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