jbd2: fix use after free in jbd2_log_do_checkpoint()

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: dc6e8d669cf5 ("jbd2: don't call get_bh() before calling __jbd2_journal_remove_checkpoint()")
Fixes: be1158cc615f ("jbd2: fold __process_buffer() into jbd2_log_do_checkpoint()")
Reported-by: syzbot+7f4a27091759e2fe7453@syzkaller.appspotmail.com
CC: stable@vger.kernel.org
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
index c125d662777c6737e33b9a605973f0808aeb33f3..26f8d7e46462e1ba402ff7fc3050650aac1f096a 100644 (file)
--- a/fs/jbd2/checkpoint.c
+++ b/fs/jbd2/checkpoint.c
@@ -251,8 +251,8 @@ restart:
                bh = jh2bh(jh);
 
                if (buffer_locked(bh)) {
-                       spin_unlock(&journal->j_list_lock);
                        get_bh(bh);
+                       spin_unlock(&journal->j_list_lock);
                        wait_on_buffer(bh);
                        /* the journal_head may have gone by now */
                        BUFFER_TRACE(bh, "brelse");
@@ -333,8 +333,8 @@ restart2:
                jh = transaction->t_checkpoint_io_list;
                bh = jh2bh(jh);
                if (buffer_locked(bh)) {
-                       spin_unlock(&journal->j_list_lock);
                        get_bh(bh);
+                       spin_unlock(&journal->j_list_lock);
                        wait_on_buffer(bh);
                        /* the journal_head may have gone by now */
                        BUFFER_TRACE(bh, "brelse");