1 /*-
2 * SPDX-License-Identifier: BSD-2-Clause
3 *
4 * Copyright (c) 2008 Michael J. Silbersack.
5 * All rights reserved.
6 *
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
9 * are met:
10 * 1. Redistributions of source code must retain the above copyright
11 * notice unmodified, this list of conditions, and the following
12 * disclaimer.
13 * 2. Redistributions in binary form must reproduce the above copyright
14 * notice, this list of conditions and the following disclaimer in the
15 * documentation and/or other materials provided with the distribution.
16 *
17 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
18 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
19 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
20 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
21 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
22 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
23 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
24 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
26 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
27 */
28
29 #include <sys/cdefs.h>
30 /*
31 * IP ID generation is a fascinating topic.
32 *
33 * In order to avoid ID collisions during packet reassembly, common sense
34 * dictates that the period between reuse of IDs be as large as possible.
35 * This leads to the classic implementation of a system-wide counter, thereby
36 * ensuring that IDs repeat only once every 2^16 packets.
37 *
38 * Subsequent security researchers have pointed out that using a global
39 * counter makes ID values predictable. This predictability allows traffic
40 * analysis, idle scanning, and even packet injection in specific cases.
41 * These results suggest that IP IDs should be as random as possible.
42 *
43 * The "searchable queues" algorithm used in this IP ID implementation was
44 * proposed by Amit Klein. It is a compromise between the above two
45 * viewpoints that has provable behavior that can be tuned to the user's
46 * requirements.
47 *
48 * The basic concept is that we supplement a standard random number generator
49 * with a queue of the last L IDs that we have handed out to ensure that all
50 * IDs have a period of at least L.
51 *
52 * To efficiently implement this idea, we keep two data structures: a
53 * circular array of IDs of size L and a bitstring of 65536 bits.
54 *
55 * To start, we ask the RNG for a new ID. A quick index into the bitstring
56 * is used to determine if this is a recently used value. The process is
57 * repeated until a value is returned that is not in the bitstring.
58 *
59 * Having found a usable ID, we remove the ID stored at the current position
60 * in the queue from the bitstring and replace it with our new ID. Our new
61 * ID is then added to the bitstring and the queue pointer is incremented.
62 *
63 * The lower limit of 512 was chosen because there doesn't seem to be much
64 * point to having a smaller value. The upper limit of 32768 was chosen for
65 * two reasons. First, every step above 32768 decreases the entropy. Taken
66 * to an extreme, 65533 would offer 1 bit of entropy. Second, the number of
67 * attempts it takes the algorithm to find an unused ID drastically
68 * increases, killing performance. The default value of 8192 was chosen
69 * because it provides a good tradeoff between randomness and non-repetition.
70 *
71 * With L=8192, the queue will use 16K of memory. The bitstring always
72 * uses 8K of memory. No memory is allocated until the use of random ids is
73 * enabled.
74 */
75
76 #include <sys/param.h>
77 #include <sys/systm.h>
78 #include <sys/counter.h>
79 #include <sys/kernel.h>
80 #include <sys/malloc.h>
81 #include <sys/lock.h>
82 #include <sys/mutex.h>
83 #include <sys/random.h>
84 #include <sys/smp.h>
85 #include <sys/sysctl.h>
86 #include <sys/bitstring.h>
87
88 #include <net/vnet.h>
89
90 #include <netinet/in.h>
91 #include <netinet/ip.h>
92 #include <netinet/ip_var.h>
93
94 /*
95 * By default we generate IP ID only for non-atomic datagrams, as
96 * suggested by RFC6864. We use per-CPU counter for that, or if
97 * user wants to, we can turn on random ID generation.
98 */
99 VNET_DEFINE_STATIC(int, ip_rfc6864) = 1;
100 VNET_DEFINE_STATIC(int, ip_do_randomid) = 0;
101 #define V_ip_rfc6864 VNET(ip_rfc6864)
102 #define V_ip_do_randomid VNET(ip_do_randomid)
103
104 /*
105 * Random ID state engine.
106 */
107 static MALLOC_DEFINE(M_IPID, "ipid", "randomized ip id state");
108 VNET_DEFINE_STATIC(uint16_t *, id_array);
109 VNET_DEFINE_STATIC(bitstr_t *, id_bits);
110 VNET_DEFINE_STATIC(int, array_ptr);
111 VNET_DEFINE_STATIC(int, array_size);
112 VNET_DEFINE_STATIC(int, random_id_collisions);
113 VNET_DEFINE_STATIC(int, random_id_total);
114 VNET_DEFINE_STATIC(struct mtx, ip_id_mtx);
115 #define V_id_array VNET(id_array)
116 #define V_id_bits VNET(id_bits)
117 #define V_array_ptr VNET(array_ptr)
118 #define V_array_size VNET(array_size)
119 #define V_random_id_collisions VNET(random_id_collisions)
120 #define V_random_id_total VNET(random_id_total)
121 #define V_ip_id_mtx VNET(ip_id_mtx)
122
123 /*
124 * Non-random ID state engine is simply a per-cpu counter.
125 */
126 VNET_DEFINE_STATIC(counter_u64_t, ip_id);
127 #define V_ip_id VNET(ip_id)
128
129 static int sysctl_ip_randomid(SYSCTL_HANDLER_ARGS);
130 static int sysctl_ip_id_change(SYSCTL_HANDLER_ARGS);
131 static void ip_initid(int);
132 static uint16_t ip_randomid(void);
133 static void ipid_sysinit(void);
134 static void ipid_sysuninit(void);
135
136 SYSCTL_DECL(_net_inet_ip);
137 SYSCTL_PROC(_net_inet_ip, OID_AUTO, random_id,
138 CTLTYPE_INT | CTLFLAG_VNET | CTLFLAG_RW | CTLFLAG_MPSAFE,
139 &VNET_NAME(ip_do_randomid), 0, sysctl_ip_randomid, "IU",
140 "Assign random ip_id values");
141 SYSCTL_INT(_net_inet_ip, OID_AUTO, rfc6864, CTLFLAG_VNET | CTLFLAG_RW,
142 &VNET_NAME(ip_rfc6864), 0,
143 "Use constant IP ID for atomic datagrams");
144 SYSCTL_PROC(_net_inet_ip, OID_AUTO, random_id_period,
145 CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_VNET | CTLFLAG_MPSAFE,
146 &VNET_NAME(array_size), 0, sysctl_ip_id_change, "IU", "IP ID Array size");
147 SYSCTL_INT(_net_inet_ip, OID_AUTO, random_id_collisions,
148 CTLFLAG_RD | CTLFLAG_VNET,
149 &VNET_NAME(random_id_collisions), 0, "Count of IP ID collisions");
150 SYSCTL_INT(_net_inet_ip, OID_AUTO, random_id_total, CTLFLAG_RD | CTLFLAG_VNET,
151 &VNET_NAME(random_id_total), 0, "Count of IP IDs created");
152
153 static int
sysctl_ip_randomid(SYSCTL_HANDLER_ARGS)154 sysctl_ip_randomid(SYSCTL_HANDLER_ARGS)
155 {
156 int error, new;
157
158 new = V_ip_do_randomid;
159 error = sysctl_handle_int(oidp, &new, 0, req);
160 if (error || req->newptr == NULL)
161 return (error);
162 if (new != 0 && new != 1)
163 return (EINVAL);
164 if (new == V_ip_do_randomid)
165 return (0);
166 if (new == 1 && V_ip_do_randomid == 0)
167 ip_initid(8192);
168 /* We don't free memory when turning random ID off, due to race. */
169 V_ip_do_randomid = new;
170 return (0);
171 }
172
173 static int
sysctl_ip_id_change(SYSCTL_HANDLER_ARGS)174 sysctl_ip_id_change(SYSCTL_HANDLER_ARGS)
175 {
176 int error, new;
177
178 new = V_array_size;
179 error = sysctl_handle_int(oidp, &new, 0, req);
180 if (error == 0 && req->newptr) {
181 if (new >= 512 && new <= 32768)
182 ip_initid(new);
183 else
184 error = EINVAL;
185 }
186 return (error);
187 }
188
189 static void
ip_initid(int new_size)190 ip_initid(int new_size)
191 {
192 uint16_t *new_array;
193 bitstr_t *new_bits;
194
195 new_array = malloc(new_size * sizeof(uint16_t), M_IPID,
196 M_WAITOK | M_ZERO);
197 new_bits = malloc(bitstr_size(65536), M_IPID, M_WAITOK | M_ZERO);
198
199 mtx_lock(&V_ip_id_mtx);
200 if (V_id_array != NULL) {
201 free(V_id_array, M_IPID);
202 free(V_id_bits, M_IPID);
203 }
204 V_id_array = new_array;
205 V_id_bits = new_bits;
206 V_array_size = new_size;
207 V_array_ptr = 0;
208 V_random_id_collisions = 0;
209 V_random_id_total = 0;
210 mtx_unlock(&V_ip_id_mtx);
211 }
212
213 static uint16_t
ip_randomid(void)214 ip_randomid(void)
215 {
216 uint16_t new_id;
217
218 mtx_lock(&V_ip_id_mtx);
219 /*
220 * To avoid a conflict with the zeros that the array is initially
221 * filled with, we never hand out an id of zero.
222 */
223 new_id = 0;
224 do {
225 if (new_id != 0)
226 V_random_id_collisions++;
227 arc4rand(&new_id, sizeof(new_id), 0);
228 } while (bit_test(V_id_bits, new_id) || new_id == 0);
229 bit_clear(V_id_bits, V_id_array[V_array_ptr]);
230 bit_set(V_id_bits, new_id);
231 V_id_array[V_array_ptr] = new_id;
232 V_array_ptr++;
233 if (V_array_ptr == V_array_size)
234 V_array_ptr = 0;
235 V_random_id_total++;
236 mtx_unlock(&V_ip_id_mtx);
237 return (new_id);
238 }
239
240 void
ip_fillid(struct ip * ip)241 ip_fillid(struct ip *ip)
242 {
243
244 /*
245 * Per RFC6864 Section 4
246 *
247 * o Atomic datagrams: (DF==1) && (MF==0) && (frag_offset==0)
248 * o Non-atomic datagrams: (DF==0) || (MF==1) || (frag_offset>0)
249 */
250 if (V_ip_rfc6864 && (ip->ip_off & htons(IP_DF)) == htons(IP_DF))
251 ip->ip_id = 0;
252 else if (V_ip_do_randomid)
253 ip->ip_id = ip_randomid();
254 else {
255 counter_u64_add(V_ip_id, 1);
256 /*
257 * There are two issues about this trick, to be kept in mind.
258 * 1) We can migrate between counter_u64_add() and next
259 * line, and grab counter from other CPU, resulting in too
260 * quick ID reuse. This is tolerable in our particular case,
261 * since probability of such event is much lower then reuse
262 * of ID due to legitimate overflow, that at modern Internet
263 * speeds happens all the time.
264 * 2) We are relying on the fact that counter(9) is based on
265 * UMA_ZONE_PCPU uma(9) zone. We also take only last
266 * sixteen bits of a counter, so we don't care about the
267 * fact that machines with 32-bit word update their counters
268 * not atomically.
269 */
270 ip->ip_id = htons((*(uint64_t *)zpcpu_get(V_ip_id)) & 0xffff);
271 }
272 }
273
274 static void
ipid_sysinit(void)275 ipid_sysinit(void)
276 {
277 int i;
278
279 mtx_init(&V_ip_id_mtx, "ip_id_mtx", NULL, MTX_DEF);
280 V_ip_id = counter_u64_alloc(M_WAITOK);
281
282 CPU_FOREACH(i)
283 arc4rand(zpcpu_get_cpu(V_ip_id, i), sizeof(uint64_t), 0);
284 }
285 VNET_SYSINIT(ip_id, SI_SUB_PROTO_DOMAIN, SI_ORDER_ANY, ipid_sysinit, NULL);
286
287 static void
ipid_sysuninit(void)288 ipid_sysuninit(void)
289 {
290
291 if (V_id_array != NULL) {
292 free(V_id_array, M_IPID);
293 free(V_id_bits, M_IPID);
294 }
295 counter_u64_free(V_ip_id);
296 mtx_destroy(&V_ip_id_mtx);
297 }
298 VNET_SYSUNINIT(ip_id, SI_SUB_PROTO_DOMAIN, SI_ORDER_THIRD, ipid_sysuninit, NULL);
299