Commit ccd3c437 authored Oct 05, 2018 by Jan Kara Committed by Theodore Ts'o Oct 05, 2018

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: dc6e8d66 ("jbd2: don't call get_bh() before calling __jbd2_journal_remove_checkpoint()")
Fixes: be1158cc ("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>

parent 182a79e0

fs/jbd2/checkpoint.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -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");

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