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1// SPDX-License-Identifier: BSD-3-Clause
2/*
3 * Simple Landlock sandbox manager able to launch a process restricted by a
4 * user-defined filesystem access control policy.
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 goto out_free_name;
124 }
125 if (fstat(path_beneath.parent_fd, &statbuf)) {
126 close(path_beneath.parent_fd);
127 goto out_free_name;
128 }
129 path_beneath.allowed_access = allowed_access;
130 if (!S_ISDIR(statbuf.st_mode))
131 path_beneath.allowed_access &= ACCESS_FILE;
132 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
133 &path_beneath, 0)) {
134 fprintf(stderr,
135 "Failed to update the ruleset with \"%s\": %s\n",
136 path_list[i], strerror(errno));
137 close(path_beneath.parent_fd);
138 goto out_free_name;
139 }
140 close(path_beneath.parent_fd);
141 }
142 ret = 0;
143
144out_free_name:
145 free(path_list);
146 free(env_path_name);
147 return ret;
148}
149
150static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
151 const __u64 allowed_access)
152{
153 int ret = 1;
154 char *env_port_name, *strport;
155 struct landlock_net_port_attr net_port = {
156 .allowed_access = allowed_access,
157 .port = 0,
158 };
159
160 env_port_name = getenv(env_var);
161 if (!env_port_name)
162 return 0;
163 env_port_name = strdup(env_port_name);
164 unsetenv(env_var);
165
166 while ((strport = strsep(&env_port_name, ENV_DELIMITER))) {
167 net_port.port = atoi(strport);
168 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT,
169 &net_port, 0)) {
170 fprintf(stderr,
171 "Failed to update the ruleset with port \"%llu\": %s\n",
172 net_port.port, strerror(errno));
173 goto out_free_name;
174 }
175 }
176 ret = 0;
177
178out_free_name:
179 free(env_port_name);
180 return ret;
181}
182
183/* clang-format off */
184
185#define ACCESS_FS_ROUGHLY_READ ( \
186 LANDLOCK_ACCESS_FS_EXECUTE | \
187 LANDLOCK_ACCESS_FS_READ_FILE | \
188 LANDLOCK_ACCESS_FS_READ_DIR)
189
190#define ACCESS_FS_ROUGHLY_WRITE ( \
191 LANDLOCK_ACCESS_FS_WRITE_FILE | \
192 LANDLOCK_ACCESS_FS_REMOVE_DIR | \
193 LANDLOCK_ACCESS_FS_REMOVE_FILE | \
194 LANDLOCK_ACCESS_FS_MAKE_CHAR | \
195 LANDLOCK_ACCESS_FS_MAKE_DIR | \
196 LANDLOCK_ACCESS_FS_MAKE_REG | \
197 LANDLOCK_ACCESS_FS_MAKE_SOCK | \
198 LANDLOCK_ACCESS_FS_MAKE_FIFO | \
199 LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
200 LANDLOCK_ACCESS_FS_MAKE_SYM | \
201 LANDLOCK_ACCESS_FS_REFER | \
202 LANDLOCK_ACCESS_FS_TRUNCATE)
203
204/* clang-format on */
205
206#define LANDLOCK_ABI_LAST 4
207
208int main(const int argc, char *const argv[], char *const *const envp)
209{
210 const char *cmd_path;
211 char *const *cmd_argv;
212 int ruleset_fd, abi;
213 char *env_port_name;
214 __u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
215 access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
216
217 struct landlock_ruleset_attr ruleset_attr = {
218 .handled_access_fs = access_fs_rw,
219 .handled_access_net = LANDLOCK_ACCESS_NET_BIND_TCP |
220 LANDLOCK_ACCESS_NET_CONNECT_TCP,
221 };
222
223 if (argc < 2) {
224 fprintf(stderr,
225 "usage: %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\"%s "
226 "<cmd> [args]...\n\n",
227 ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
228 ENV_TCP_CONNECT_NAME, argv[0]);
229 fprintf(stderr,
230 "Launch a command in a restricted environment.\n\n");
231 fprintf(stderr,
232 "Environment variables containing paths and ports "
233 "each separated by a colon:\n");
234 fprintf(stderr,
235 "* %s: list of paths allowed to be used in a read-only way.\n",
236 ENV_FS_RO_NAME);
237 fprintf(stderr,
238 "* %s: list of paths allowed to be used in a read-write way.\n\n",
239 ENV_FS_RW_NAME);
240 fprintf(stderr,
241 "Environment variables containing ports are optional "
242 "and could be skipped.\n");
243 fprintf(stderr,
244 "* %s: list of ports allowed to bind (server).\n",
245 ENV_TCP_BIND_NAME);
246 fprintf(stderr,
247 "* %s: list of ports allowed to connect (client).\n",
248 ENV_TCP_CONNECT_NAME);
249 fprintf(stderr,
250 "\nexample:\n"
251 "%s=\"/bin:/lib:/usr:/proc:/etc:/dev/urandom\" "
252 "%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
253 "%s=\"9418\" "
254 "%s=\"80:443\" "
255 "%s bash -i\n\n",
256 ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
257 ENV_TCP_CONNECT_NAME, argv[0]);
258 fprintf(stderr,
259 "This sandboxer can use Landlock features "
260 "up to ABI version %d.\n",
261 LANDLOCK_ABI_LAST);
262 return 1;
263 }
264
265 abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
266 if (abi < 0) {
267 const int err = errno;
268
269 perror("Failed to check Landlock compatibility");
270 switch (err) {
271 case ENOSYS:
272 fprintf(stderr,
273 "Hint: Landlock is not supported by the current kernel. "
274 "To support it, build the kernel with "
275 "CONFIG_SECURITY_LANDLOCK=y and prepend "
276 "\"landlock,\" to the content of CONFIG_LSM.\n");
277 break;
278 case EOPNOTSUPP:
279 fprintf(stderr,
280 "Hint: Landlock is currently disabled. "
281 "It can be enabled in the kernel configuration by "
282 "prepending \"landlock,\" to the content of CONFIG_LSM, "
283 "or at boot time by setting the same content to the "
284 "\"lsm\" kernel parameter.\n");
285 break;
286 }
287 return 1;
288 }
289
290 /* Best-effort security. */
291 switch (abi) {
292 case 1:
293 /*
294 * Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
295 *
296 * Note: The "refer" operations (file renaming and linking
297 * across different directories) are always forbidden when using
298 * Landlock with ABI 1.
299 *
300 * If only ABI 1 is available, this sandboxer knowingly forbids
301 * refer operations.
302 *
303 * If a program *needs* to do refer operations after enabling
304 * Landlock, it can not use Landlock at ABI level 1. To be
305 * compatible with different kernel versions, such programs
306 * should then fall back to not restrict themselves at all if
307 * the running kernel only supports ABI 1.
308 */
309 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
310 __attribute__((fallthrough));
311 case 2:
312 /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
313 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
314 __attribute__((fallthrough));
315 case 3:
316 /* Removes network support for ABI < 4 */
317 ruleset_attr.handled_access_net &=
318 ~(LANDLOCK_ACCESS_NET_BIND_TCP |
319 LANDLOCK_ACCESS_NET_CONNECT_TCP);
320 fprintf(stderr,
321 "Hint: You should update the running kernel "
322 "to leverage Landlock features "
323 "provided by ABI version %d (instead of %d).\n",
324 LANDLOCK_ABI_LAST, abi);
325 __attribute__((fallthrough));
326 case LANDLOCK_ABI_LAST:
327 break;
328 default:
329 fprintf(stderr,
330 "Hint: You should update this sandboxer "
331 "to leverage Landlock features "
332 "provided by ABI version %d (instead of %d).\n",
333 abi, LANDLOCK_ABI_LAST);
334 }
335 access_fs_ro &= ruleset_attr.handled_access_fs;
336 access_fs_rw &= ruleset_attr.handled_access_fs;
337
338 /* Removes bind access attribute if not supported by a user. */
339 env_port_name = getenv(ENV_TCP_BIND_NAME);
340 if (!env_port_name) {
341 ruleset_attr.handled_access_net &=
342 ~LANDLOCK_ACCESS_NET_BIND_TCP;
343 }
344 /* Removes connect access attribute if not supported by a user. */
345 env_port_name = getenv(ENV_TCP_CONNECT_NAME);
346 if (!env_port_name) {
347 ruleset_attr.handled_access_net &=
348 ~LANDLOCK_ACCESS_NET_CONNECT_TCP;
349 }
350
351 ruleset_fd =
352 landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
353 if (ruleset_fd < 0) {
354 perror("Failed to create a ruleset");
355 return 1;
356 }
357
358 if (populate_ruleset_fs(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
359 goto err_close_ruleset;
360 }
361 if (populate_ruleset_fs(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
362 goto err_close_ruleset;
363 }
364
365 if (populate_ruleset_net(ENV_TCP_BIND_NAME, ruleset_fd,
366 LANDLOCK_ACCESS_NET_BIND_TCP)) {
367 goto err_close_ruleset;
368 }
369 if (populate_ruleset_net(ENV_TCP_CONNECT_NAME, ruleset_fd,
370 LANDLOCK_ACCESS_NET_CONNECT_TCP)) {
371 goto err_close_ruleset;
372 }
373
374 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
375 perror("Failed to restrict privileges");
376 goto err_close_ruleset;
377 }
378 if (landlock_restrict_self(ruleset_fd, 0)) {
379 perror("Failed to enforce ruleset");
380 goto err_close_ruleset;
381 }
382 close(ruleset_fd);
383
384 cmd_path = argv[1];
385 cmd_argv = argv + 1;
386 execvpe(cmd_path, cmd_argv, envp);
387 fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
388 strerror(errno));
389 fprintf(stderr, "Hint: access to the binary, the interpreter or "
390 "shared libraries may be denied.\n");
391 return 1;
392
393err_close_ruleset:
394 close(ruleset_fd);
395 return 1;
396}