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

[Greg KH <greg@kroah.com>]

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.
> 
> Fixes: dc6e8d669cf5cb3ff84707c372c0a2a8a5e80845
> Fixes: be1158cc615fd723552f0d9912087423c7cadda5

Nit, this normally looks like:

Fixes: dc6e8d669cf5 ("jbd2: don't call get_bh() before calling __jbd2_journal_remove_checkpoint()")

And this is created by:
	git show -s --abbrev-commit --abbrev=12 --pretty=format:"%h (\"%s\")%n"

thanks

greg k-h