jbd2: fix use after free in jbd2_journal_dirty_metadata()

jbd2_journal_dirty_metadata() didn't get a reference to journal_head it
was working with. This is OK in most of the cases since the journal head
should be attached to a transaction but in rare occasions when we are
journalling data, __ext4_journalled_writepage() can race with
jbd2_journal_invalidatepage() stripping buffers from a page and thus
journal head can be freed under hands of jbd2_journal_dirty_metadata().

Fix the problem by getting own journal head reference in
jbd2_journal_dirty_metadata() (and also in jbd2_journal_set_triggers()
which can possibly have the same issue).

Reported-by: Zheng Liu <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>
Signed-off-by: "Theodore Ts'o" <[e-mail]>
Cc: [e-mail]

fs/jbd2/transaction.c

···
 void jbd2_journal_set_triggers(struct buffer_head *bh,
 			       struct jbd2_buffer_trigger_type *type)
 {
-	struct journal_head *jh = bh2jh(bh);
+	struct journal_head *jh = jbd2_journal_grab_journal_head(bh);
 
+	if (WARN_ON(!jh))
+		return;
 	jh->b_triggers = type;
+	jbd2_journal_put_journal_head(jh);
 }
 
 void jbd2_buffer_frozen_trigger(struct journal_head *jh, void *mapped_data,
···
 {
 	transaction_t *transaction = handle->h_transaction;
 	journal_t *journal = transaction->t_journal;
-	struct journal_head *jh = bh2jh(bh);
+	struct journal_head *jh;
 	int ret = 0;
 
-	jbd_debug(5, "journal_head %p\n", jh);
-	JBUFFER_TRACE(jh, "entry");
 	if (is_handle_aborted(handle))
 		goto out;
-	if (!buffer_jbd(bh)) {
+	jh = jbd2_journal_grab_journal_head(bh);
+	if (!jh) {
 		ret = -EUCLEAN;
 		goto out;
 	}
+	jbd_debug(5, "journal_head %p\n", jh);
+	JBUFFER_TRACE(jh, "entry");
 
 	jbd_lock_bh_state(bh);
 
···
 	spin_unlock(&journal->j_list_lock);
 out_unlock_bh:
 	jbd_unlock_bh_state(bh);
+	jbd2_journal_put_journal_head(jh);
 out:
 	JBUFFER_TRACE(jh, "exit");
 	WARN_ON(ret);	/* All errors are bugs, so dump the stack */