netfilter: ctnetlink: disable helper autoassign

When userspace, e.g. conntrackd, inserts an entry with a specified helper,
its possible that the helper is lost immediately after its added:

ctnetlink_create_conntrack
  -> nf_ct_helper_ext_add + assign helper
    -> ctnetlink_setup_nat
      -> ctnetlink_parse_nat_setup
         -> parse_nat_setup -> nfnetlink_parse_nat_setup
	                       -> nf_nat_setup_info
                                 -> nf_conntrack_alter_reply
                                   -> __nf_ct_try_assign_helper

... and __nf_ct_try_assign_helper will zero the helper again.

Set IPS_HELPER bit to bypass auto-assign logic, its unwanted, just like
when helper is assigned via ruleset.

Dropped old 'not strictly necessary' comment, it referred to use of
rcu_assign_pointer() before it got replaced by RCU_INIT_POINTER().

NB: Fixes tag intentionally incorrect, this extends the referenced commit,
but this change won't build without IPS_HELPER introduced there.

Fixes: 6714cf54 ("netfilter: nf_conntrack: fix explicit helper attachment and NAT")
Reported-by: Pham Thanh Tuyen <phamtyn@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/nf_conntrack_common.h

@@ -106,7 +106,7 @@ enum ip_conntrack_status {
 	IPS_NAT_CLASH = IPS_UNTRACKED,
 #endif
 
-	/* Conntrack got a helper explicitly attached via CT target. */
+	/* Conntrack got a helper explicitly attached (ruleset, ctnetlink). */
 	IPS_HELPER_BIT = 13,
 	IPS_HELPER = (1 << IPS_HELPER_BIT),
 

net/netfilter/nf_conntrack_netlink.c

@@ -2311,7 +2311,8 @@ ctnetlink_create_conntrack(struct net *net,
 			if (helper->from_nlattr)
 				helper->from_nlattr(helpinfo, ct);
 
-			/* not in hash table yet so not strictly necessary */
+			/* disable helper auto-assignment for this entry */
+			ct->status |= IPS_HELPER;
 			RCU_INIT_POINTER(help->helper, helper);
 		}
 	} else {