root/net/ipv4/netfilter/nf_nat_snmp_basic_main.c

DEFINITIONS

1. fast_csum
2. snmp_version
3. snmp_helper
4. snmp_translate
5. help
6. nf_nat_snmp_basic_init
7. nf_nat_snmp_basic_fini

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /*
   3  * nf_nat_snmp_basic.c
   4  *
   5  * Basic SNMP Application Layer Gateway
   6  *
   7  * This IP NAT module is intended for use with SNMP network
   8  * discovery and monitoring applications where target networks use
   9  * conflicting private address realms.
  10  *
  11  * Static NAT is used to remap the networks from the view of the network
  12  * management system at the IP layer, and this module remaps some application
  13  * layer addresses to match.
  14  *
  15  * The simplest form of ALG is performed, where only tagged IP addresses
  16  * are modified.  The module does not need to be MIB aware and only scans
  17  * messages at the ASN.1/BER level.
  18  *
  19  * Currently, only SNMPv1 and SNMPv2 are supported.
  20  *
  21  * More information on ALG and associated issues can be found in
  22  * RFC 2962
  23  *
  24  * The ASB.1/BER parsing code is derived from the gxsnmp package by Gregory
  25  * McLean & Jochen Friedrich, stripped down for use in the kernel.
  26  *
  27  * Copyright (c) 2000 RP Internet (www.rpi.net.au).
  28  *
  29  * Author: James Morris <jmorris@intercode.com.au>
  30  *
  31  * Copyright (c) 2006-2010 Patrick McHardy <kaber@trash.net>
  32  */
  33 #include <linux/module.h>
  34 #include <linux/moduleparam.h>
  35 #include <linux/types.h>
  36 #include <linux/kernel.h>
  37 #include <linux/in.h>
  38 #include <linux/ip.h>
  39 #include <linux/udp.h>
  40 #include <net/checksum.h>
  41 #include <net/udp.h>
  42 
  43 #include <net/netfilter/nf_nat.h>
  44 #include <net/netfilter/nf_conntrack_expect.h>
  45 #include <net/netfilter/nf_conntrack_helper.h>
  46 #include <linux/netfilter/nf_conntrack_snmp.h>
  47 #include "nf_nat_snmp_basic.asn1.h"
  48 
  49 MODULE_LICENSE("GPL");
  50 MODULE_AUTHOR("James Morris <jmorris@intercode.com.au>");
  51 MODULE_DESCRIPTION("Basic SNMP Application Layer Gateway");
  52 MODULE_ALIAS("ip_nat_snmp_basic");
  53 MODULE_ALIAS_NFCT_HELPER("snmp_trap");
  54 
  55 #define SNMP_PORT 161
  56 #define SNMP_TRAP_PORT 162
  57 
  58 static DEFINE_SPINLOCK(snmp_lock);
  59 
  60 struct snmp_ctx {
  61         unsigned char *begin;
  62         __sum16 *check;
  63         __be32 from;
  64         __be32 to;
  65 };
  66 
  67 static void fast_csum(struct snmp_ctx *ctx, unsigned char offset)
  68 {
  69         unsigned char s[12] = {0,};
  70         int size;
  71 
  72         if (offset & 1) {
  73                 memcpy(&s[1], &ctx->from, 4);
  74                 memcpy(&s[7], &ctx->to, 4);
  75                 s[0] = ~0;
  76                 s[1] = ~s[1];
  77                 s[2] = ~s[2];
  78                 s[3] = ~s[3];
  79                 s[4] = ~s[4];
  80                 s[5] = ~0;
  81                 size = 12;
  82         } else {
  83                 memcpy(&s[0], &ctx->from, 4);
  84                 memcpy(&s[4], &ctx->to, 4);
  85                 s[0] = ~s[0];
  86                 s[1] = ~s[1];
  87                 s[2] = ~s[2];
  88                 s[3] = ~s[3];
  89                 size = 8;
  90         }
  91         *ctx->check = csum_fold(csum_partial(s, size,
  92                                              ~csum_unfold(*ctx->check)));
  93 }
  94 
  95 int snmp_version(void *context, size_t hdrlen, unsigned char tag,
  96                  const void *data, size_t datalen)
  97 {
  98         if (datalen != 1)
  99                 return -EINVAL;
 100         if (*(unsigned char *)data > 1)
 101                 return -ENOTSUPP;
 102         return 1;
 103 }
 104 
 105 int snmp_helper(void *context, size_t hdrlen, unsigned char tag,
 106                 const void *data, size_t datalen)
 107 {
 108         struct snmp_ctx *ctx = (struct snmp_ctx *)context;
 109         __be32 *pdata;
 110 
 111         if (datalen != 4)
 112                 return -EINVAL;
 113         pdata = (__be32 *)data;
 114         if (*pdata == ctx->from) {
 115                 pr_debug("%s: %pI4 to %pI4\n", __func__,
 116                          (void *)&ctx->from, (void *)&ctx->to);
 117 
 118                 if (*ctx->check)
 119                         fast_csum(ctx, (unsigned char *)data - ctx->begin);
 120                 *pdata = ctx->to;
 121         }
 122 
 123         return 1;
 124 }
 125 
 126 static int snmp_translate(struct nf_conn *ct, int dir, struct sk_buff *skb)
 127 {
 128         struct iphdr *iph = ip_hdr(skb);
 129         struct udphdr *udph = (struct udphdr *)((__be32 *)iph + iph->ihl);
 130         u16 datalen = ntohs(udph->len) - sizeof(struct udphdr);
 131         char *data = (unsigned char *)udph + sizeof(struct udphdr);
 132         struct snmp_ctx ctx;
 133         int ret;
 134 
 135         if (dir == IP_CT_DIR_ORIGINAL) {
 136                 ctx.from = ct->tuplehash[dir].tuple.src.u3.ip;
 137                 ctx.to = ct->tuplehash[!dir].tuple.dst.u3.ip;
 138         } else {
 139                 ctx.from = ct->tuplehash[!dir].tuple.src.u3.ip;
 140                 ctx.to = ct->tuplehash[dir].tuple.dst.u3.ip;
 141         }
 142 
 143         if (ctx.from == ctx.to)
 144                 return NF_ACCEPT;
 145 
 146         ctx.begin = (unsigned char *)udph + sizeof(struct udphdr);
 147         ctx.check = &udph->check;
 148         ret = asn1_ber_decoder(&nf_nat_snmp_basic_decoder, &ctx, data, datalen);
 149         if (ret < 0) {
 150                 nf_ct_helper_log(skb, ct, "parser failed\n");
 151                 return NF_DROP;
 152         }
 153 
 154         return NF_ACCEPT;
 155 }
 156 
 157 /* We don't actually set up expectations, just adjust internal IP
 158  * addresses if this is being NATted
 159  */
 160 static int help(struct sk_buff *skb, unsigned int protoff,
 161                 struct nf_conn *ct,
 162                 enum ip_conntrack_info ctinfo)
 163 {
 164         int dir = CTINFO2DIR(ctinfo);
 165         unsigned int ret;
 166         const struct iphdr *iph = ip_hdr(skb);
 167         const struct udphdr *udph = (struct udphdr *)((__be32 *)iph + iph->ihl);
 168 
 169         /* SNMP replies and originating SNMP traps get mangled */
 170         if (udph->source == htons(SNMP_PORT) && dir != IP_CT_DIR_REPLY)
 171                 return NF_ACCEPT;
 172         if (udph->dest == htons(SNMP_TRAP_PORT) && dir != IP_CT_DIR_ORIGINAL)
 173                 return NF_ACCEPT;
 174 
 175         /* No NAT? */
 176         if (!(ct->status & IPS_NAT_MASK))
 177                 return NF_ACCEPT;
 178 
 179         /* Make sure the packet length is ok.  So far, we were only guaranteed
 180          * to have a valid length IP header plus 8 bytes, which means we have
 181          * enough room for a UDP header.  Just verify the UDP length field so we
 182          * can mess around with the payload.
 183          */
 184         if (ntohs(udph->len) != skb->len - (iph->ihl << 2)) {
 185                 nf_ct_helper_log(skb, ct, "dropping malformed packet\n");
 186                 return NF_DROP;
 187         }
 188 
 189         if (skb_ensure_writable(skb, skb->len)) {
 190                 nf_ct_helper_log(skb, ct, "cannot mangle packet");
 191                 return NF_DROP;
 192         }
 193 
 194         spin_lock_bh(&snmp_lock);
 195         ret = snmp_translate(ct, dir, skb);
 196         spin_unlock_bh(&snmp_lock);
 197         return ret;
 198 }
 199 
 200 static const struct nf_conntrack_expect_policy snmp_exp_policy = {
 201         .max_expected   = 0,
 202         .timeout        = 180,
 203 };
 204 
 205 static struct nf_conntrack_helper snmp_trap_helper __read_mostly = {
 206         .me                     = THIS_MODULE,
 207         .help                   = help,
 208         .expect_policy          = &snmp_exp_policy,
 209         .name                   = "snmp_trap",
 210         .tuple.src.l3num        = AF_INET,
 211         .tuple.src.u.udp.port   = cpu_to_be16(SNMP_TRAP_PORT),
 212         .tuple.dst.protonum     = IPPROTO_UDP,
 213 };
 214 
 215 static int __init nf_nat_snmp_basic_init(void)
 216 {
 217         BUG_ON(nf_nat_snmp_hook != NULL);
 218         RCU_INIT_POINTER(nf_nat_snmp_hook, help);
 219 
 220         return nf_conntrack_helper_register(&snmp_trap_helper);
 221 }
 222 
 223 static void __exit nf_nat_snmp_basic_fini(void)
 224 {
 225         RCU_INIT_POINTER(nf_nat_snmp_hook, NULL);
 226         synchronize_rcu();
 227         nf_conntrack_helper_unregister(&snmp_trap_helper);
 228 }
 229 
 230 module_init(nf_nat_snmp_basic_init);
 231 module_exit(nf_nat_snmp_basic_fini);