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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#include <errno.h>
12#include <fcntl.h>
13#include <linux/landlock.h>
14#include <linux/prctl.h>
15#include <stddef.h>
16#include <stdio.h>
17#include <stdlib.h>
18#include <string.h>
19#include <sys/prctl.h>
20#include <sys/stat.h>
21#include <sys/syscall.h>
22#include <unistd.h>
23
24#ifndef landlock_create_ruleset
25static inline int
26landlock_create_ruleset(const struct landlock_ruleset_attr *const attr,
27 const size_t size, const __u32 flags)
28{
29 return syscall(__NR_landlock_create_ruleset, attr, size, flags);
30}
31#endif
32
33#ifndef landlock_add_rule
34static inline int landlock_add_rule(const int ruleset_fd,
35 const enum landlock_rule_type rule_type,
36 const void *const rule_attr,
37 const __u32 flags)
38{
39 return syscall(__NR_landlock_add_rule, ruleset_fd, rule_type, rule_attr,
40 flags);
41}
42#endif
43
44#ifndef landlock_restrict_self
45static inline int landlock_restrict_self(const int ruleset_fd,
46 const __u32 flags)
47{
48 return syscall(__NR_landlock_restrict_self, ruleset_fd, flags);
49}
50#endif
51
52#define ENV_FS_RO_NAME "LL_FS_RO"
53#define ENV_FS_RW_NAME "LL_FS_RW"
54#define ENV_PATH_TOKEN ":"
55
56static int parse_path(char *env_path, const char ***const path_list)
57{
58 int i, num_paths = 0;
59
60 if (env_path) {
61 num_paths++;
62 for (i = 0; env_path[i]; i++) {
63 if (env_path[i] == ENV_PATH_TOKEN[0])
64 num_paths++;
65 }
66 }
67 *path_list = malloc(num_paths * sizeof(**path_list));
68 for (i = 0; i < num_paths; i++)
69 (*path_list)[i] = strsep(&env_path, ENV_PATH_TOKEN);
70
71 return num_paths;
72}
73
74/* clang-format off */
75
76#define ACCESS_FILE ( \
77 LANDLOCK_ACCESS_FS_EXECUTE | \
78 LANDLOCK_ACCESS_FS_WRITE_FILE | \
79 LANDLOCK_ACCESS_FS_READ_FILE | \
80 LANDLOCK_ACCESS_FS_TRUNCATE)
81
82/* clang-format on */
83
84static int populate_ruleset(const char *const env_var, const int ruleset_fd,
85 const __u64 allowed_access)
86{
87 int num_paths, i, ret = 1;
88 char *env_path_name;
89 const char **path_list = NULL;
90 struct landlock_path_beneath_attr path_beneath = {
91 .parent_fd = -1,
92 };
93
94 env_path_name = getenv(env_var);
95 if (!env_path_name) {
96 /* Prevents users to forget a setting. */
97 fprintf(stderr, "Missing environment variable %s\n", env_var);
98 return 1;
99 }
100 env_path_name = strdup(env_path_name);
101 unsetenv(env_var);
102 num_paths = parse_path(env_path_name, &path_list);
103 if (num_paths == 1 && path_list[0][0] == '\0') {
104 /*
105 * Allows to not use all possible restrictions (e.g. use
106 * LL_FS_RO without LL_FS_RW).
107 */
108 ret = 0;
109 goto out_free_name;
110 }
111
112 for (i = 0; i < num_paths; i++) {
113 struct stat statbuf;
114
115 path_beneath.parent_fd = open(path_list[i], O_PATH | O_CLOEXEC);
116 if (path_beneath.parent_fd < 0) {
117 fprintf(stderr, "Failed to open \"%s\": %s\n",
118 path_list[i], strerror(errno));
119 goto out_free_name;
120 }
121 if (fstat(path_beneath.parent_fd, &statbuf)) {
122 close(path_beneath.parent_fd);
123 goto out_free_name;
124 }
125 path_beneath.allowed_access = allowed_access;
126 if (!S_ISDIR(statbuf.st_mode))
127 path_beneath.allowed_access &= ACCESS_FILE;
128 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
129 &path_beneath, 0)) {
130 fprintf(stderr,
131 "Failed to update the ruleset with \"%s\": %s\n",
132 path_list[i], strerror(errno));
133 close(path_beneath.parent_fd);
134 goto out_free_name;
135 }
136 close(path_beneath.parent_fd);
137 }
138 ret = 0;
139
140out_free_name:
141 free(path_list);
142 free(env_path_name);
143 return ret;
144}
145
146/* clang-format off */
147
148#define ACCESS_FS_ROUGHLY_READ ( \
149 LANDLOCK_ACCESS_FS_EXECUTE | \
150 LANDLOCK_ACCESS_FS_READ_FILE | \
151 LANDLOCK_ACCESS_FS_READ_DIR)
152
153#define ACCESS_FS_ROUGHLY_WRITE ( \
154 LANDLOCK_ACCESS_FS_WRITE_FILE | \
155 LANDLOCK_ACCESS_FS_REMOVE_DIR | \
156 LANDLOCK_ACCESS_FS_REMOVE_FILE | \
157 LANDLOCK_ACCESS_FS_MAKE_CHAR | \
158 LANDLOCK_ACCESS_FS_MAKE_DIR | \
159 LANDLOCK_ACCESS_FS_MAKE_REG | \
160 LANDLOCK_ACCESS_FS_MAKE_SOCK | \
161 LANDLOCK_ACCESS_FS_MAKE_FIFO | \
162 LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
163 LANDLOCK_ACCESS_FS_MAKE_SYM | \
164 LANDLOCK_ACCESS_FS_REFER | \
165 LANDLOCK_ACCESS_FS_TRUNCATE)
166
167/* clang-format on */
168
169#define LANDLOCK_ABI_LAST 3
170
171int main(const int argc, char *const argv[], char *const *const envp)
172{
173 const char *cmd_path;
174 char *const *cmd_argv;
175 int ruleset_fd, abi;
176 __u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
177 access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
178 struct landlock_ruleset_attr ruleset_attr = {
179 .handled_access_fs = access_fs_rw,
180 };
181
182 if (argc < 2) {
183 fprintf(stderr,
184 "usage: %s=\"...\" %s=\"...\" %s <cmd> [args]...\n\n",
185 ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
186 fprintf(stderr,
187 "Launch a command in a restricted environment.\n\n");
188 fprintf(stderr, "Environment variables containing paths, "
189 "each separated by a colon:\n");
190 fprintf(stderr,
191 "* %s: list of paths allowed to be used in a read-only way.\n",
192 ENV_FS_RO_NAME);
193 fprintf(stderr,
194 "* %s: list of paths allowed to be used in a read-write way.\n",
195 ENV_FS_RW_NAME);
196 fprintf(stderr,
197 "\nexample:\n"
198 "%s=\"/bin:/lib:/usr:/proc:/etc:/dev/urandom\" "
199 "%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
200 "%s bash -i\n\n",
201 ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
202 fprintf(stderr,
203 "This sandboxer can use Landlock features "
204 "up to ABI version %d.\n",
205 LANDLOCK_ABI_LAST);
206 return 1;
207 }
208
209 abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
210 if (abi < 0) {
211 const int err = errno;
212
213 perror("Failed to check Landlock compatibility");
214 switch (err) {
215 case ENOSYS:
216 fprintf(stderr,
217 "Hint: Landlock is not supported by the current kernel. "
218 "To support it, build the kernel with "
219 "CONFIG_SECURITY_LANDLOCK=y and prepend "
220 "\"landlock,\" to the content of CONFIG_LSM.\n");
221 break;
222 case EOPNOTSUPP:
223 fprintf(stderr,
224 "Hint: Landlock is currently disabled. "
225 "It can be enabled in the kernel configuration by "
226 "prepending \"landlock,\" to the content of CONFIG_LSM, "
227 "or at boot time by setting the same content to the "
228 "\"lsm\" kernel parameter.\n");
229 break;
230 }
231 return 1;
232 }
233
234 /* Best-effort security. */
235 switch (abi) {
236 case 1:
237 /*
238 * Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
239 *
240 * Note: The "refer" operations (file renaming and linking
241 * across different directories) are always forbidden when using
242 * Landlock with ABI 1.
243 *
244 * If only ABI 1 is available, this sandboxer knowingly forbids
245 * refer operations.
246 *
247 * If a program *needs* to do refer operations after enabling
248 * Landlock, it can not use Landlock at ABI level 1. To be
249 * compatible with different kernel versions, such programs
250 * should then fall back to not restrict themselves at all if
251 * the running kernel only supports ABI 1.
252 */
253 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
254 __attribute__((fallthrough));
255 case 2:
256 /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
257 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
258
259 fprintf(stderr,
260 "Hint: You should update the running kernel "
261 "to leverage Landlock features "
262 "provided by ABI version %d (instead of %d).\n",
263 LANDLOCK_ABI_LAST, abi);
264 __attribute__((fallthrough));
265 case LANDLOCK_ABI_LAST:
266 break;
267 default:
268 fprintf(stderr,
269 "Hint: You should update this sandboxer "
270 "to leverage Landlock features "
271 "provided by ABI version %d (instead of %d).\n",
272 abi, LANDLOCK_ABI_LAST);
273 }
274 access_fs_ro &= ruleset_attr.handled_access_fs;
275 access_fs_rw &= ruleset_attr.handled_access_fs;
276
277 ruleset_fd =
278 landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
279 if (ruleset_fd < 0) {
280 perror("Failed to create a ruleset");
281 return 1;
282 }
283 if (populate_ruleset(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
284 goto err_close_ruleset;
285 }
286 if (populate_ruleset(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
287 goto err_close_ruleset;
288 }
289 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
290 perror("Failed to restrict privileges");
291 goto err_close_ruleset;
292 }
293 if (landlock_restrict_self(ruleset_fd, 0)) {
294 perror("Failed to enforce ruleset");
295 goto err_close_ruleset;
296 }
297 close(ruleset_fd);
298
299 cmd_path = argv[1];
300 cmd_argv = argv + 1;
301 execvpe(cmd_path, cmd_argv, envp);
302 fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
303 strerror(errno));
304 fprintf(stderr, "Hint: access to the binary, the interpreter or "
305 "shared libraries may be denied.\n");
306 return 1;
307
308err_close_ruleset:
309 close(ruleset_fd);
310 return 1;
311}