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>

diff --git a/net/netfilter/nf_conntrack_helper.c b/net/netfilter/nf_conntrack_helper.c
index 2918ec2e450974ffba9fb5354a3f3640871187a2..c4bc637feb76b3e542ac0eaac9c76198e38188cf 100644 (file)
--- a/net/netfilter/nf_conntrack_helper.c
+++ b/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);