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

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 Reported-by: syzbot+7f4a27091759e2fe7453@syzkaller.appspotmail.com CC: stable@vger.kernel.org Signed-off-by: Jan Kara jack@suse.cz

Applied, thanks (with the Fixes field adjusted).

- Ted