Re: [PATCH V2] netfilter: ctnetlink: force null nat binding on insert

[Pablo Neira Ayuso <pablo@netfilter.org>]

[-- Attachment #1: Type: text/plain, Size: 2225 bytes --]

On Mon, Feb 17, 2014 at 12:15:19PM +0100, Florian Westphal wrote:
> Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> > > > I think we should always do the module autoloading for nf-nat and
> > > > nf-nat-ipvX modules depending on nf_ct_l3num(ct), then return EAGAIN
> > > > to give another retry. Now, this needs to happen in any case, not only
> > > > when calling ctnetlink_parse_nat_setup().
> > >
> > > Not sure what you mean.  If we enter nf_nat_setup_info without ctnetlink
> > > involvement the nf-nat protocol should already be there (else, how can
> > > we end up in nf_nat_setup_info? NAT/MASQUERADE depends on nf-nat).
> > >
> > > What use-case did you have in mind? (or, to put it differently, where
> > > do you think the module probing logic should be)?
> > 
> > If __nf_nat_l3proto_find returns NULL before trying to attach the null
> > binding, I think you should call the routine to autoload the modules
> > before returning EAGAIN.
> > 
> >         proto = __nf_nat_l3proto_find(nf_ct_l3num(ct));
> >         if (proto == NULL) {
> >                 ... release locks
> >                 request_module(...);
> >                 ... acquire locks again
> >                 return -EAGAIN;
> >         }
> 
> The patch alters ctnetlink to call ctnetlink_parse_nat_setup even when
> NAT attr == NULL.
> 
> nfnetlink_attach_null_binding() returns EAGAIN; this return value is
> propagated back to ctnetlink_parse_nat_setup.
> 
> That will then request_module(), nfnetlink will replay the message.
> 
> running
> conntrack -I -p tcp -s 1.1.1.1 -d 2.2.2.2 --timeout 100 --state \
> 	ESTABLISHED --sport 10 --dport 20
> on newly booted machine works, lsmod pre/post
> shows:
> +nf_conntrack_ipv4      14808  1 
> +nf_defrag_ipv4         12702  1 nf_conntrack_ipv4
> +nf_nat_ipv4            13199  0 
> +nf_nat                 20926  1 nf_nat_ipv4
> 
> Which is the desired behaviour afaiu.

Right, your patch looks good.

> [ If you think calling ctnetlink_parse_nat_setup with NULL attr
>   is abuse, please let me know and I will try to come up with something
>   different ]

I think we can simplify this, the nfnetlink_parse_nat_setup() hook
function is always called under rcu_read_lock. See patch attached.

[-- Attachment #2: florian.patch --]
[-- Type: text/x-diff, Size: 4852 bytes --]

diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index bb322d0..b9f0e03 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -1310,27 +1310,22 @@ ctnetlink_change_status(struct nf_conn *ct, const struct nlattr * const cda[])
 }
 
 static int
-ctnetlink_change_nat(struct nf_conn *ct, const struct nlattr * const cda[])
+ctnetlink_setup_nat(struct nf_conn *ct, const struct nlattr * const cda[])
 {
 #ifdef CONFIG_NF_NAT_NEEDED
 	int ret;
 
-	if (cda[CTA_NAT_DST]) {
-		ret = ctnetlink_parse_nat_setup(ct,
-						NF_NAT_MANIP_DST,
-						cda[CTA_NAT_DST]);
-		if (ret < 0)
-			return ret;
-	}
-	if (cda[CTA_NAT_SRC]) {
-		ret = ctnetlink_parse_nat_setup(ct,
-						NF_NAT_MANIP_SRC,
-						cda[CTA_NAT_SRC]);
-		if (ret < 0)
-			return ret;
-	}
-	return 0;
+	ret = ctnetlink_parse_nat_setup(ct, NF_NAT_MANIP_DST,
+					cda[CTA_NAT_DST]);
+	if (ret < 0)
+		return ret;
+
+	ret = ctnetlink_parse_nat_setup(ct, NF_NAT_MANIP_SRC,
+					cda[CTA_NAT_SRC]);
+	return ret;
 #else
+	if (!cda[CTA_NAT_DST] && !cda[CTA_NAT_SRC])
+		return 0;
 	return -EOPNOTSUPP;
 #endif
 }
@@ -1659,11 +1654,9 @@ ctnetlink_create_conntrack(struct net *net, u16 zone,
 			goto err2;
 	}
 
-	if (cda[CTA_NAT_SRC] || cda[CTA_NAT_DST]) {
-		err = ctnetlink_change_nat(ct, cda);
-		if (err < 0)
-			goto err2;
-	}
+	err = ctnetlink_setup_nat(ct, cda);
+	if (err < 0)
+		goto err2;
 
 	nf_ct_acct_ext_add(ct, GFP_ATOMIC);
 	nf_ct_tstamp_ext_add(ct, GFP_ATOMIC);
diff --git a/net/netfilter/nf_nat_core.c b/net/netfilter/nf_nat_core.c
index d3f5cd6..52ca952 100644
--- a/net/netfilter/nf_nat_core.c
+++ b/net/netfilter/nf_nat_core.c
@@ -432,15 +432,15 @@ nf_nat_setup_info(struct nf_conn *ct,
 }
 EXPORT_SYMBOL(nf_nat_setup_info);
 
-unsigned int
-nf_nat_alloc_null_binding(struct nf_conn *ct, unsigned int hooknum)
+static unsigned int
+__nf_nat_alloc_null_binding(struct nf_conn *ct, enum nf_nat_manip_type manip)
 {
 	/* Force range to this IP; let proto decide mapping for
 	 * per-proto parts (hence not IP_NAT_RANGE_PROTO_SPECIFIED).
 	 * Use reply in case it's already been mangled (eg local packet).
 	 */
 	union nf_inet_addr ip =
-		(HOOK2MANIP(hooknum) == NF_NAT_MANIP_SRC ?
+		(manip == NF_NAT_MANIP_SRC ?
 		ct->tuplehash[IP_CT_DIR_REPLY].tuple.dst.u3 :
 		ct->tuplehash[IP_CT_DIR_REPLY].tuple.src.u3);
 	struct nf_nat_range range = {
@@ -448,7 +448,13 @@ nf_nat_alloc_null_binding(struct nf_conn *ct, unsigned int hooknum)
 		.min_addr	= ip,
 		.max_addr	= ip,
 	};
-	return nf_nat_setup_info(ct, &range, HOOK2MANIP(hooknum));
+	return nf_nat_setup_info(ct, &range, manip);
+}
+
+unsigned int
+nf_nat_alloc_null_binding(struct nf_conn *ct, unsigned int hooknum)
+{
+	return __nf_nat_alloc_null_binding(ct, HOOK2MANIP(hooknum));
 }
 EXPORT_SYMBOL_GPL(nf_nat_alloc_null_binding);
 
@@ -702,9 +708,9 @@ static const struct nla_policy nat_nla_policy[CTA_NAT_MAX+1] = {
 
 static int
 nfnetlink_parse_nat(const struct nlattr *nat,
-		    const struct nf_conn *ct, struct nf_nat_range *range)
+		    const struct nf_conn *ct, struct nf_nat_range *range,
+		    const struct nf_nat_l3proto *l3proto)
 {
-	const struct nf_nat_l3proto *l3proto;
 	struct nlattr *tb[CTA_NAT_MAX+1];
 	int err;
 
@@ -714,38 +720,46 @@ nfnetlink_parse_nat(const struct nlattr *nat,
 	if (err < 0)
 		return err;
 
-	rcu_read_lock();
-	l3proto = __nf_nat_l3proto_find(nf_ct_l3num(ct));
-	if (l3proto == NULL) {
-		err = -EAGAIN;
-		goto out;
-	}
 	err = l3proto->nlattr_to_range(tb, range);
 	if (err < 0)
-		goto out;
+		return err;
 
 	if (!tb[CTA_NAT_PROTO])
-		goto out;
+		return 0;
 
-	err = nfnetlink_parse_nat_proto(tb[CTA_NAT_PROTO], ct, range);
-out:
-	rcu_read_unlock();
-	return err;
+	return nfnetlink_parse_nat_proto(tb[CTA_NAT_PROTO], ct, range);
 }
 
+/* This function is called under rcu_read_lock() */
 static int
 nfnetlink_parse_nat_setup(struct nf_conn *ct,
 			  enum nf_nat_manip_type manip,
 			  const struct nlattr *attr)
 {
 	struct nf_nat_range range;
+	const struct nf_nat_l3proto *l3proto;
 	int err;
 
-	err = nfnetlink_parse_nat(attr, ct, &range);
+	/* Should not happen, restricted to creating new conntracks
+	 * via ctnetlink.
+	 */
+	if (WARN_ON_ONCE(nf_nat_initialized(ct, manip)))
+		return -EEXIST;
+
+	/* Make sure that L3 NAT is there by when we call nf_nat_setup_info to
+	 * attach the null binding, otherwise this may oops.
+	 */
+	l3proto = __nf_nat_l3proto_find(nf_ct_l3num(ct));
+	if (l3proto == NULL)
+		return -EAGAIN;
+
+	/* No NAT information has been passed, allocate the null-binding */
+	if (attr == NULL)
+		return __nf_nat_alloc_null_binding(ct, manip);
+
+	err = nfnetlink_parse_nat(attr, ct, &range, l3proto);
 	if (err < 0)
 		return err;
-	if (nf_nat_initialized(ct, manip))
-		return -EEXIST;
 
 	return nf_nat_setup_info(ct, &range, manip);
 }
-- 
1.7.10.4