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
15static int nf_reject_iphdr_validate(struct sk_buff *skb)
16{
17	struct iphdr *iph;
18	u32 len;
19
20	if (!pskb_may_pull(skb, sizeof(struct iphdr)))
21		return 0;
22
23	iph = ip_hdr(skb);
24	if (iph->ihl < 5 || iph->version != 4)
25		return 0;
26
27	len = ntohs(iph->tot_len);
28	if (skb->len < len)
29		return 0;
30	else if (len < (iph->ihl*4))
31		return 0;
32
33	if (!pskb_may_pull(skb, iph->ihl*4))
34		return 0;
35
36	return 1;
37}
38
39struct sk_buff *nf_reject_skb_v4_tcp_reset(struct net *net,
40					   struct sk_buff *oldskb,
41					   const struct net_device *dev,
42					   int hook)
43{
44	const struct tcphdr *oth;
45	struct sk_buff *nskb;
46	struct iphdr *niph;
47	struct tcphdr _oth;
48
49	if (!nf_reject_iphdr_validate(oldskb))
50		return NULL;
51
52	oth = nf_reject_ip_tcphdr_get(oldskb, &_oth, hook);
53	if (!oth)
54		return NULL;
55
56	nskb = alloc_skb(sizeof(struct iphdr) + sizeof(struct tcphdr) +
57			 LL_MAX_HEADER, GFP_ATOMIC);
58	if (!nskb)
59		return NULL;
60
61	nskb->dev = (struct net_device *)dev;
62
63	skb_reserve(nskb, LL_MAX_HEADER);
64	niph = nf_reject_iphdr_put(nskb, oldskb, IPPROTO_TCP,
65				   READ_ONCE(net->ipv4.sysctl_ip_default_ttl));
66	nf_reject_ip_tcphdr_put(nskb, oldskb, oth);
67	niph->tot_len = htons(nskb->len);
68	ip_send_check(niph);
69
70	return nskb;
71}
72EXPORT_SYMBOL_GPL(nf_reject_skb_v4_tcp_reset);
73
74struct sk_buff *nf_reject_skb_v4_unreach(struct net *net,
75					 struct sk_buff *oldskb,
76					 const struct net_device *dev,
77					 int hook, u8 code)
78{
79	struct sk_buff *nskb;
80	struct iphdr *niph;
81	struct icmphdr *icmph;
82	unsigned int len;
83	int dataoff;
84	__wsum csum;
85	u8 proto;
86
87	if (!nf_reject_iphdr_validate(oldskb))
88		return NULL;
89
90	/* IP header checks: fragment. */
91	if (ip_hdr(oldskb)->frag_off & htons(IP_OFFSET))
92		return NULL;
93
94	/* RFC says return as much as we can without exceeding 576 bytes. */
95	len = min_t(unsigned int, 536, oldskb->len);
96
97	if (!pskb_may_pull(oldskb, len))
98		return NULL;
99
100	if (pskb_trim_rcsum(oldskb, ntohs(ip_hdr(oldskb)->tot_len)))
101		return NULL;
102
103	dataoff = ip_hdrlen(oldskb);
104	proto = ip_hdr(oldskb)->protocol;
105
106	if (!skb_csum_unnecessary(oldskb) &&
107	    nf_reject_verify_csum(oldskb, dataoff, proto) &&
108	    nf_ip_checksum(oldskb, hook, ip_hdrlen(oldskb), proto))
109		return NULL;
110
111	nskb = alloc_skb(sizeof(struct iphdr) + sizeof(struct icmphdr) +
112			 LL_MAX_HEADER + len, GFP_ATOMIC);
113	if (!nskb)
114		return NULL;
115
116	nskb->dev = (struct net_device *)dev;
117
118	skb_reserve(nskb, LL_MAX_HEADER);
119	niph = nf_reject_iphdr_put(nskb, oldskb, IPPROTO_ICMP,
120				   READ_ONCE(net->ipv4.sysctl_ip_default_ttl));
121
122	skb_reset_transport_header(nskb);
123	icmph = skb_put_zero(nskb, sizeof(struct icmphdr));
124	icmph->type     = ICMP_DEST_UNREACH;
125	icmph->code	= code;
126
127	skb_put_data(nskb, skb_network_header(oldskb), len);
128
129	csum = csum_partial((void *)icmph, len + sizeof(struct icmphdr), 0);
130	icmph->checksum = csum_fold(csum);
131
132	niph->tot_len	= htons(nskb->len);
133	ip_send_check(niph);
134
135	return nskb;
136}
137EXPORT_SYMBOL_GPL(nf_reject_skb_v4_unreach);
138
139const struct tcphdr *nf_reject_ip_tcphdr_get(struct sk_buff *oldskb,
140					     struct tcphdr *_oth, int hook)
141{
142	const struct tcphdr *oth;
143
144	/* IP header checks: fragment. */
145	if (ip_hdr(oldskb)->frag_off & htons(IP_OFFSET))
146		return NULL;
147
148	if (ip_hdr(oldskb)->protocol != IPPROTO_TCP)
149		return NULL;
150
151	oth = skb_header_pointer(oldskb, ip_hdrlen(oldskb),
152				 sizeof(struct tcphdr), _oth);
153	if (oth == NULL)
154		return NULL;
155
156	/* No RST for RST. */
157	if (oth->rst)
158		return NULL;
159
160	/* Check checksum */
161	if (nf_ip_checksum(oldskb, hook, ip_hdrlen(oldskb), IPPROTO_TCP))
162		return NULL;
163
164	return oth;
165}
166EXPORT_SYMBOL_GPL(nf_reject_ip_tcphdr_get);
167
168struct iphdr *nf_reject_iphdr_put(struct sk_buff *nskb,
169				  const struct sk_buff *oldskb,
170				  __u8 protocol, int ttl)
171{
172	struct iphdr *niph, *oiph = ip_hdr(oldskb);
173
174	skb_reset_network_header(nskb);
175	niph = skb_put(nskb, sizeof(struct iphdr));
176	niph->version	= 4;
177	niph->ihl	= sizeof(struct iphdr) / 4;
178	niph->tos	= 0;
179	niph->id	= 0;
180	niph->frag_off	= htons(IP_DF);
181	niph->protocol	= protocol;
182	niph->check	= 0;
183	niph->saddr	= oiph->daddr;
184	niph->daddr	= oiph->saddr;
185	niph->ttl	= ttl;
186
187	nskb->protocol = htons(ETH_P_IP);
188
189	return niph;
190}
191EXPORT_SYMBOL_GPL(nf_reject_iphdr_put);
192
193void nf_reject_ip_tcphdr_put(struct sk_buff *nskb, const struct sk_buff *oldskb,
194			  const struct tcphdr *oth)
195{
196	struct iphdr *niph = ip_hdr(nskb);
197	struct tcphdr *tcph;
198
199	skb_reset_transport_header(nskb);
200	tcph = skb_put_zero(nskb, sizeof(struct tcphdr));
201	tcph->source	= oth->dest;
202	tcph->dest	= oth->source;
203	tcph->doff	= sizeof(struct tcphdr) / 4;
204
205	if (oth->ack) {
206		tcph->seq = oth->ack_seq;
207	} else {
208		tcph->ack_seq = htonl(ntohl(oth->seq) + oth->syn + oth->fin +
209				      oldskb->len - ip_hdrlen(oldskb) -
210				      (oth->doff << 2));
211		tcph->ack = 1;
212	}
213
214	tcph->rst	= 1;
215	tcph->check = ~tcp_v4_check(sizeof(struct tcphdr), niph->saddr,
216				    niph->daddr, 0);
217	nskb->ip_summed = CHECKSUM_PARTIAL;
218	nskb->csum_start = (unsigned char *)tcph - nskb->head;
219	nskb->csum_offset = offsetof(struct tcphdr, check);
220}
221EXPORT_SYMBOL_GPL(nf_reject_ip_tcphdr_put);
222
223static int nf_reject_fill_skb_dst(struct sk_buff *skb_in)
224{
225	struct dst_entry *dst = NULL;
226	struct flowi fl;
227
228	memset(&fl, 0, sizeof(struct flowi));
229	fl.u.ip4.daddr = ip_hdr(skb_in)->saddr;
230	nf_ip_route(dev_net(skb_in->dev), &dst, &fl, false);
231	if (!dst)
232		return -1;
233
234	skb_dst_set(skb_in, dst);
235	return 0;
236}
237
238/* Send RST reply */
239void nf_send_reset(struct net *net, struct sock *sk, struct sk_buff *oldskb,
240		   int hook)
241{
242	struct sk_buff *nskb;
243	struct iphdr *niph;
244	const struct tcphdr *oth;
245	struct tcphdr _oth;
246
247	oth = nf_reject_ip_tcphdr_get(oldskb, &_oth, hook);
248	if (!oth)
249		return;
250
251	if ((hook == NF_INET_PRE_ROUTING || hook == NF_INET_INGRESS) &&
252	    nf_reject_fill_skb_dst(oldskb) < 0)
253		return;
254
255	if (skb_rtable(oldskb)->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST))
256		return;
257
258	nskb = alloc_skb(sizeof(struct iphdr) + sizeof(struct tcphdr) +
259			 LL_MAX_HEADER, GFP_ATOMIC);
260	if (!nskb)
261		return;
262
263	/* ip_route_me_harder expects skb->dst to be set */
264	skb_dst_set_noref(nskb, skb_dst(oldskb));
265
266	nskb->mark = IP4_REPLY_MARK(net, oldskb->mark);
267
268	skb_reserve(nskb, LL_MAX_HEADER);
269	niph = nf_reject_iphdr_put(nskb, oldskb, IPPROTO_TCP,
270				   ip4_dst_hoplimit(skb_dst(nskb)));
271	nf_reject_ip_tcphdr_put(nskb, oldskb, oth);
272	if (ip_route_me_harder(net, sk, nskb, RTN_UNSPEC))
273		goto free_nskb;
274
275	niph = ip_hdr(nskb);
276
277	/* "Never happens" */
278	if (nskb->len > dst_mtu(skb_dst(nskb)))
279		goto free_nskb;
280
281	nf_ct_attach(nskb, oldskb);
282	nf_ct_set_closing(skb_nfct(oldskb));
283
284#if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
285	/* If we use ip_local_out for bridged traffic, the MAC source on
286	 * the RST will be ours, instead of the destination's.  This confuses
287	 * some routers/firewalls, and they drop the packet.  So we need to
288	 * build the eth header using the original destination's MAC as the
289	 * source, and send the RST packet directly.
290	 */
291	if (nf_bridge_info_exists(oldskb)) {
292		struct ethhdr *oeth = eth_hdr(oldskb);
293		struct net_device *br_indev;
294
295		br_indev = nf_bridge_get_physindev(oldskb, net);
296		if (!br_indev)
297			goto free_nskb;
298
299		nskb->dev = br_indev;
300		niph->tot_len = htons(nskb->len);
301		ip_send_check(niph);
302		if (dev_hard_header(nskb, nskb->dev, ntohs(nskb->protocol),
303				    oeth->h_source, oeth->h_dest, nskb->len) < 0)
304			goto free_nskb;
305		dev_queue_xmit(nskb);
306	} else
307#endif
308		ip_local_out(net, nskb->sk, nskb);
309
310	return;
311
312 free_nskb:
313	kfree_skb(nskb);
314}
315EXPORT_SYMBOL_GPL(nf_send_reset);
316
317void nf_send_unreach(struct sk_buff *skb_in, int code, int hook)
318{
319	struct iphdr *iph = ip_hdr(skb_in);
320	int dataoff = ip_hdrlen(skb_in);
321	u8 proto = iph->protocol;
322
323	if (iph->frag_off & htons(IP_OFFSET))
324		return;
325
326	if ((hook == NF_INET_PRE_ROUTING || hook == NF_INET_INGRESS) &&
327	    nf_reject_fill_skb_dst(skb_in) < 0)
328		return;
329
330	if (skb_csum_unnecessary(skb_in) ||
331	    !nf_reject_verify_csum(skb_in, dataoff, proto)) {
332		icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
333		return;
334	}
335
336	if (nf_ip_checksum(skb_in, hook, dataoff, proto) == 0)
337		icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
338}
339EXPORT_SYMBOL_GPL(nf_send_unreach);
340
341MODULE_LICENSE("GPL");
342MODULE_DESCRIPTION("IPv4 packet rejection core");
343