sets task to TASK_RUNNING state) we would still be
potentially looping in that loop burning cpu...
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
