Re: [PATCH] jbd2: Fix use after free in jbd2_log_do_checkpoint()

[Jan Kara <jack@suse.cz>]

On Thu 04-10-18 13:50:12, Lukas Czerner wrote:
> On Thu, Oct 04, 2018 at 12:46:40PM +0200, Jan Kara wrote:
> > The code cleaning transaction's lists of checkpoint buffers has a bug
> > where it increases bh refcount only after releasing
> > journal->j_list_lock. Thus the following race is possible:
> > 
> > CPU0					CPU1
> > jbd2_log_do_checkpoint()
> > 					jbd2_journal_try_to_free_buffers()
> > 					  __journal_try_to_free_buffer(bh)
> >   ...
> >   while (transaction->t_checkpoint_io_list)
> >   ...
> >     if (buffer_locked(bh)) {
> > 
> > <-- IO completes now, buffer gets unlocked -->
> > 
> >       spin_unlock(&journal->j_list_lock);
> > 					    spin_lock(&journal->j_list_lock);
> > 					    __jbd2_journal_remove_checkpoint(jh);
> > 					    spin_unlock(&journal->j_list_lock);
> > 					  try_to_free_buffers(page);
> >       get_bh(bh) <-- accesses freed bh
> > 
> > Fix the problem by grabbing bh reference before unlocking
> > journal->j_list_lock.
> 
> Hi Jan,
> 
> nice catch. The patch looks good, you can add
> 
> Reviewed-by: Lukas Czerner <lczerner@redhat.com>
> 
> Btw, do you by any chance have a reproducer for this ?

No, syzbot hit it but the race window is really small so I don't think you
can create reasonably reliable reproducer...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR