gfs2: fix use-after-free in trans_drain

This patch adds code to function trans_drain to remove drained
bd elements from the ail lists, if queued, before freeing the bd.
If we don't remove the bd from the ail, function ail_drain will
try to reference the bd after it has been freed by trans_drain.

Thanks to Andy Price for his analysis of the problem.
Reported-by: Andy Price <anprice@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/log.c

@@ -998,12 +998,16 @@ static void trans_drain(struct gfs2_trans *tr)
 	while (!list_empty(head)) {
 		bd = list_first_entry(head, struct gfs2_bufdata, bd_list);
 		list_del_init(&bd->bd_list);
+		if (!list_empty(&bd->bd_ail_st_list))
+			gfs2_remove_from_ail(bd);
 		kmem_cache_free(gfs2_bufdata_cachep, bd);
 	}
 	head = &tr->tr_databuf;
 	while (!list_empty(head)) {
 		bd = list_first_entry(head, struct gfs2_bufdata, bd_list);
 		list_del_init(&bd->bd_list);
+		if (!list_empty(&bd->bd_ail_st_list))
+			gfs2_remove_from_ail(bd);
 		kmem_cache_free(gfs2_bufdata_cachep, bd);
 	}
 }

fs/gfs2/trans.c

@@ -169,6 +169,8 @@ static struct gfs2_bufdata *gfs2_alloc_bufdata(struct gfs2_glock *gl,
 	bd->bd_bh = bh;
 	bd->bd_gl = gl;
 	INIT_LIST_HEAD(&bd->bd_list);
+	INIT_LIST_HEAD(&bd->bd_ail_st_list);
+	INIT_LIST_HEAD(&bd->bd_ail_gl_list);
 	bh->b_private = bd;
 	return bd;
 }