1 // SPDX-License-Identifier: BSD-3-Clause
2 /*
3 * Simple Landlock sandbox manager able to execute a process restricted by
4 * user-defined file system and network access control policies.
5 *
6 * Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
7 * Copyright © 2020 ANSSI
8 */
9
10 #define _GNU_SOURCE
11 #define __SANE_USERSPACE_TYPES__
12 #include <arpa/inet.h>
13 #include <errno.h>
14 #include <fcntl.h>
15 #include <linux/landlock.h>
16 #include <linux/prctl.h>
17 #include <stddef.h>
18 #include <stdio.h>
19 #include <stdlib.h>
20 #include <string.h>
21 #include <sys/prctl.h>
22 #include <sys/stat.h>
23 #include <sys/syscall.h>
24 #include <unistd.h>
25
26 #ifndef landlock_create_ruleset
27 static inline int
28 landlock_create_ruleset(const struct landlock_ruleset_attr *const attr,
29 const size_t size, const __u32 flags)
30 {
31 return syscall(__NR_landlock_create_ruleset, attr, size, flags);
32 }
33 #endif
34
35 #ifndef landlock_add_rule
36 static inline int landlock_add_rule(const int ruleset_fd,
37 const enum landlock_rule_type rule_type,
38 const void *const rule_attr,
39 const __u32 flags)
40 {
41 return syscall(__NR_landlock_add_rule, ruleset_fd, rule_type, rule_attr,
42 flags);
43 }
44 #endif
45
46 #ifndef landlock_restrict_self
47 static inline int landlock_restrict_self(const int ruleset_fd,
48 const __u32 flags)
49 {
50 return syscall(__NR_landlock_restrict_self, ruleset_fd, flags);
51 }
52 #endif
53
54 #define ENV_FS_RO_NAME "LL_FS_RO"
55 #define ENV_FS_RW_NAME "LL_FS_RW"
56 #define ENV_TCP_BIND_NAME "LL_TCP_BIND"
57 #define ENV_TCP_CONNECT_NAME "LL_TCP_CONNECT"
58 #define ENV_DELIMITER ":"
59
60 static int parse_path(char *env_path, const char ***const path_list)
61 {
62 int i, num_paths = 0;
63
64 if (env_path) {
65 num_paths++;
66 for (i = 0; env_path[i]; i++) {
67 if (env_path[i] == ENV_DELIMITER[0])
68 num_paths++;
69 }
70 }
71 *path_list = malloc(num_paths * sizeof(**path_list));
72 for (i = 0; i < num_paths; i++)
73 (*path_list)[i] = strsep(&env_path, ENV_DELIMITER);
74
75 return num_paths;
76 }
77
78 /* clang-format off */
79
80 #define ACCESS_FILE ( \
81 LANDLOCK_ACCESS_FS_EXECUTE | \
82 LANDLOCK_ACCESS_FS_WRITE_FILE | \
83 LANDLOCK_ACCESS_FS_READ_FILE | \
84 LANDLOCK_ACCESS_FS_TRUNCATE | \
85 LANDLOCK_ACCESS_FS_IOCTL_DEV)
86
87 /* clang-format on */
88
89 static int populate_ruleset_fs(const char *const env_var, const int ruleset_fd,
90 const __u64 allowed_access)
91 {
92 int num_paths, i, ret = 1;
93 char *env_path_name;
94 const char **path_list = NULL;
95 struct landlock_path_beneath_attr path_beneath = {
96 .parent_fd = -1,
97 };
98
99 env_path_name = getenv(env_var);
100 if (!env_path_name) {
101 /* Prevents users to forget a setting. */
102 fprintf(stderr, "Missing environment variable %s\n", env_var);
103 return 1;
104 }
105 env_path_name = strdup(env_path_name);
106 unsetenv(env_var);
107 num_paths = parse_path(env_path_name, &path_list);
108 if (num_paths == 1 && path_list[0][0] == '\0') {
109 /*
110 * Allows to not use all possible restrictions (e.g. use
111 * LL_FS_RO without LL_FS_RW).
112 */
113 ret = 0;
114 goto out_free_name;
115 }
116
117 for (i = 0; i < num_paths; i++) {
118 struct stat statbuf;
119
120 path_beneath.parent_fd = open(path_list[i], O_PATH | O_CLOEXEC);
121 if (path_beneath.parent_fd < 0) {
122 fprintf(stderr, "Failed to open \"%s\": %s\n",
123 path_list[i], strerror(errno));
124 continue;
125 }
126 if (fstat(path_beneath.parent_fd, &statbuf)) {
127 fprintf(stderr, "Failed to stat \"%s\": %s\n",
128 path_list[i], strerror(errno));
129 close(path_beneath.parent_fd);
130 goto out_free_name;
131 }
132 path_beneath.allowed_access = allowed_access;
133 if (!S_ISDIR(statbuf.st_mode))
134 path_beneath.allowed_access &= ACCESS_FILE;
135 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
136 &path_beneath, 0)) {
137 fprintf(stderr,
138 "Failed to update the ruleset with \"%s\": %s\n",
139 path_list[i], strerror(errno));
140 close(path_beneath.parent_fd);
141 goto out_free_name;
142 }
143 close(path_beneath.parent_fd);
144 }
145 ret = 0;
146
147 out_free_name:
148 free(path_list);
149 free(env_path_name);
150 return ret;
151 }
152
153 static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
154 const __u64 allowed_access)
155 {
156 int ret = 1;
157 char *env_port_name, *env_port_name_next, *strport;
158 struct landlock_net_port_attr net_port = {
159 .allowed_access = allowed_access,
160 .port = 0,
161 };
162
163 env_port_name = getenv(env_var);
164 if (!env_port_name)
165 return 0;
166 env_port_name = strdup(env_port_name);
167 unsetenv(env_var);
168
169 env_port_name_next = env_port_name;
170 while ((strport = strsep(&env_port_name_next, ENV_DELIMITER))) {
171 net_port.port = atoi(strport);
172 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT,
173 &net_port, 0)) {
174 fprintf(stderr,
175 "Failed to update the ruleset with port \"%llu\": %s\n",
176 net_port.port, strerror(errno));
177 goto out_free_name;
178 }
179 }
180 ret = 0;
181
182 out_free_name:
183 free(env_port_name);
184 return ret;
185 }
186
187 /* clang-format off */
188
189 #define ACCESS_FS_ROUGHLY_READ ( \
190 LANDLOCK_ACCESS_FS_EXECUTE | \
191 LANDLOCK_ACCESS_FS_READ_FILE | \
192 LANDLOCK_ACCESS_FS_READ_DIR)
193
194 #define ACCESS_FS_ROUGHLY_WRITE ( \
195 LANDLOCK_ACCESS_FS_WRITE_FILE | \
196 LANDLOCK_ACCESS_FS_REMOVE_DIR | \
197 LANDLOCK_ACCESS_FS_REMOVE_FILE | \
198 LANDLOCK_ACCESS_FS_MAKE_CHAR | \
199 LANDLOCK_ACCESS_FS_MAKE_DIR | \
200 LANDLOCK_ACCESS_FS_MAKE_REG | \
201 LANDLOCK_ACCESS_FS_MAKE_SOCK | \
202 LANDLOCK_ACCESS_FS_MAKE_FIFO | \
203 LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
204 LANDLOCK_ACCESS_FS_MAKE_SYM | \
205 LANDLOCK_ACCESS_FS_REFER | \
206 LANDLOCK_ACCESS_FS_TRUNCATE | \
207 LANDLOCK_ACCESS_FS_IOCTL_DEV)
208
209 /* clang-format on */
210
211 #define LANDLOCK_ABI_LAST 5
212
213 int main(const int argc, char *const argv[], char *const *const envp)
214 {
215 const char *cmd_path;
216 char *const *cmd_argv;
217 int ruleset_fd, abi;
218 char *env_port_name;
219 __u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
220 access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
221
222 struct landlock_ruleset_attr ruleset_attr = {
223 .handled_access_fs = access_fs_rw,
224 .handled_access_net = LANDLOCK_ACCESS_NET_BIND_TCP |
225 LANDLOCK_ACCESS_NET_CONNECT_TCP,
226 };
227
228 if (argc < 2) {
229 fprintf(stderr,
230 "usage: %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\"%s "
231 "<cmd> [args]...\n\n",
232 ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
233 ENV_TCP_CONNECT_NAME, argv[0]);
234 fprintf(stderr,
235 "Execute a command in a restricted environment.\n\n");
236 fprintf(stderr,
237 "Environment variables containing paths and ports "
238 "each separated by a colon:\n");
239 fprintf(stderr,
240 "* %s: list of paths allowed to be used in a read-only way.\n",
241 ENV_FS_RO_NAME);
242 fprintf(stderr,
243 "* %s: list of paths allowed to be used in a read-write way.\n\n",
244 ENV_FS_RW_NAME);
245 fprintf(stderr,
246 "Environment variables containing ports are optional "
247 "and could be skipped.\n");
248 fprintf(stderr,
249 "* %s: list of ports allowed to bind (server).\n",
250 ENV_TCP_BIND_NAME);
251 fprintf(stderr,
252 "* %s: list of ports allowed to connect (client).\n",
253 ENV_TCP_CONNECT_NAME);
254 fprintf(stderr,
255 "\nexample:\n"
256 "%s=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
257 "%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
258 "%s=\"9418\" "
259 "%s=\"80:443\" "
260 "%s bash -i\n\n",
261 ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
262 ENV_TCP_CONNECT_NAME, argv[0]);
263 fprintf(stderr,
264 "This sandboxer can use Landlock features "
265 "up to ABI version %d.\n",
266 LANDLOCK_ABI_LAST);
267 return 1;
268 }
269
270 abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
271 if (abi < 0) {
272 const int err = errno;
273
274 perror("Failed to check Landlock compatibility");
275 switch (err) {
276 case ENOSYS:
277 fprintf(stderr,
278 "Hint: Landlock is not supported by the current kernel. "
279 "To support it, build the kernel with "
280 "CONFIG_SECURITY_LANDLOCK=y and prepend "
281 "\"landlock,\" to the content of CONFIG_LSM.\n");
282 break;
283 case EOPNOTSUPP:
284 fprintf(stderr,
285 "Hint: Landlock is currently disabled. "
286 "It can be enabled in the kernel configuration by "
287 "prepending \"landlock,\" to the content of CONFIG_LSM, "
288 "or at boot time by setting the same content to the "
289 "\"lsm\" kernel parameter.\n");
290 break;
291 }
292 return 1;
293 }
294
295 /* Best-effort security. */
296 switch (abi) {
297 case 1:
298 /*
299 * Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
300 *
301 * Note: The "refer" operations (file renaming and linking
302 * across different directories) are always forbidden when using
303 * Landlock with ABI 1.
304 *
305 * If only ABI 1 is available, this sandboxer knowingly forbids
306 * refer operations.
307 *
308 * If a program *needs* to do refer operations after enabling
309 * Landlock, it can not use Landlock at ABI level 1. To be
310 * compatible with different kernel versions, such programs
311 * should then fall back to not restrict themselves at all if
312 * the running kernel only supports ABI 1.
313 */
314 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
315 __attribute__((fallthrough));
316 case 2:
317 /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
318 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
319 __attribute__((fallthrough));
320 case 3:
321 /* Removes network support for ABI < 4 */
322 ruleset_attr.handled_access_net &=
323 ~(LANDLOCK_ACCESS_NET_BIND_TCP |
324 LANDLOCK_ACCESS_NET_CONNECT_TCP);
325 __attribute__((fallthrough));
326 case 4:
327 /* Removes LANDLOCK_ACCESS_FS_IOCTL_DEV for ABI < 5 */
328 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_IOCTL_DEV;
329
330 fprintf(stderr,
331 "Hint: You should update the running kernel "
332 "to leverage Landlock features "
333 "provided by ABI version %d (instead of %d).\n",
334 LANDLOCK_ABI_LAST, abi);
335 __attribute__((fallthrough));
336 case LANDLOCK_ABI_LAST:
337 break;
338 default:
339 fprintf(stderr,
340 "Hint: You should update this sandboxer "
341 "to leverage Landlock features "
342 "provided by ABI version %d (instead of %d).\n",
343 abi, LANDLOCK_ABI_LAST);
344 }
345 access_fs_ro &= ruleset_attr.handled_access_fs;
346 access_fs_rw &= ruleset_attr.handled_access_fs;
347
348 /* Removes bind access attribute if not supported by a user. */
349 env_port_name = getenv(ENV_TCP_BIND_NAME);
350 if (!env_port_name) {
351 ruleset_attr.handled_access_net &=
352 ~LANDLOCK_ACCESS_NET_BIND_TCP;
353 }
354 /* Removes connect access attribute if not supported by a user. */
355 env_port_name = getenv(ENV_TCP_CONNECT_NAME);
356 if (!env_port_name) {
357 ruleset_attr.handled_access_net &=
358 ~LANDLOCK_ACCESS_NET_CONNECT_TCP;
359 }
360
361 ruleset_fd =
362 landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
363 if (ruleset_fd < 0) {
364 perror("Failed to create a ruleset");
365 return 1;
366 }
367
368 if (populate_ruleset_fs(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
369 goto err_close_ruleset;
370 }
371 if (populate_ruleset_fs(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
372 goto err_close_ruleset;
373 }
374
375 if (populate_ruleset_net(ENV_TCP_BIND_NAME, ruleset_fd,
376 LANDLOCK_ACCESS_NET_BIND_TCP)) {
377 goto err_close_ruleset;
378 }
379 if (populate_ruleset_net(ENV_TCP_CONNECT_NAME, ruleset_fd,
380 LANDLOCK_ACCESS_NET_CONNECT_TCP)) {
381 goto err_close_ruleset;
382 }
383
384 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
385 perror("Failed to restrict privileges");
386 goto err_close_ruleset;
387 }
388 if (landlock_restrict_self(ruleset_fd, 0)) {
389 perror("Failed to enforce ruleset");
390 goto err_close_ruleset;
391 }
392 close(ruleset_fd);
393
394 cmd_path = argv[1];
395 cmd_argv = argv + 1;
396 fprintf(stderr, "Executing the sandboxed command...\n");
397 execvpe(cmd_path, cmd_argv, envp);
398 fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
399 strerror(errno));
400 fprintf(stderr, "Hint: access to the binary, the interpreter or "
401 "shared libraries may be denied.\n");
402 return 1;
403
404 err_close_ruleset:
405 close(ruleset_fd);
406 return 1;
407 }
408
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.