net/ipv4/netfilter/nf_reject_ipv4.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /* (C) 1999-2001 Paul `Rusty' Russell
   3  * (C) 2002-2004 Netfilter Core Team <coreteam@netfilter.org>
   4  */
   5
   6 #include <linux/module.h>
   7 #include <net/ip.h>
   8 #include <net/tcp.h>
   9 #include <net/route.h>
  10 #include <net/dst.h>
  11 #include <net/netfilter/ipv4/nf_reject.h>
  12 #include <linux/netfilter_ipv4.h>
  13 #include <linux/netfilter_bridge.h>
  14
  15 const struct tcphdr *nf_reject_ip_tcphdr_get(struct sk_buff *oldskb,
  16                                              struct tcphdr *_oth, int hook)
  17 {
  18         const struct tcphdr *oth;
  19
  20         /* IP header checks: fragment. */
  21         if (ip_hdr(oldskb)->frag_off & htons(IP_OFFSET))
  22                 return NULL;
  23
  24         if (ip_hdr(oldskb)->protocol != IPPROTO_TCP)
  25                 return NULL;
  26
  27         oth = skb_header_pointer(oldskb, ip_hdrlen(oldskb),
  28                                  sizeof(struct tcphdr), _oth);
  29         if (oth == NULL)
  30                 return NULL;
  31
  32         /* No RST for RST. */
  33         if (oth->rst)
  34                 return NULL;
  35
  36         /* Check checksum */
  37         if (nf_ip_checksum(oldskb, hook, ip_hdrlen(oldskb), IPPROTO_TCP))
  38                 return NULL;
  39
  40         return oth;
  41 }
  42 EXPORT_SYMBOL_GPL(nf_reject_ip_tcphdr_get);
  43
  44 struct iphdr *nf_reject_iphdr_put(struct sk_buff *nskb,
  45                                   const struct sk_buff *oldskb,
  46                                   __u8 protocol, int ttl)
  47 {
  48         struct iphdr *niph, *oiph = ip_hdr(oldskb);
  49
  50         skb_reset_network_header(nskb);
  51         niph = skb_put(nskb, sizeof(struct iphdr));
  52         niph->version   = 4;
  53         niph->ihl       = sizeof(struct iphdr) / 4;
  54         niph->tos       = 0;
  55         niph->id        = 0;
  56         niph->frag_off  = htons(IP_DF);
  57         niph->protocol  = protocol;
  58         niph->check     = 0;
  59         niph->saddr     = oiph->daddr;
  60         niph->daddr     = oiph->saddr;
  61         niph->ttl       = ttl;
  62
  63         nskb->protocol = htons(ETH_P_IP);
  64
  65         return niph;
  66 }
  67 EXPORT_SYMBOL_GPL(nf_reject_iphdr_put);
  68
  69 void nf_reject_ip_tcphdr_put(struct sk_buff *nskb, const struct sk_buff *oldskb,
  70                           const struct tcphdr *oth)
  71 {
  72         struct iphdr *niph = ip_hdr(nskb);
  73         struct tcphdr *tcph;
  74
  75         skb_reset_transport_header(nskb);
  76         tcph = skb_put_zero(nskb, sizeof(struct tcphdr));
  77         tcph->source    = oth->dest;
  78         tcph->dest      = oth->source;
  79         tcph->doff      = sizeof(struct tcphdr) / 4;
  80
  81         if (oth->ack) {
  82                 tcph->seq = oth->ack_seq;
  83         } else {
  84                 tcph->ack_seq = htonl(ntohl(oth->seq) + oth->syn + oth->fin +
  85                                       oldskb->len - ip_hdrlen(oldskb) -
  86                                       (oth->doff << 2));
  87                 tcph->ack = 1;
  88         }
  89
  90         tcph->rst       = 1;
  91         tcph->check = ~tcp_v4_check(sizeof(struct tcphdr), niph->saddr,
  92                                     niph->daddr, 0);
  93         nskb->ip_summed = CHECKSUM_PARTIAL;
  94         nskb->csum_start = (unsigned char *)tcph - nskb->head;
  95         nskb->csum_offset = offsetof(struct tcphdr, check);
  96 }
  97 EXPORT_SYMBOL_GPL(nf_reject_ip_tcphdr_put);
  98
  99 static int nf_reject_fill_skb_dst(struct sk_buff *skb_in)
 100 {
 101         struct dst_entry *dst = NULL;
 102         struct flowi fl;
 103
 104         memset(&fl, 0, sizeof(struct flowi));
 105         fl.u.ip4.daddr = ip_hdr(skb_in)->saddr;
 106         nf_ip_route(dev_net(skb_in->dev), &dst, &fl, false);
 107         if (!dst)
 108                 return -1;
 109
 110         skb_dst_set(skb_in, dst);
 111         return 0;
 112 }
 113
 114 /* Send RST reply */
 115 void nf_send_reset(struct net *net, struct sk_buff *oldskb, int hook)
 116 {
 117         struct net_device *br_indev __maybe_unused;
 118         struct sk_buff *nskb;
 119         struct iphdr *niph;
 120         const struct tcphdr *oth;
 121         struct tcphdr _oth;
 122
 123         oth = nf_reject_ip_tcphdr_get(oldskb, &_oth, hook);
 124         if (!oth)
 125                 return;
 126
 127         if (hook == NF_INET_PRE_ROUTING && nf_reject_fill_skb_dst(oldskb))
 128                 return;
 129
 130         if (skb_rtable(oldskb)->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST))
 131                 return;
 132
 133         nskb = alloc_skb(sizeof(struct iphdr) + sizeof(struct tcphdr) +
 134                          LL_MAX_HEADER, GFP_ATOMIC);
 135         if (!nskb)
 136                 return;
 137
 138         /* ip_route_me_harder expects skb->dst to be set */
 139         skb_dst_set_noref(nskb, skb_dst(oldskb));
 140
 141         nskb->mark = IP4_REPLY_MARK(net, oldskb->mark);
 142
 143         skb_reserve(nskb, LL_MAX_HEADER);
 144         niph = nf_reject_iphdr_put(nskb, oldskb, IPPROTO_TCP,
 145                                    ip4_dst_hoplimit(skb_dst(nskb)));
 146         nf_reject_ip_tcphdr_put(nskb, oldskb, oth);
 147
 148         if (ip_route_me_harder(net, nskb->sk, nskb, RTN_UNSPEC))
 149                 goto free_nskb;
 150
 151         niph = ip_hdr(nskb);
 152
 153         /* "Never happens" */
 154         if (nskb->len > dst_mtu(skb_dst(nskb)))
 155                 goto free_nskb;
 156
 157         nf_ct_attach(nskb, oldskb);
 158
 159 #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
 160         /* If we use ip_local_out for bridged traffic, the MAC source on
 161          * the RST will be ours, instead of the destination's.  This confuses
 162          * some routers/firewalls, and they drop the packet.  So we need to
 163          * build the eth header using the original destination's MAC as the
 164          * source, and send the RST packet directly.
 165          */
 166         br_indev = nf_bridge_get_physindev(oldskb);
 167         if (br_indev) {
 168                 struct ethhdr *oeth = eth_hdr(oldskb);
 169
 170                 nskb->dev = br_indev;
 171                 niph->tot_len = htons(nskb->len);
 172                 ip_send_check(niph);
 173                 if (dev_hard_header(nskb, nskb->dev, ntohs(nskb->protocol),
 174                                     oeth->h_source, oeth->h_dest, nskb->len) < 0)
 175                         goto free_nskb;
 176                 dev_queue_xmit(nskb);
 177         } else
 178 #endif
 179                 ip_local_out(net, nskb->sk, nskb);
 180
 181         return;
 182
 183  free_nskb:
 184         kfree_skb(nskb);
 185 }
 186 EXPORT_SYMBOL_GPL(nf_send_reset);
 187
 188 void nf_send_unreach(struct sk_buff *skb_in, int code, int hook)
 189 {
 190         struct iphdr *iph = ip_hdr(skb_in);
 191         u8 proto = iph->protocol;
 192
 193         if (iph->frag_off & htons(IP_OFFSET))
 194                 return;
 195
 196         if (hook == NF_INET_PRE_ROUTING && nf_reject_fill_skb_dst(skb_in))
 197                 return;
 198
 199         if (skb_csum_unnecessary(skb_in) || !nf_reject_verify_csum(proto)) {
 200                 icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
 201                 return;
 202         }
 203
 204         if (nf_ip_checksum(skb_in, hook, ip_hdrlen(skb_in), proto) == 0)
 205                 icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
 206 }
 207 EXPORT_SYMBOL_GPL(nf_send_unreach);
 208
 209 MODULE_LICENSE("GPL");