netfilter: nf_ct_helper: disable automatic helper re-assignment of different type

This patch modifies __nf_ct_try_assign_helper in a way that invalidates support
for the following scenario:

1) attach the helper A for first time when the conntrack is created
2) attach new (different) helper B due to changes the reply tuple caused by NAT

eg. port redirection from TCP/21 to TCP/5060 with both FTP and SIP helpers
loaded, which seems to be a quite unorthodox scenario.

I can provide a more elaborated patch to support this scenario but explicit
helper attachment provides a better solution for this since now the use can
attach the helpers consistently, without relying on the automatic helper
lookup magic.

This patch fixes a possible out of bound zeroing of the conntrack helper
extension if the helper B uses more memory for its private data than
helper A.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_helper.c

@@ -229,7 +229,13 @@ int __nf_ct_try_assign_helper(struct nf_conn *ct, struct nf_conn *tmpl,
 			goto out;
 		}
 	} else {
-		memset(help->data, 0, helper->data_len);
+		/* We only allow helper re-assignment of the same sort since
+		 * we cannot reallocate the helper extension area.
+		 */
+		if (help->helper != helper) {
+			RCU_INIT_POINTER(help->helper, NULL);
+			goto out;
+		}
 	}
 
 	rcu_assign_pointer(help->helper, helper);