Subject: Re: Oops in nf_nat_core.c:find_appropriate_src(), kernel 2.6.25.4

From: Patrick McHardy <[email-masked]>
Date: Tue, 10 Jun 2008 16:00:29 +0200

Krzysztof Oledzki wrote:
> On Tue, 10 Jun 2008, Patrick McHardy wrote:
> 
>> We found the reason for that crash and I've queued these two
>> patches. Please let me know whether they also fix the problem
>> from the redhat bugzilla.
> 
> There is a one thing that still bugs me. This patch removes setting the 
> nat->ct pointer to NULL:
> 
> --- a/net/ipv4/netfilter/nf_nat_core.c
> +++ b/net/ipv4/netfilter/nf_nat_core.c
> @@ -556,7 +556,6 @@ static void nf_nat_cleanup_conntrack(struct nf_conn 
> *ct)
> 
>         spin_lock_bh(&nf_nat_lock);
>         hlist_del_rcu(&nat->bysource);
> -       nat->ct = NULL;
>         spin_unlock_bh(&nf_nat_lock);
>  }
> 
> After this patch the whole function looks like this:
> 
> /* Noone using conntrack by the time this called. */
> static void nf_nat_cleanup_conntrack(struct nf_conn *ct)
> {
>         struct nf_conn_nat *nat = nf_ct_ext_find(ct, NF_CT_EXT_NAT);
> 
>         if (nat == NULL || nat->ct == NULL)
>                 return;
> 
>         NF_CT_ASSERT(nat->ct->status & IPS_NAT_DONE_MASK);
> 
>         spin_lock_bh(&nf_nat_lock);
>         hlist_del_rcu(&nat->bysource);
>         spin_unlock_bh(&nf_nat_lock);
> }
> 
> As you can see we still check if nat->ct is NULL here. So, or the check 
> is now unnecessary, or it is still possible that nat->ct may become 
> NULL. If the second statement is true than we may need to check ct 
> before calling same_src in the find_appropriate_src function.

No, the nf_nat_cleanup_nat function can be called for a NAT extension
that isn't in the hash yet (and thus has nat->ct == NULL) when the
nf_conntrack_alter_reply() call in nf_nat_setup_info() allocates a
helper extension and need to realloc the NAT extension space.