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 /* Send RST reply */
 100 void nf_send_reset(struct net *net, struct sk_buff *oldskb, int hook)
 101 {
 102         struct net_device *br_indev __maybe_unused;
 103         struct sk_buff *nskb;
 104         struct iphdr *niph;
 105         const struct tcphdr *oth;
 106         struct tcphdr _oth;
 107
 108         oth = nf_reject_ip_tcphdr_get(oldskb, &_oth, hook);
 109         if (!oth)
 110                 return;
 111
 112         if (skb_rtable(oldskb)->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST))
 113                 return;
 114
 115         nskb = alloc_skb(sizeof(struct iphdr) + sizeof(struct tcphdr) +
 116                          LL_MAX_HEADER, GFP_ATOMIC);
 117         if (!nskb)
 118                 return;
 119
 120         /* ip_route_me_harder expects skb->dst to be set */
 121         skb_dst_set_noref(nskb, skb_dst(oldskb));
 122
 123         nskb->mark = IP4_REPLY_MARK(net, oldskb->mark);
 124
 125         skb_reserve(nskb, LL_MAX_HEADER);
 126         niph = nf_reject_iphdr_put(nskb, oldskb, IPPROTO_TCP,
 127                                    ip4_dst_hoplimit(skb_dst(nskb)));
 128         nf_reject_ip_tcphdr_put(nskb, oldskb, oth);
 129
 130         if (ip_route_me_harder(net, nskb, RTN_UNSPEC))
 131                 goto free_nskb;
 132
 133         niph = ip_hdr(nskb);
 134
 135         /* "Never happens" */
 136         if (nskb->len > dst_mtu(skb_dst(nskb)))
 137                 goto free_nskb;
 138
 139         nf_ct_attach(nskb, oldskb);
 140
 141 #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
 142         /* If we use ip_local_out for bridged traffic, the MAC source on
 143          * the RST will be ours, instead of the destination's.  This confuses
 144          * some routers/firewalls, and they drop the packet.  So we need to
 145          * build the eth header using the original destination's MAC as the
 146          * source, and send the RST packet directly.
 147          */
 148         br_indev = nf_bridge_get_physindev(oldskb);
 149         if (br_indev) {
 150                 struct ethhdr *oeth = eth_hdr(oldskb);
 151
 152                 nskb->dev = br_indev;
 153                 niph->tot_len = htons(nskb->len);
 154                 ip_send_check(niph);
 155                 if (dev_hard_header(nskb, nskb->dev, ntohs(nskb->protocol),
 156                                     oeth->h_source, oeth->h_dest, nskb->len) < 0)
 157                         goto free_nskb;
 158                 dev_queue_xmit(nskb);
 159         } else
 160 #endif
 161                 ip_local_out(net, nskb->sk, nskb);
 162
 163         return;
 164
 165  free_nskb:
 166         kfree_skb(nskb);
 167 }
 168 EXPORT_SYMBOL_GPL(nf_send_reset);
 169
 170 void nf_send_unreach(struct sk_buff *skb_in, int code, int hook)
 171 {
 172         struct iphdr *iph = ip_hdr(skb_in);
 173         u8 proto = iph->protocol;
 174
 175         if (iph->frag_off & htons(IP_OFFSET))
 176                 return;
 177
 178         if (skb_csum_unnecessary(skb_in) || !nf_reject_verify_csum(proto)) {
 179                 icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
 180                 return;
 181         }
 182
 183         if (nf_ip_checksum(skb_in, hook, ip_hdrlen(skb_in), proto) == 0)
 184                 icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
 185 }
 186 EXPORT_SYMBOL_GPL(nf_send_unreach);
 187
 188 MODULE_LICENSE("GPL");