tc: oops in em_meta

If userspace passes a unknown match index into em_meta, then em_meta_change will return an error and the data for the match will not be set. This then causes an null pointer dereference when the cleanup is done in the error path via tcf_em_tree_destroy. Since the tree structure comes kzalloc, it is initialized to NULL.

Discovered when testing a new version of tc command against an accidental older kernel.

---

Patch against net-2.6.25 tree, should be backported to earlier stable kernels as well.

@@ -735,11 +735,13 @@ static int em_meta_match(struct sk_buff 
 
 static inline void meta_delete(struct meta_match *meta)
 {
-	struct meta_type_ops *ops = meta_type_ops(&meta->lvalue);
+	if (meta) {
+		struct meta_type_ops *ops = meta_type_ops(&meta->lvalue);
 
-	if (ops && ops->destroy) {
-		ops->destroy(&meta->lvalue);
-		ops->destroy(&meta->rvalue);
+		if (ops && ops->destroy) {
+			ops->destroy(&meta->lvalue);
+			ops->destroy(&meta->rvalue);
+		}
 	}
 
 	kfree(meta);