netfilter: nf_nat_snmp_basic: use asn1 decoder library

The basic SNMP ALG parse snmp ASN.1 payload
however, since 2012 linux kernel provide ASN.1 decoder library.
If we use ASN.1 decoder in the /lib/asn1_decoder.c, we can remove
about 1000 line of ASN.1 parsing routine.

To use asn1_decoder.c, we should write mib file(nf_nat_snmp_basic.asn1)
then /script/asn1_compiler.c makes *-asn1.c and *-asn1.h file
at the compiletime.(nf_nat_snmp_basic-asn1.c, nf_nat_snmp_basic-asn1.h)
The nf_nat_snmp_basic.asn1 is made by RFC1155, RFC1157, RFC1902, RFC1905,
RFC2578, RFC3416. of course that mib file supports only the basic SNMP ALG.

Previous SNMP ALG mangles only first octet of IPv4 address.
but after this patch, the SNMP ALG mangles whole IPv4 Address.
And SNMPv3 is not supported.

I tested with snmp commands such ans snmpd, snmpwalk, snmptrap.
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/Kconfig

@@ -158,6 +158,7 @@ config NF_NAT_SNMP_BASIC
 	depends on NF_CONNTRACK_SNMP
 	depends on NETFILTER_ADVANCED
 	default NF_NAT && NF_CONNTRACK_SNMP
+	select ASN1
 	---help---
 
 	  This module implements an Application Layer Gateway (ALG) for

net/ipv4/netfilter/Makefile

@@ -27,9 +27,15 @@ obj-$(CONFIG_NF_REJECT_IPV4) += nf_reject_ipv4.o
 # NAT helpers (nf_conntrack)
 obj-$(CONFIG_NF_NAT_H323) += nf_nat_h323.o
 obj-$(CONFIG_NF_NAT_PPTP) += nf_nat_pptp.o
+
+nf_nat_snmp_basic-y := nf_nat_snmp_basic-asn1.o nf_nat_snmp_basic_main.o
+nf_nat_snmp_basic-y : nf_nat_snmp_basic-asn1.h nf_nat_snmp_basic-asn1.c
 obj-$(CONFIG_NF_NAT_SNMP_BASIC) += nf_nat_snmp_basic.o
+clean-files := nf_nat_snmp_basic-asn1.c nf_nat_snmp_basic-asn1.h
+
 obj-$(CONFIG_NF_NAT_MASQUERADE_IPV4) += nf_nat_masquerade_ipv4.o
 
+
 # NAT protocols (nf_nat)
 obj-$(CONFIG_NF_NAT_PROTO_GRE) += nf_nat_proto_gre.o
 

net/ipv4/netfilter/nf_nat_snmp_basic.asn1

+Message ::=
+	SEQUENCE {
+		version
+			INTEGER ({snmp_version}),
+
+		community
+			OCTET STRING,
+
+		pdu
+			PDUs
+	}
+
+
+ObjectName ::=
+	OBJECT IDENTIFIER
+
+ObjectSyntax ::=
+	CHOICE {
+		simple
+			SimpleSyntax,
+
+		application-wide
+			ApplicationSyntax
+	}
+
+SimpleSyntax ::=
+	CHOICE {
+		integer-value
+			INTEGER,
+
+		string-value
+			OCTET STRING,
+
+		objectID-value
+			OBJECT IDENTIFIER
+	}
+
+ApplicationSyntax ::=
+	CHOICE {
+		ipAddress-value
+			IpAddress,
+
+		counter-value
+			Counter32,
+
+		timeticks-value
+			TimeTicks,
+
+		arbitrary-value
+			Opaque,
+
+		big-counter-value
+			Counter64,
+
+		unsigned-integer-value
+			Unsigned32
+	}
+
+IpAddress ::=
+	[APPLICATION 0]
+		IMPLICIT OCTET STRING OPTIONAL ({snmp_helper})
+
+Counter32 ::=
+	[APPLICATION 1]
+		IMPLICIT INTEGER OPTIONAL
+
+Unsigned32 ::=
+	[APPLICATION 2]
+		IMPLICIT INTEGER OPTIONAL
+
+Gauge32 ::= Unsigned32 OPTIONAL
+
+TimeTicks ::=
+	[APPLICATION 3]
+		IMPLICIT INTEGER OPTIONAL
+
+Opaque ::=
+	[APPLICATION 4]
+		IMPLICIT OCTET STRING OPTIONAL
+
+Counter64 ::=
+	[APPLICATION 6]
+		IMPLICIT INTEGER OPTIONAL
+
+PDUs ::=
+	CHOICE {
+		get-request
+			GetRequest-PDU,
+
+		get-next-request
+			GetNextRequest-PDU,
+
+		get-bulk-request
+			GetBulkRequest-PDU,
+
+		response
+			Response-PDU,
+
+		set-request
+			SetRequest-PDU,
+
+		inform-request
+			InformRequest-PDU,
+
+		snmpV2-trap
+			SNMPv2-Trap-PDU,
+
+		report
+			Report-PDU
+	}
+
+GetRequest-PDU ::=
+	[0] IMPLICIT PDU OPTIONAL
+
+GetNextRequest-PDU ::=
+	[1] IMPLICIT PDU OPTIONAL
+
+Response-PDU ::=
+	[2] IMPLICIT PDU OPTIONAL
+
+SetRequest-PDU ::=
+	[3] IMPLICIT PDU OPTIONAL
+
+-- [4] is obsolete
+
+GetBulkRequest-PDU ::=
+	[5] IMPLICIT PDU OPTIONAL
+
+InformRequest-PDU ::=
+	[6] IMPLICIT PDU OPTIONAL
+
+SNMPv2-Trap-PDU ::=
+	[7] IMPLICIT PDU OPTIONAL
+
+Report-PDU ::=
+	[8] IMPLICIT PDU OPTIONAL
+
+PDU ::=
+	SEQUENCE {
+		request-id
+			INTEGER,
+
+		error-status
+			INTEGER,
+
+		error-index
+			INTEGER,
+
+		variable-bindings
+			VarBindList
+	}
+
+
+VarBind ::=
+	SEQUENCE {
+		name
+			ObjectName,
+
+	CHOICE {
+		value
+			ObjectSyntax,
+
+		unSpecified
+			NULL,
+
+		noSuchObject
+			[0] IMPLICIT NULL,
+
+		noSuchInstance
+			[1] IMPLICIT NULL,
+
+		endOfMibView
+			[2] IMPLICIT NULL
+	}
+}
+
+VarBindList ::= SEQUENCE OF VarBind

net/ipv4/netfilter/nf_nat_snmp_basic_main.c

+/*
+ * nf_nat_snmp_basic.c
+ *
+ * Basic SNMP Application Layer Gateway
+ *
+ * This IP NAT module is intended for use with SNMP network
+ * discovery and monitoring applications where target networks use
+ * conflicting private address realms.
+ *
+ * Static NAT is used to remap the networks from the view of the network
+ * management system at the IP layer, and this module remaps some application
+ * layer addresses to match.
+ *
+ * The simplest form of ALG is performed, where only tagged IP addresses
+ * are modified.  The module does not need to be MIB aware and only scans
+ * messages at the ASN.1/BER level.
+ *
+ * Currently, only SNMPv1 and SNMPv2 are supported.
+ *
+ * More information on ALG and associated issues can be found in
+ * RFC 2962
+ *
+ * The ASB.1/BER parsing code is derived from the gxsnmp package by Gregory
+ * McLean & Jochen Friedrich, stripped down for use in the kernel.
+ *
+ * Copyright (c) 2000 RP Internet (www.rpi.net.au).
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: James Morris <jmorris@intercode.com.au>
+ *
+ * Copyright (c) 2006-2010 Patrick McHardy <kaber@trash.net>
+ */
+#include <linux/module.h>
+#include <linux/moduleparam.h>
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/in.h>
+#include <linux/ip.h>
+#include <linux/udp.h>
+#include <net/checksum.h>
+#include <net/udp.h>
+
+#include <net/netfilter/nf_nat.h>
+#include <net/netfilter/nf_conntrack_expect.h>
+#include <net/netfilter/nf_conntrack_helper.h>
+#include <linux/netfilter/nf_conntrack_snmp.h>
+#include "nf_nat_snmp_basic-asn1.h"
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("James Morris <jmorris@intercode.com.au>");
+MODULE_DESCRIPTION("Basic SNMP Application Layer Gateway");
+MODULE_ALIAS("ip_nat_snmp_basic");
+
+#define SNMP_PORT 161
+#define SNMP_TRAP_PORT 162
+
+static DEFINE_SPINLOCK(snmp_lock);
+
+struct snmp_ctx {
+	unsigned char *begin;
+	__sum16 *check;
+	__be32 from;
+	__be32 to;
+};
+
+static void fast_csum(struct snmp_ctx *ctx, unsigned char offset)
+{
+	unsigned char s[12] = {0,};
+	int size;
+
+	if (offset & 1) {
+		memcpy(&s[1], &ctx->from, 4);
+		memcpy(&s[7], &ctx->to, 4);
+		s[0] = ~0;
+		s[1] = ~s[1];
+		s[2] = ~s[2];
+		s[3] = ~s[3];
+		s[4] = ~s[4];
+		s[5] = ~0;
+		size = 12;
+	} else {
+		memcpy(&s[0], &ctx->from, 4);
+		memcpy(&s[4], &ctx->to, 4);
+		s[0] = ~s[0];
+		s[1] = ~s[1];
+		s[2] = ~s[2];
+		s[3] = ~s[3];
+		size = 8;
+	}
+	*ctx->check = csum_fold(csum_partial(s, size,
+					     ~csum_unfold(*ctx->check)));
+}
+
+int snmp_version(void *context, size_t hdrlen, unsigned char tag,
+		 const void *data, size_t datalen)
+{
+	if (*(unsigned char *)data > 1)
+		return -ENOTSUPP;
+	return 1;
+}
+
+int snmp_helper(void *context, size_t hdrlen, unsigned char tag,
+		const void *data, size_t datalen)
+{
+	struct snmp_ctx *ctx = (struct snmp_ctx *)context;
+	__be32 *pdata = (__be32 *)data;
+
+	if (*pdata == ctx->from) {
+		pr_debug("%s: %pI4 to %pI4\n", __func__,
+			 (void *)&ctx->from, (void *)&ctx->to);
+
+		if (*ctx->check)
+			fast_csum(ctx, (unsigned char *)data - ctx->begin);
+		*pdata = ctx->to;
+	}
+
+	return 1;
+}
+
+static int snmp_translate(struct nf_conn *ct, int dir, struct sk_buff *skb)
+{
+	struct iphdr *iph = ip_hdr(skb);
+	struct udphdr *udph = (struct udphdr *)((__be32 *)iph + iph->ihl);
+	u16 datalen = ntohs(udph->len) - sizeof(struct udphdr);
+	char *data = (unsigned char *)udph + sizeof(struct udphdr);
+	struct snmp_ctx ctx;
+	int ret;
+
+	if (dir == IP_CT_DIR_ORIGINAL) {
+		ctx.from = ct->tuplehash[dir].tuple.src.u3.ip;
+		ctx.to = ct->tuplehash[!dir].tuple.dst.u3.ip;
+	} else {
+		ctx.from = ct->tuplehash[!dir].tuple.src.u3.ip;
+		ctx.to = ct->tuplehash[dir].tuple.dst.u3.ip;
+	}
+
+	if (ctx.from == ctx.to)
+		return NF_ACCEPT;
+
+	ctx.begin = (unsigned char *)udph + sizeof(struct udphdr);
+	ctx.check = &udph->check;
+	ret = asn1_ber_decoder(&nf_nat_snmp_basic_decoder, &ctx, data, datalen);
+	if (ret < 0) {
+		nf_ct_helper_log(skb, ct, "parser failed\n");
+		return NF_DROP;
+	}
+
+	return NF_ACCEPT;
+}
+
+/* We don't actually set up expectations, just adjust internal IP
+ * addresses if this is being NATted
+ */
+static int help(struct sk_buff *skb, unsigned int protoff,
+		struct nf_conn *ct,
+		enum ip_conntrack_info ctinfo)
+{
+	int dir = CTINFO2DIR(ctinfo);
+	unsigned int ret;
+	const struct iphdr *iph = ip_hdr(skb);
+	const struct udphdr *udph = (struct udphdr *)((__be32 *)iph + iph->ihl);
+
+	/* SNMP replies and originating SNMP traps get mangled */
+	if (udph->source == htons(SNMP_PORT) && dir != IP_CT_DIR_REPLY)
+		return NF_ACCEPT;
+	if (udph->dest == htons(SNMP_TRAP_PORT) && dir != IP_CT_DIR_ORIGINAL)
+		return NF_ACCEPT;
+
+	/* No NAT? */
+	if (!(ct->status & IPS_NAT_MASK))
+		return NF_ACCEPT;
+
+	/* Make sure the packet length is ok.  So far, we were only guaranteed
+	 * to have a valid length IP header plus 8 bytes, which means we have
+	 * enough room for a UDP header.  Just verify the UDP length field so we
+	 * can mess around with the payload.
+	 */
+	if (ntohs(udph->len) != skb->len - (iph->ihl << 2)) {
+		nf_ct_helper_log(skb, ct, "dropping malformed packet\n");
+		return NF_DROP;
+	}
+
+	if (!skb_make_writable(skb, skb->len)) {
+		nf_ct_helper_log(skb, ct, "cannot mangle packet");
+		return NF_DROP;
+	}
+
+	spin_lock_bh(&snmp_lock);
+	ret = snmp_translate(ct, dir, skb);
+	spin_unlock_bh(&snmp_lock);
+	return ret;
+}
+
+static const struct nf_conntrack_expect_policy snmp_exp_policy = {
+	.max_expected	= 0,
+	.timeout	= 180,
+};
+
+static struct nf_conntrack_helper snmp_trap_helper __read_mostly = {
+	.me			= THIS_MODULE,
+	.help			= help,
+	.expect_policy		= &snmp_exp_policy,
+	.name			= "snmp_trap",
+	.tuple.src.l3num	= AF_INET,
+	.tuple.src.u.udp.port	= cpu_to_be16(SNMP_TRAP_PORT),
+	.tuple.dst.protonum	= IPPROTO_UDP,
+};
+
+static int __init nf_nat_snmp_basic_init(void)
+{
+	BUG_ON(nf_nat_snmp_hook != NULL);
+	RCU_INIT_POINTER(nf_nat_snmp_hook, help);
+
+	return nf_conntrack_helper_register(&snmp_trap_helper);
+}
+
+static void __exit nf_nat_snmp_basic_fini(void)
+{
+	RCU_INIT_POINTER(nf_nat_snmp_hook, NULL);
+	synchronize_rcu();
+	nf_conntrack_helper_unregister(&snmp_trap_helper);
+}
+
+module_init(nf_nat_snmp_basic_init);
+module_exit(nf_nat_snmp_basic_fini);