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 <linux/socket.h>
18 #include <stddef.h>
19 #include <stdio.h>
20 #include <stdlib.h>
21 #include <string.h>
22 #include <sys/prctl.h>
23 #include <sys/stat.h>
24 #include <sys/syscall.h>
25 #include <unistd.h>
26 #include <stdbool.h>
27
28 #ifndef landlock_create_ruleset
29 static inline int
landlock_create_ruleset(const struct landlock_ruleset_attr * const attr,const size_t size,const __u32 flags)30 landlock_create_ruleset(const struct landlock_ruleset_attr *const attr,
31 const size_t size, const __u32 flags)
32 {
33 return syscall(__NR_landlock_create_ruleset, attr, size, flags);
34 }
35 #endif
36
37 #ifndef landlock_add_rule
landlock_add_rule(const int ruleset_fd,const enum landlock_rule_type rule_type,const void * const rule_attr,const __u32 flags)38 static inline int landlock_add_rule(const int ruleset_fd,
39 const enum landlock_rule_type rule_type,
40 const void *const rule_attr,
41 const __u32 flags)
42 {
43 return syscall(__NR_landlock_add_rule, ruleset_fd, rule_type, rule_attr,
44 flags);
45 }
46 #endif
47
48 #ifndef landlock_restrict_self
landlock_restrict_self(const int ruleset_fd,const __u32 flags)49 static inline int landlock_restrict_self(const int ruleset_fd,
50 const __u32 flags)
51 {
52 return syscall(__NR_landlock_restrict_self, ruleset_fd, flags);
53 }
54 #endif
55
56 #define ENV_FS_RO_NAME "LL_FS_RO"
57 #define ENV_FS_RW_NAME "LL_FS_RW"
58 #define ENV_TCP_BIND_NAME "LL_TCP_BIND"
59 #define ENV_TCP_CONNECT_NAME "LL_TCP_CONNECT"
60 #define ENV_SCOPED_NAME "LL_SCOPED"
61 #define ENV_FORCE_LOG_NAME "LL_FORCE_LOG"
62 #define ENV_DELIMITER ":"
63
str2num(const char * numstr,__u64 * num_dst)64 static int str2num(const char *numstr, __u64 *num_dst)
65 {
66 char *endptr = NULL;
67 int err = 0;
68 __u64 num;
69
70 errno = 0;
71 num = strtoull(numstr, &endptr, 10);
72 if (errno != 0)
73 err = errno;
74 /* Was the string empty, or not entirely parsed successfully? */
75 else if ((*numstr == '\0') || (*endptr != '\0'))
76 err = EINVAL;
77 else
78 *num_dst = num;
79
80 return err;
81 }
82
parse_path(char * env_path,const char *** const path_list)83 static int parse_path(char *env_path, const char ***const path_list)
84 {
85 int i, num_paths = 0;
86
87 if (env_path) {
88 num_paths++;
89 for (i = 0; env_path[i]; i++) {
90 if (env_path[i] == ENV_DELIMITER[0])
91 num_paths++;
92 }
93 }
94 *path_list = malloc(num_paths * sizeof(**path_list));
95 if (!*path_list)
96 return -1;
97
98 for (i = 0; i < num_paths; i++)
99 (*path_list)[i] = strsep(&env_path, ENV_DELIMITER);
100
101 return num_paths;
102 }
103
104 /* clang-format off */
105
106 #define ACCESS_FILE ( \
107 LANDLOCK_ACCESS_FS_EXECUTE | \
108 LANDLOCK_ACCESS_FS_WRITE_FILE | \
109 LANDLOCK_ACCESS_FS_READ_FILE | \
110 LANDLOCK_ACCESS_FS_TRUNCATE | \
111 LANDLOCK_ACCESS_FS_IOCTL_DEV)
112
113 /* clang-format on */
114
populate_ruleset_fs(const char * const env_var,const int ruleset_fd,const __u64 allowed_access)115 static int populate_ruleset_fs(const char *const env_var, const int ruleset_fd,
116 const __u64 allowed_access)
117 {
118 int num_paths, i, ret = 1;
119 char *env_path_name;
120 const char **path_list = NULL;
121 struct landlock_path_beneath_attr path_beneath = {
122 .parent_fd = -1,
123 };
124
125 env_path_name = getenv(env_var);
126 if (!env_path_name) {
127 /* Prevents users to forget a setting. */
128 fprintf(stderr, "Missing environment variable %s\n", env_var);
129 return 1;
130 }
131 env_path_name = strdup(env_path_name);
132 unsetenv(env_var);
133 num_paths = parse_path(env_path_name, &path_list);
134 if (num_paths < 0) {
135 fprintf(stderr, "Failed to allocate memory\n");
136 goto out_free_name;
137 }
138 if (num_paths == 1 && path_list[0][0] == '\0') {
139 /*
140 * Allows to not use all possible restrictions (e.g. use
141 * LL_FS_RO without LL_FS_RW).
142 */
143 ret = 0;
144 goto out_free_name;
145 }
146
147 for (i = 0; i < num_paths; i++) {
148 struct stat statbuf;
149
150 path_beneath.parent_fd = open(path_list[i], O_PATH | O_CLOEXEC);
151 if (path_beneath.parent_fd < 0) {
152 fprintf(stderr, "Failed to open \"%s\": %s\n",
153 path_list[i], strerror(errno));
154 continue;
155 }
156 if (fstat(path_beneath.parent_fd, &statbuf)) {
157 fprintf(stderr, "Failed to stat \"%s\": %s\n",
158 path_list[i], strerror(errno));
159 close(path_beneath.parent_fd);
160 goto out_free_name;
161 }
162 path_beneath.allowed_access = allowed_access;
163 if (!S_ISDIR(statbuf.st_mode))
164 path_beneath.allowed_access &= ACCESS_FILE;
165 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
166 &path_beneath, 0)) {
167 fprintf(stderr,
168 "Failed to update the ruleset with \"%s\": %s\n",
169 path_list[i], strerror(errno));
170 close(path_beneath.parent_fd);
171 goto out_free_name;
172 }
173 close(path_beneath.parent_fd);
174 }
175 ret = 0;
176
177 out_free_name:
178 free(path_list);
179 free(env_path_name);
180 return ret;
181 }
182
populate_ruleset_net(const char * const env_var,const int ruleset_fd,const __u64 allowed_access)183 static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
184 const __u64 allowed_access)
185 {
186 int ret = 1;
187 char *env_port_name, *env_port_name_next, *strport;
188 struct landlock_net_port_attr net_port = {
189 .allowed_access = allowed_access,
190 };
191
192 env_port_name = getenv(env_var);
193 if (!env_port_name)
194 return 0;
195 env_port_name = strdup(env_port_name);
196 unsetenv(env_var);
197
198 env_port_name_next = env_port_name;
199 while ((strport = strsep(&env_port_name_next, ENV_DELIMITER))) {
200 __u64 port;
201
202 if (strcmp(strport, "") == 0)
203 continue;
204
205 if (str2num(strport, &port)) {
206 fprintf(stderr, "Failed to parse port at \"%s\"\n",
207 strport);
208 goto out_free_name;
209 }
210 net_port.port = port;
211 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT,
212 &net_port, 0)) {
213 fprintf(stderr,
214 "Failed to update the ruleset with port \"%llu\": %s\n",
215 net_port.port, strerror(errno));
216 goto out_free_name;
217 }
218 }
219 ret = 0;
220
221 out_free_name:
222 free(env_port_name);
223 return ret;
224 }
225
226 /* Returns true on error, false otherwise. */
check_ruleset_scope(const char * const env_var,struct landlock_ruleset_attr * ruleset_attr)227 static bool check_ruleset_scope(const char *const env_var,
228 struct landlock_ruleset_attr *ruleset_attr)
229 {
230 char *env_type_scope, *env_type_scope_next, *ipc_scoping_name;
231 bool error = false;
232 bool abstract_scoping = false;
233 bool signal_scoping = false;
234
235 /* Scoping is not supported by Landlock ABI */
236 if (!(ruleset_attr->scoped &
237 (LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET | LANDLOCK_SCOPE_SIGNAL)))
238 goto out_unset;
239
240 env_type_scope = getenv(env_var);
241 /* Scoping is not supported by the user */
242 if (!env_type_scope || strcmp("", env_type_scope) == 0)
243 goto out_unset;
244
245 env_type_scope = strdup(env_type_scope);
246 env_type_scope_next = env_type_scope;
247 while ((ipc_scoping_name =
248 strsep(&env_type_scope_next, ENV_DELIMITER))) {
249 if (strcmp("a", ipc_scoping_name) == 0 && !abstract_scoping) {
250 abstract_scoping = true;
251 } else if (strcmp("s", ipc_scoping_name) == 0 &&
252 !signal_scoping) {
253 signal_scoping = true;
254 } else {
255 fprintf(stderr, "Unknown or duplicate scope \"%s\"\n",
256 ipc_scoping_name);
257 error = true;
258 goto out_free_name;
259 }
260 }
261
262 out_free_name:
263 free(env_type_scope);
264
265 out_unset:
266 if (!abstract_scoping)
267 ruleset_attr->scoped &= ~LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET;
268 if (!signal_scoping)
269 ruleset_attr->scoped &= ~LANDLOCK_SCOPE_SIGNAL;
270
271 unsetenv(env_var);
272 return error;
273 }
274
275 /* clang-format off */
276
277 #define ACCESS_FS_ROUGHLY_READ ( \
278 LANDLOCK_ACCESS_FS_EXECUTE | \
279 LANDLOCK_ACCESS_FS_READ_FILE | \
280 LANDLOCK_ACCESS_FS_READ_DIR)
281
282 #define ACCESS_FS_ROUGHLY_WRITE ( \
283 LANDLOCK_ACCESS_FS_WRITE_FILE | \
284 LANDLOCK_ACCESS_FS_REMOVE_DIR | \
285 LANDLOCK_ACCESS_FS_REMOVE_FILE | \
286 LANDLOCK_ACCESS_FS_MAKE_CHAR | \
287 LANDLOCK_ACCESS_FS_MAKE_DIR | \
288 LANDLOCK_ACCESS_FS_MAKE_REG | \
289 LANDLOCK_ACCESS_FS_MAKE_SOCK | \
290 LANDLOCK_ACCESS_FS_MAKE_FIFO | \
291 LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
292 LANDLOCK_ACCESS_FS_MAKE_SYM | \
293 LANDLOCK_ACCESS_FS_REFER | \
294 LANDLOCK_ACCESS_FS_TRUNCATE | \
295 LANDLOCK_ACCESS_FS_IOCTL_DEV)
296
297 /* clang-format on */
298
299 #define LANDLOCK_ABI_LAST 7
300
301 #define XSTR(s) #s
302 #define STR(s) XSTR(s)
303
304 /* clang-format off */
305
306 static const char help[] =
307 "usage: " ENV_FS_RO_NAME "=\"...\" " ENV_FS_RW_NAME "=\"...\" "
308 "[other environment variables] %1$s <cmd> [args]...\n"
309 "\n"
310 "Execute the given command in a restricted environment.\n"
311 "Multi-valued settings (lists of ports, paths, scopes) are colon-delimited.\n"
312 "\n"
313 "Mandatory settings:\n"
314 "* " ENV_FS_RO_NAME ": paths allowed to be used in a read-only way\n"
315 "* " ENV_FS_RW_NAME ": paths allowed to be used in a read-write way\n"
316 "\n"
317 "Optional settings (when not set, their associated access check "
318 "is always allowed, which is different from an empty string which "
319 "means an empty list):\n"
320 "* " ENV_TCP_BIND_NAME ": ports allowed to bind (server)\n"
321 "* " ENV_TCP_CONNECT_NAME ": ports allowed to connect (client)\n"
322 "* " ENV_SCOPED_NAME ": actions denied on the outside of the landlock domain\n"
323 " - \"a\" to restrict opening abstract unix sockets\n"
324 " - \"s\" to restrict sending signals\n"
325 "\n"
326 "A sandboxer should not log denied access requests to avoid spamming logs, "
327 "but to test audit we can set " ENV_FORCE_LOG_NAME "=1\n"
328 "\n"
329 "Example:\n"
330 ENV_FS_RO_NAME "=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
331 ENV_FS_RW_NAME "=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
332 ENV_TCP_BIND_NAME "=\"9418\" "
333 ENV_TCP_CONNECT_NAME "=\"80:443\" "
334 ENV_SCOPED_NAME "=\"a:s\" "
335 "%1$s bash -i\n"
336 "\n"
337 "This sandboxer can use Landlock features up to ABI version "
338 STR(LANDLOCK_ABI_LAST) ".\n";
339
340 /* clang-format on */
341
main(const int argc,char * const argv[],char * const * const envp)342 int main(const int argc, char *const argv[], char *const *const envp)
343 {
344 const char *cmd_path;
345 char *const *cmd_argv;
346 int ruleset_fd, abi;
347 char *env_port_name, *env_force_log;
348 __u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
349 access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
350
351 struct landlock_ruleset_attr ruleset_attr = {
352 .handled_access_fs = access_fs_rw,
353 .handled_access_net = LANDLOCK_ACCESS_NET_BIND_TCP |
354 LANDLOCK_ACCESS_NET_CONNECT_TCP,
355 .scoped = LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
356 LANDLOCK_SCOPE_SIGNAL,
357 };
358 int supported_restrict_flags = LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
359 int set_restrict_flags = 0;
360
361 if (argc < 2) {
362 fprintf(stderr, help, argv[0]);
363 return 1;
364 }
365
366 abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
367 if (abi < 0) {
368 const int err = errno;
369
370 perror("Failed to check Landlock compatibility");
371 switch (err) {
372 case ENOSYS:
373 fprintf(stderr,
374 "Hint: Landlock is not supported by the current kernel. "
375 "To support it, build the kernel with "
376 "CONFIG_SECURITY_LANDLOCK=y and prepend "
377 "\"landlock,\" to the content of CONFIG_LSM.\n");
378 break;
379 case EOPNOTSUPP:
380 fprintf(stderr,
381 "Hint: Landlock is currently disabled. "
382 "It can be enabled in the kernel configuration by "
383 "prepending \"landlock,\" to the content of CONFIG_LSM, "
384 "or at boot time by setting the same content to the "
385 "\"lsm\" kernel parameter.\n");
386 break;
387 }
388 return 1;
389 }
390
391 /* Best-effort security. */
392 switch (abi) {
393 case 1:
394 /*
395 * Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
396 *
397 * Note: The "refer" operations (file renaming and linking
398 * across different directories) are always forbidden when using
399 * Landlock with ABI 1.
400 *
401 * If only ABI 1 is available, this sandboxer knowingly forbids
402 * refer operations.
403 *
404 * If a program *needs* to do refer operations after enabling
405 * Landlock, it can not use Landlock at ABI level 1. To be
406 * compatible with different kernel versions, such programs
407 * should then fall back to not restrict themselves at all if
408 * the running kernel only supports ABI 1.
409 */
410 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
411 __attribute__((fallthrough));
412 case 2:
413 /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
414 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
415 __attribute__((fallthrough));
416 case 3:
417 /* Removes network support for ABI < 4 */
418 ruleset_attr.handled_access_net &=
419 ~(LANDLOCK_ACCESS_NET_BIND_TCP |
420 LANDLOCK_ACCESS_NET_CONNECT_TCP);
421 __attribute__((fallthrough));
422 case 4:
423 /* Removes LANDLOCK_ACCESS_FS_IOCTL_DEV for ABI < 5 */
424 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_IOCTL_DEV;
425
426 __attribute__((fallthrough));
427 case 5:
428 /* Removes LANDLOCK_SCOPE_* for ABI < 6 */
429 ruleset_attr.scoped &= ~(LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
430 LANDLOCK_SCOPE_SIGNAL);
431 __attribute__((fallthrough));
432 case 6:
433 /* Removes LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON for ABI < 7 */
434 supported_restrict_flags &=
435 ~LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
436
437 /* Must be printed for any ABI < LANDLOCK_ABI_LAST. */
438 fprintf(stderr,
439 "Hint: You should update the running kernel "
440 "to leverage Landlock features "
441 "provided by ABI version %d (instead of %d).\n",
442 LANDLOCK_ABI_LAST, abi);
443 __attribute__((fallthrough));
444 case LANDLOCK_ABI_LAST:
445 break;
446 default:
447 fprintf(stderr,
448 "Hint: You should update this sandboxer "
449 "to leverage Landlock features "
450 "provided by ABI version %d (instead of %d).\n",
451 abi, LANDLOCK_ABI_LAST);
452 }
453 access_fs_ro &= ruleset_attr.handled_access_fs;
454 access_fs_rw &= ruleset_attr.handled_access_fs;
455
456 /* Removes bind access attribute if not supported by a user. */
457 env_port_name = getenv(ENV_TCP_BIND_NAME);
458 if (!env_port_name) {
459 ruleset_attr.handled_access_net &=
460 ~LANDLOCK_ACCESS_NET_BIND_TCP;
461 }
462 /* Removes connect access attribute if not supported by a user. */
463 env_port_name = getenv(ENV_TCP_CONNECT_NAME);
464 if (!env_port_name) {
465 ruleset_attr.handled_access_net &=
466 ~LANDLOCK_ACCESS_NET_CONNECT_TCP;
467 }
468
469 if (check_ruleset_scope(ENV_SCOPED_NAME, &ruleset_attr))
470 return 1;
471
472 /* Enables optional logs. */
473 env_force_log = getenv(ENV_FORCE_LOG_NAME);
474 if (env_force_log) {
475 if (strcmp(env_force_log, "1") != 0) {
476 fprintf(stderr, "Unknown value for " ENV_FORCE_LOG_NAME
477 " (only \"1\" is handled)\n");
478 return 1;
479 }
480 if (!(supported_restrict_flags &
481 LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON)) {
482 fprintf(stderr,
483 "Audit logs not supported by current kernel\n");
484 return 1;
485 }
486 set_restrict_flags |= LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
487 unsetenv(ENV_FORCE_LOG_NAME);
488 }
489
490 ruleset_fd =
491 landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
492 if (ruleset_fd < 0) {
493 perror("Failed to create a ruleset");
494 return 1;
495 }
496
497 if (populate_ruleset_fs(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
498 goto err_close_ruleset;
499 }
500 if (populate_ruleset_fs(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
501 goto err_close_ruleset;
502 }
503
504 if (populate_ruleset_net(ENV_TCP_BIND_NAME, ruleset_fd,
505 LANDLOCK_ACCESS_NET_BIND_TCP)) {
506 goto err_close_ruleset;
507 }
508 if (populate_ruleset_net(ENV_TCP_CONNECT_NAME, ruleset_fd,
509 LANDLOCK_ACCESS_NET_CONNECT_TCP)) {
510 goto err_close_ruleset;
511 }
512
513 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
514 perror("Failed to restrict privileges");
515 goto err_close_ruleset;
516 }
517 if (landlock_restrict_self(ruleset_fd, set_restrict_flags)) {
518 perror("Failed to enforce ruleset");
519 goto err_close_ruleset;
520 }
521 close(ruleset_fd);
522
523 cmd_path = argv[1];
524 cmd_argv = argv + 1;
525 fprintf(stderr, "Executing the sandboxed command...\n");
526 execvpe(cmd_path, cmd_argv, envp);
527 fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
528 strerror(errno));
529 fprintf(stderr, "Hint: access to the binary, the interpreter or "
530 "shared libraries may be denied.\n");
531 return 1;
532
533 err_close_ruleset:
534 close(ruleset_fd);
535 return 1;
536 }
537