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 
71 #ifdef CONFIG_NET_SWITCHDEV
72 	if (skb->offload_l3_fwd_mark) {
73 		consume_skb(skb);
74 		return 0;
75 	}
76 #endif
77 
78 	if (unlikely(opt->optlen))
79 		ip_forward_options(skb);
80 
81 	skb_clear_tstamp(skb);
82 	return dst_output(net, sk, skb);
83 }
84 
ip_forward(struct sk_buff * skb)85 int ip_forward(struct sk_buff *skb)
86 {
87 	u32 mtu;
88 	struct iphdr *iph;	/* Our header */
89 	struct rtable *rt;	/* Route we use */
90 	struct ip_options *opt	= &(IPCB(skb)->opt);
91 	struct net *net;
92 	SKB_DR(reason);
93 
94 	/* that should never happen */
95 	if (skb->pkt_type != PACKET_HOST)
96 		goto drop;
97 
98 	if (unlikely(skb->sk))
99 		goto drop;
100 
101 	if (skb_warn_if_lro(skb))
102 		goto drop;
103 
104 	if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb)) {
105 		SKB_DR_SET(reason, XFRM_POLICY);
106 		goto drop;
107 	}
108 
109 	if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
110 		return NET_RX_SUCCESS;
111 
112 	skb_forward_csum(skb);
113 	net = dev_net(skb->dev);
114 
115 	/*
116 	 *	According to the RFC, we must first decrease the TTL field. If
117 	 *	that reaches zero, we must reply an ICMP control message telling
118 	 *	that the packet's lifetime expired.
119 	 */
120 	if (ip_hdr(skb)->ttl <= 1)
121 		goto too_many_hops;
122 
123 	if (!xfrm4_route_forward(skb)) {
124 		SKB_DR_SET(reason, XFRM_POLICY);
125 		goto drop;
126 	}
127 
128 	rt = skb_rtable(skb);
129 
130 	if (opt->is_strictroute && rt->rt_uses_gateway)
131 		goto sr_failed;
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 
166 sr_failed:
167 	/*
168 	 *	Strict routing permits no gatewaying
169 	 */
170 	 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
171 	 goto drop;
172 
173 too_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);
178 drop:
179 	kfree_skb_reason(skb, reason);
180 	return NET_RX_DROP;
181 }
182