receive-pack: check object type of sha1 before using them as commits

Signed-off-by: Martin Koegler <mkoegler@auto.tuwien.ac.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

receive-pack.c

@@ -178,11 +178,21 @@ static const char *update(struct command *cmd)
 	if (deny_non_fast_forwards && !is_null_sha1(new_sha1) &&
 	    !is_null_sha1(old_sha1) &&
 	    !prefixcmp(name, "refs/heads/")) {
+		struct object *old_object, *new_object;
 		struct commit *old_commit, *new_commit;
 		struct commit_list *bases, *ent;
 
-		old_commit = (struct commit *)parse_object(old_sha1);
-		new_commit = (struct commit *)parse_object(new_sha1);
+		old_object = parse_object(old_sha1);
+		new_object = parse_object(new_sha1);
+
+		if (!old_object || !new_object ||
+		    old_object->type != OBJ_COMMIT ||
+		    new_object->type != OBJ_COMMIT) {
+			error("bad sha1 objects for %s", name);
+			return "bad ref";
+		}
+		old_commit = (struct commit *)old_object;
+		new_commit = (struct commit *)new_object;
 		bases = get_merge_bases(old_commit, new_commit, 1);
 		for (ent = bases; ent; ent = ent->next)
 			if (!hashcmp(old_sha1, ent->item->object.sha1))