1 // SPDX-License-Identifier: GPL-2.0-only
2 /* nf_nat_helper.c - generic support functions for NAT helpers
3 *
4 * (C) 2000-2002 Harald Welte <laforge@netfilter.org>
5 * (C) 2003-2006 Netfilter Core Team <coreteam@netfilter.org>
6 * (C) 2007-2012 Patrick McHardy <kaber@trash.net>
7 */
8 #include <linux/module.h>
9 #include <linux/gfp.h>
10 #include <linux/types.h>
11 #include <linux/skbuff.h>
12 #include <linux/tcp.h>
13 #include <linux/udp.h>
14 #include <net/tcp.h>
15
16 #include <net/netfilter/nf_conntrack.h>
17 #include <net/netfilter/nf_conntrack_helper.h>
18 #include <net/netfilter/nf_conntrack_ecache.h>
19 #include <net/netfilter/nf_conntrack_expect.h>
20 #include <net/netfilter/nf_conntrack_seqadj.h>
21 #include <net/netfilter/nf_nat.h>
22 #include <net/netfilter/nf_nat_helper.h>
23
24 /* Frobs data inside this packet, which is linear. */
25 static void mangle_contents(struct sk_buff *skb,
26 unsigned int dataoff,
27 unsigned int match_offset,
28 unsigned int match_len,
29 const char *rep_buffer,
30 unsigned int rep_len)
31 {
32 unsigned char *data;
33
34 SKB_LINEAR_ASSERT(skb);
35 data = skb_network_header(skb) + dataoff;
36
37 /* move post-replacement */
38 memmove(data + match_offset + rep_len,
39 data + match_offset + match_len,
40 skb_tail_pointer(skb) - (skb_network_header(skb) + dataoff +
41 match_offset + match_len));
42
43 /* insert data from buffer */
44 memcpy(data + match_offset, rep_buffer, rep_len);
45
46 /* update skb info */
47 if (rep_len > match_len) {
48 pr_debug("nf_nat_mangle_packet: Extending packet by "
49 "%u from %u bytes\n", rep_len - match_len, skb->len);
50 skb_put(skb, rep_len - match_len);
51 } else {
52 pr_debug("nf_nat_mangle_packet: Shrinking packet from "
53 "%u from %u bytes\n", match_len - rep_len, skb->len);
54 __skb_trim(skb, skb->len + rep_len - match_len);
55 }
56
57 if (nf_ct_l3num((struct nf_conn *)skb_nfct(skb)) == NFPROTO_IPV4) {
58 /* fix IP hdr checksum information */
59 ip_hdr(skb)->tot_len = htons(skb->len);
60 ip_send_check(ip_hdr(skb));
61 } else
62 ipv6_hdr(skb)->payload_len =
63 htons(skb->len - sizeof(struct ipv6hdr));
64 }
65
66 /* Unusual, but possible case. */
67 static bool enlarge_skb(struct sk_buff *skb, unsigned int extra)
68 {
69 if (skb->len + extra > 65535)
70 return false;
71
72 if (pskb_expand_head(skb, 0, extra - skb_tailroom(skb), GFP_ATOMIC))
73 return false;
74
75 return true;
76 }
77
78 /* Generic function for mangling variable-length address changes inside
79 * NATed TCP connections (like the PORT XXX,XXX,XXX,XXX,XXX,XXX
80 * command in FTP).
81 *
82 * Takes care about all the nasty sequence number changes, checksumming,
83 * skb enlargement, ...
84 *
85 * */
86 bool __nf_nat_mangle_tcp_packet(struct sk_buff *skb,
87 struct nf_conn *ct,
88 enum ip_conntrack_info ctinfo,
89 unsigned int protoff,
90 unsigned int match_offset,
91 unsigned int match_len,
92 const char *rep_buffer,
93 unsigned int rep_len, bool adjust)
94 {
95 struct tcphdr *tcph;
96 int oldlen, datalen;
97
98 if (skb_ensure_writable(skb, skb->len))
99 return false;
100
101 if (rep_len > match_len &&
102 rep_len - match_len > skb_tailroom(skb) &&
103 !enlarge_skb(skb, rep_len - match_len))
104 return false;
105
106 tcph = (void *)skb->data + protoff;
107
108 oldlen = skb->len - protoff;
109 mangle_contents(skb, protoff + tcph->doff*4,
110 match_offset, match_len, rep_buffer, rep_len);
111
112 datalen = skb->len - protoff;
113
114 nf_nat_csum_recalc(skb, nf_ct_l3num(ct), IPPROTO_TCP,
115 tcph, &tcph->check, datalen, oldlen);
116
117 if (adjust && rep_len != match_len)
118 nf_ct_seqadj_set(ct, ctinfo, tcph->seq,
119 (int)rep_len - (int)match_len);
120
121 return true;
122 }
123 EXPORT_SYMBOL(__nf_nat_mangle_tcp_packet);
124
125 /* Generic function for mangling variable-length address changes inside
126 * NATed UDP connections (like the CONNECT DATA XXXXX MESG XXXXX INDEX XXXXX
127 * command in the Amanda protocol)
128 *
129 * Takes care about all the nasty sequence number changes, checksumming,
130 * skb enlargement, ...
131 *
132 * XXX - This function could be merged with nf_nat_mangle_tcp_packet which
133 * should be fairly easy to do.
134 */
135 bool
136 nf_nat_mangle_udp_packet(struct sk_buff *skb,
137 struct nf_conn *ct,
138 enum ip_conntrack_info ctinfo,
139 unsigned int protoff,
140 unsigned int match_offset,
141 unsigned int match_len,
142 const char *rep_buffer,
143 unsigned int rep_len)
144 {
145 struct udphdr *udph;
146 int datalen, oldlen;
147
148 if (skb_ensure_writable(skb, skb->len))
149 return false;
150
151 if (rep_len > match_len &&
152 rep_len - match_len > skb_tailroom(skb) &&
153 !enlarge_skb(skb, rep_len - match_len))
154 return false;
155
156 udph = (void *)skb->data + protoff;
157
158 oldlen = skb->len - protoff;
159 mangle_contents(skb, protoff + sizeof(*udph),
160 match_offset, match_len, rep_buffer, rep_len);
161
162 /* update the length of the UDP packet */
163 datalen = skb->len - protoff;
164 udph->len = htons(datalen);
165
166 /* fix udp checksum if udp checksum was previously calculated */
167 if (!udph->check && skb->ip_summed != CHECKSUM_PARTIAL)
168 return true;
169
170 nf_nat_csum_recalc(skb, nf_ct_l3num(ct), IPPROTO_UDP,
171 udph, &udph->check, datalen, oldlen);
172
173 return true;
174 }
175 EXPORT_SYMBOL(nf_nat_mangle_udp_packet);
176
177 /* Setup NAT on this expected conntrack so it follows master. */
178 /* If we fail to get a free NAT slot, we'll get dropped on confirm */
179 void nf_nat_follow_master(struct nf_conn *ct,
180 struct nf_conntrack_expect *exp)
181 {
182 struct nf_nat_range2 range;
183
184 /* This must be a fresh one. */
185 BUG_ON(ct->status & IPS_NAT_DONE_MASK);
186
187 /* Change src to where master sends to */
188 range.flags = NF_NAT_RANGE_MAP_IPS;
189 range.min_addr = range.max_addr
190 = ct->master->tuplehash[!exp->dir].tuple.dst.u3;
191 nf_nat_setup_info(ct, &range, NF_NAT_MANIP_SRC);
192
193 /* For DST manip, map port here to where it's expected. */
194 range.flags = (NF_NAT_RANGE_MAP_IPS | NF_NAT_RANGE_PROTO_SPECIFIED);
195 range.min_proto = range.max_proto = exp->saved_proto;
196 range.min_addr = range.max_addr
197 = ct->master->tuplehash[!exp->dir].tuple.src.u3;
198 nf_nat_setup_info(ct, &range, NF_NAT_MANIP_DST);
199 }
200 EXPORT_SYMBOL(nf_nat_follow_master);
201
202 u16 nf_nat_exp_find_port(struct nf_conntrack_expect *exp, u16 port)
203 {
204 static const unsigned int max_attempts = 128;
205 int range, attempts_left;
206 u16 min = port;
207
208 range = USHRT_MAX - port;
209 attempts_left = range;
210
211 if (attempts_left > max_attempts)
212 attempts_left = max_attempts;
213
214 /* Try to get same port: if not, try to change it. */
215 for (;;) {
216 int res;
217
218 exp->tuple.dst.u.tcp.port = htons(port);
219 res = nf_ct_expect_related(exp, 0);
220 if (res == 0)
221 return port;
222
223 if (res != -EBUSY || (--attempts_left < 0))
224 break;
225
226 port = min + get_random_u32_below(range);
227 }
228
229 return 0;
230 }
231 EXPORT_SYMBOL_GPL(nf_nat_exp_find_port);
232
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