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
43static 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
65static 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#ifdef CONFIG_NET_SWITCHDEV
70	if (skb->offload_l3_fwd_mark) {
71		consume_skb(skb);
72		return 0;
73	}
74#endif
75
76	if (unlikely(opt->optlen))
77		ip_forward_options(skb);
78
79	skb_clear_tstamp(skb);
80	return dst_output(net, sk, skb);
81}
82
83int ip_forward(struct sk_buff *skb)
84{
85	u32 mtu;
86	struct iphdr *iph;	/* Our header */
87	struct rtable *rt;	/* Route we use */
88	struct ip_options *opt	= &(IPCB(skb)->opt);
89	struct net *net;
90	SKB_DR(reason);
91
92	/* that should never happen */
93	if (skb->pkt_type != PACKET_HOST)
94		goto drop;
95
96	if (unlikely(skb->sk))
97		goto drop;
98
99	if (skb_warn_if_lro(skb))
100		goto drop;
101
102	if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb)) {
103		SKB_DR_SET(reason, XFRM_POLICY);
104		goto drop;
105	}
106
107	if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
108		return NET_RX_SUCCESS;
109
110	skb_forward_csum(skb);
111	net = dev_net(skb->dev);
112
113	/*
114	 *	According to the RFC, we must first decrease the TTL field. If
115	 *	that reaches zero, we must reply an ICMP control message telling
116	 *	that the packet's lifetime expired.
117	 */
118	if (ip_hdr(skb)->ttl <= 1)
119		goto too_many_hops;
120
121	if (!xfrm4_route_forward(skb)) {
122		SKB_DR_SET(reason, XFRM_POLICY);
123		goto drop;
124	}
125
126	rt = skb_rtable(skb);
127
128	if (opt->is_strictroute && rt->rt_uses_gateway)
129		goto sr_failed;
130
131	__IP_INC_STATS(net, IPSTATS_MIB_OUTFORWDATAGRAMS);
132
133	IPCB(skb)->flags |= IPSKB_FORWARDED;
134	mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
135	if (ip_exceeds_mtu(skb, mtu)) {
136		IP_INC_STATS(net, IPSTATS_MIB_FRAGFAILS);
137		icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
138			  htonl(mtu));
139		SKB_DR_SET(reason, PKT_TOO_BIG);
140		goto drop;
141	}
142
143	/* We are about to mangle packet. Copy it! */
144	if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
145		goto drop;
146	iph = ip_hdr(skb);
147
148	/* Decrease ttl after skb cow done */
149	ip_decrease_ttl(iph);
150
151	/*
152	 *	We now generate an ICMP HOST REDIRECT giving the route
153	 *	we calculated.
154	 */
155	if (IPCB(skb)->flags & IPSKB_DOREDIRECT && !opt->srr &&
156	    !skb_sec_path(skb))
157		ip_rt_send_redirect(skb);
158
159	if (READ_ONCE(net->ipv4.sysctl_ip_fwd_update_priority))
160		skb->priority = rt_tos2priority(iph->tos);
161
162	return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD,
163		       net, NULL, skb, skb->dev, rt->dst.dev,
164		       ip_forward_finish);
165
166sr_failed:
167	/*
168	 *	Strict routing permits no gatewaying
169	 */
170	 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
171	 goto drop;
172
173too_many_hops:
174	/* Tell the sender its packet died... */
175	__IP_INC_STATS(net, IPSTATS_MIB_INHDRERRORS);
176	icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
177	SKB_DR_SET(reason, IP_INHDR);
178drop:
179	kfree_skb_reason(skb, reason);
180	return NET_RX_DROP;
181}
182