netlink: allow NLA_NESTED to specify nested policy to validate

Now that we have a validation_data pointer, and the len field in
the policy is unused for NLA_NESTED, we can allow using them both
to have nested validation. This can be nice in code, although we
still have to use nla_parse_nested() or similar which would also
take a policy; however, it also serves as documentation in the
policy without requiring a look at the code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/nlattr.c

@@ -155,6 +155,20 @@ static int validate_nla(const struct nlattr *nla, int maxtype,
 		 */
 		if (attrlen == 0)
 			break;
+		if (attrlen < NLA_HDRLEN)
+			goto out_err;
+		if (pt->validation_data) {
+			err = nla_validate(nla_data(nla), nla_len(nla), pt->len,
+					   pt->validation_data, extack);
+			if (err < 0) {
+				/*
+				 * return directly to preserve the inner
+				 * error message/attribute pointer
+				 */
+				return err;
+			}
+		}
+		break;
 	default:
 		if (pt->len)
 			minlen = pt->len;