libsepol/cil: Fix heap-use-after-free when using optional blockinherit

This is based on a patch by Nicolas Iooss. He writes:
    When secilc compiles the following policy:

        (block b1
            (optional o1
                (blockinherit b1)
                (blockinherit x)
            )
        )

    it disables the optional block at pass 3 (CIL_PASS_BLKIN_LINK)
    because the block "x" does not exist.
    __cil_resolve_ast_last_child_helper() calls
    cil_tree_children_destroy() on the optional block, which destroys
    the two blockinherit statements. But the (blockinherit b1) node
    was referenced inside (block b1) node, in its block->bi_nodes list.
    Therefore, when this list is used at pass 4 (CIL_PASS_BLKIN_COPY),
    it contains a node which was freed: this triggers a use-after-free
    issue

    Fix this issue by removing blockinherit nodes from their lists of
    nodes block->bi_nodes when they are being destroyed. As
    cil_destroy_blockinherit() does not have a reference to the node
    containing the blockinherit data, implement this new logic in
    cil_tree_node_destroy().

    This issue was found while investigating a testcase from an OSS-Fuzz
    issue which seems unrelated (a Null-dereference READ in
    cil_symtab_get_datum,
    https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=29861).

Reported-by: Nicolas Iooss <nicolas.iooss@m4x.org>
Signed-off-by: James Carter <jwcart2@gmail.com>
Acked-by: Nicolas Iooss <nicolas.iooss@m4x.org>

libsepol/cil/src/cil_build_ast.c

@@ -300,6 +300,19 @@ void cil_destroy_blockinherit(struct cil_blockinherit *inherit)
 		return;
 	}
 
+	if (inherit->block != NULL && inherit->block->bi_nodes != NULL) {
+		struct cil_tree_node *node;
+		struct cil_list_item *item;
+
+		cil_list_for_each(item, inherit->block->bi_nodes) {
+			node = item->data;
+			if (node->data == inherit) {
+				cil_list_remove(inherit->block->bi_nodes, CIL_NODE, node, CIL_FALSE);
+				break;
+			}
+		}
+	}
+
 	free(inherit);
 }