1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * INET An implementation of the TCP/IP protocol suite for the LINUX
4 * operating system. INET is implemented using the BSD Socket
5 * interface as the means of communication with the user level.
6 *
7 * The IP forwarding functionality.
8 *
9 * Authors: see ip.c
10 *
11 * Fixes:
12 * Many : Split from ip.c , see ip_input.c for
13 * history.
14 * Dave Gregorich : NULL ip_rt_put fix for multicast
15 * routing.
16 * Jos Vos : Add call_out_firewall before sending,
17 * use output device for accounting.
18 * Jos Vos : Call forward firewall after routing
19 * (always use output device).
20 * Mike McLagan : Routing by source
21 */
22
23 #include <linux/types.h>
24 #include <linux/mm.h>
25 #include <linux/skbuff.h>
26 #include <linux/ip.h>
27 #include <linux/icmp.h>
28 #include <linux/netdevice.h>
29 #include <linux/slab.h>
30 #include <net/sock.h>
31 #include <net/ip.h>
32 #include <net/tcp.h>
33 #include <net/udp.h>
34 #include <net/icmp.h>
35 #include <linux/tcp.h>
36 #include <linux/udp.h>
37 #include <linux/netfilter_ipv4.h>
38 #include <net/checksum.h>
39 #include <linux/route.h>
40 #include <net/route.h>
41 #include <net/xfrm.h>
42
ip_exceeds_mtu(const struct sk_buff * skb,unsigned int mtu)43 static bool ip_exceeds_mtu(const struct sk_buff *skb, unsigned int mtu)
44 {
45 if (skb->len <= mtu)
46 return false;
47
48 if (unlikely((ip_hdr(skb)->frag_off & htons(IP_DF)) == 0))
49 return false;
50
51 /* original fragment exceeds mtu and DF is set */
52 if (unlikely(IPCB(skb)->frag_max_size > mtu))
53 return true;
54
55 if (skb->ignore_df)
56 return false;
57
58 if (skb_is_gso(skb) && skb_gso_validate_network_len(skb, mtu))
59 return false;
60
61 return true;
62 }
63
64
ip_forward_finish(struct net * net,struct sock * sk,struct sk_buff * skb)65 static int ip_forward_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
66 {
67 struct ip_options *opt = &(IPCB(skb)->opt);
68
69 __IP_INC_STATS(net, IPSTATS_MIB_OUTFORWDATAGRAMS);
70 __IP_ADD_STATS(net, IPSTATS_MIB_OUTOCTETS, skb->len);
71
72 #ifdef CONFIG_NET_SWITCHDEV
73 if (skb->offload_l3_fwd_mark) {
74 consume_skb(skb);
75 return 0;
76 }
77 #endif
78
79 if (unlikely(opt->optlen))
80 ip_forward_options(skb);
81
82 skb_clear_tstamp(skb);
83 return dst_output(net, sk, skb);
84 }
85
ip_forward(struct sk_buff * skb)86 int ip_forward(struct sk_buff *skb)
87 {
88 u32 mtu;
89 struct iphdr *iph; /* Our header */
90 struct rtable *rt; /* Route we use */
91 struct ip_options *opt = &(IPCB(skb)->opt);
92 struct net *net;
93 SKB_DR(reason);
94
95 /* that should never happen */
96 if (skb->pkt_type != PACKET_HOST)
97 goto drop;
98
99 if (unlikely(skb->sk))
100 goto drop;
101
102 if (skb_warn_if_lro(skb))
103 goto drop;
104
105 if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb)) {
106 SKB_DR_SET(reason, XFRM_POLICY);
107 goto drop;
108 }
109
110 if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
111 return NET_RX_SUCCESS;
112
113 skb_forward_csum(skb);
114 net = dev_net(skb->dev);
115
116 /*
117 * According to the RFC, we must first decrease the TTL field. If
118 * that reaches zero, we must reply an ICMP control message telling
119 * that the packet's lifetime expired.
120 */
121 if (ip_hdr(skb)->ttl <= 1)
122 goto too_many_hops;
123
124 if (!xfrm4_route_forward(skb)) {
125 SKB_DR_SET(reason, XFRM_POLICY);
126 goto drop;
127 }
128
129 rt = skb_rtable(skb);
130
131 if (opt->is_strictroute && rt->rt_uses_gateway)
132 goto sr_failed;
133
134 IPCB(skb)->flags |= IPSKB_FORWARDED;
135 mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
136 if (ip_exceeds_mtu(skb, mtu)) {
137 IP_INC_STATS(net, IPSTATS_MIB_FRAGFAILS);
138 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
139 htonl(mtu));
140 SKB_DR_SET(reason, PKT_TOO_BIG);
141 goto drop;
142 }
143
144 /* We are about to mangle packet. Copy it! */
145 if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
146 goto drop;
147 iph = ip_hdr(skb);
148
149 /* Decrease ttl after skb cow done */
150 ip_decrease_ttl(iph);
151
152 /*
153 * We now generate an ICMP HOST REDIRECT giving the route
154 * we calculated.
155 */
156 if (IPCB(skb)->flags & IPSKB_DOREDIRECT && !opt->srr &&
157 !skb_sec_path(skb))
158 ip_rt_send_redirect(skb);
159
160 if (READ_ONCE(net->ipv4.sysctl_ip_fwd_update_priority))
161 skb->priority = rt_tos2priority(iph->tos);
162
163 return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD,
164 net, NULL, skb, skb->dev, rt->dst.dev,
165 ip_forward_finish);
166
167 sr_failed:
168 /*
169 * Strict routing permits no gatewaying
170 */
171 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
172 goto drop;
173
174 too_many_hops:
175 /* Tell the sender its packet died... */
176 __IP_INC_STATS(net, IPSTATS_MIB_INHDRERRORS);
177 icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
178 SKB_DR_SET(reason, IP_INHDR);
179 drop:
180 kfree_skb_reason(skb, reason);
181 return NET_RX_DROP;
182 }
183