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