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
25 static inline int
landlock_create_ruleset(const struct landlock_ruleset_attr * const attr,const size_t size,const __u32 flags)26 landlock_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
landlock_add_rule(const int ruleset_fd,const enum landlock_rule_type rule_type,const void * const rule_attr,const __u32 flags)34 static 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
landlock_restrict_self(const int ruleset_fd,const __u32 flags)45 static 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
parse_path(char * env_path,const char *** const path_list)56 static 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 if (!*path_list)
69 return -1;
70
71 for (i = 0; i < num_paths; i++)
72 (*path_list)[i] = strsep(&env_path, ENV_PATH_TOKEN);
73
74 return num_paths;
75 }
76
77 /* clang-format off */
78
79 #define ACCESS_FILE ( \
80 LANDLOCK_ACCESS_FS_EXECUTE | \
81 LANDLOCK_ACCESS_FS_WRITE_FILE | \
82 LANDLOCK_ACCESS_FS_READ_FILE | \
83 LANDLOCK_ACCESS_FS_TRUNCATE)
84
85 /* clang-format on */
86
populate_ruleset(const char * const env_var,const int ruleset_fd,const __u64 allowed_access)87 static int populate_ruleset(const char *const env_var, const int ruleset_fd,
88 const __u64 allowed_access)
89 {
90 int num_paths, i, ret = 1;
91 char *env_path_name;
92 const char **path_list = NULL;
93 struct landlock_path_beneath_attr path_beneath = {
94 .parent_fd = -1,
95 };
96
97 env_path_name = getenv(env_var);
98 if (!env_path_name) {
99 /* Prevents users to forget a setting. */
100 fprintf(stderr, "Missing environment variable %s\n", env_var);
101 return 1;
102 }
103 env_path_name = strdup(env_path_name);
104 unsetenv(env_var);
105 num_paths = parse_path(env_path_name, &path_list);
106 if (num_paths < 0) {
107 fprintf(stderr, "Failed to allocate memory\n");
108 goto out_free_name;
109 }
110 if (num_paths == 1 && path_list[0][0] == '\0') {
111 /*
112 * Allows to not use all possible restrictions (e.g. use
113 * LL_FS_RO without LL_FS_RW).
114 */
115 ret = 0;
116 goto out_free_name;
117 }
118
119 for (i = 0; i < num_paths; i++) {
120 struct stat statbuf;
121
122 path_beneath.parent_fd = open(path_list[i], O_PATH | O_CLOEXEC);
123 if (path_beneath.parent_fd < 0) {
124 fprintf(stderr, "Failed to open \"%s\": %s\n",
125 path_list[i], strerror(errno));
126 goto out_free_name;
127 }
128 if (fstat(path_beneath.parent_fd, &statbuf)) {
129 close(path_beneath.parent_fd);
130 goto out_free_name;
131 }
132 path_beneath.allowed_access = allowed_access;
133 if (!S_ISDIR(statbuf.st_mode))
134 path_beneath.allowed_access &= ACCESS_FILE;
135 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
136 &path_beneath, 0)) {
137 fprintf(stderr,
138 "Failed to update the ruleset with \"%s\": %s\n",
139 path_list[i], strerror(errno));
140 close(path_beneath.parent_fd);
141 goto out_free_name;
142 }
143 close(path_beneath.parent_fd);
144 }
145 ret = 0;
146
147 out_free_name:
148 free(path_list);
149 free(env_path_name);
150 return ret;
151 }
152
153 /* clang-format off */
154
155 #define ACCESS_FS_ROUGHLY_READ ( \
156 LANDLOCK_ACCESS_FS_EXECUTE | \
157 LANDLOCK_ACCESS_FS_READ_FILE | \
158 LANDLOCK_ACCESS_FS_READ_DIR)
159
160 #define ACCESS_FS_ROUGHLY_WRITE ( \
161 LANDLOCK_ACCESS_FS_WRITE_FILE | \
162 LANDLOCK_ACCESS_FS_REMOVE_DIR | \
163 LANDLOCK_ACCESS_FS_REMOVE_FILE | \
164 LANDLOCK_ACCESS_FS_MAKE_CHAR | \
165 LANDLOCK_ACCESS_FS_MAKE_DIR | \
166 LANDLOCK_ACCESS_FS_MAKE_REG | \
167 LANDLOCK_ACCESS_FS_MAKE_SOCK | \
168 LANDLOCK_ACCESS_FS_MAKE_FIFO | \
169 LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
170 LANDLOCK_ACCESS_FS_MAKE_SYM | \
171 LANDLOCK_ACCESS_FS_REFER | \
172 LANDLOCK_ACCESS_FS_TRUNCATE)
173
174 /* clang-format on */
175
176 #define LANDLOCK_ABI_LAST 3
177
main(const int argc,char * const argv[],char * const * const envp)178 int main(const int argc, char *const argv[], char *const *const envp)
179 {
180 const char *cmd_path;
181 char *const *cmd_argv;
182 int ruleset_fd, abi;
183 __u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
184 access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
185 struct landlock_ruleset_attr ruleset_attr = {
186 .handled_access_fs = access_fs_rw,
187 };
188
189 if (argc < 2) {
190 fprintf(stderr,
191 "usage: %s=\"...\" %s=\"...\" %s <cmd> [args]...\n\n",
192 ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
193 fprintf(stderr,
194 "Launch a command in a restricted environment.\n\n");
195 fprintf(stderr, "Environment variables containing paths, "
196 "each separated by a colon:\n");
197 fprintf(stderr,
198 "* %s: list of paths allowed to be used in a read-only way.\n",
199 ENV_FS_RO_NAME);
200 fprintf(stderr,
201 "* %s: list of paths allowed to be used in a read-write way.\n",
202 ENV_FS_RW_NAME);
203 fprintf(stderr,
204 "\nexample:\n"
205 "%s=\"/bin:/lib:/usr:/proc:/etc:/dev/urandom\" "
206 "%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
207 "%s bash -i\n\n",
208 ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
209 fprintf(stderr,
210 "This sandboxer can use Landlock features "
211 "up to ABI version %d.\n",
212 LANDLOCK_ABI_LAST);
213 return 1;
214 }
215
216 abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
217 if (abi < 0) {
218 const int err = errno;
219
220 perror("Failed to check Landlock compatibility");
221 switch (err) {
222 case ENOSYS:
223 fprintf(stderr,
224 "Hint: Landlock is not supported by the current kernel. "
225 "To support it, build the kernel with "
226 "CONFIG_SECURITY_LANDLOCK=y and prepend "
227 "\"landlock,\" to the content of CONFIG_LSM.\n");
228 break;
229 case EOPNOTSUPP:
230 fprintf(stderr,
231 "Hint: Landlock is currently disabled. "
232 "It can be enabled in the kernel configuration by "
233 "prepending \"landlock,\" to the content of CONFIG_LSM, "
234 "or at boot time by setting the same content to the "
235 "\"lsm\" kernel parameter.\n");
236 break;
237 }
238 return 1;
239 }
240
241 /* Best-effort security. */
242 switch (abi) {
243 case 1:
244 /*
245 * Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
246 *
247 * Note: The "refer" operations (file renaming and linking
248 * across different directories) are always forbidden when using
249 * Landlock with ABI 1.
250 *
251 * If only ABI 1 is available, this sandboxer knowingly forbids
252 * refer operations.
253 *
254 * If a program *needs* to do refer operations after enabling
255 * Landlock, it can not use Landlock at ABI level 1. To be
256 * compatible with different kernel versions, such programs
257 * should then fall back to not restrict themselves at all if
258 * the running kernel only supports ABI 1.
259 */
260 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
261 __attribute__((fallthrough));
262 case 2:
263 /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
264 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
265
266 fprintf(stderr,
267 "Hint: You should update the running kernel "
268 "to leverage Landlock features "
269 "provided by ABI version %d (instead of %d).\n",
270 LANDLOCK_ABI_LAST, abi);
271 __attribute__((fallthrough));
272 case LANDLOCK_ABI_LAST:
273 break;
274 default:
275 fprintf(stderr,
276 "Hint: You should update this sandboxer "
277 "to leverage Landlock features "
278 "provided by ABI version %d (instead of %d).\n",
279 abi, LANDLOCK_ABI_LAST);
280 }
281 access_fs_ro &= ruleset_attr.handled_access_fs;
282 access_fs_rw &= ruleset_attr.handled_access_fs;
283
284 ruleset_fd =
285 landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
286 if (ruleset_fd < 0) {
287 perror("Failed to create a ruleset");
288 return 1;
289 }
290 if (populate_ruleset(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
291 goto err_close_ruleset;
292 }
293 if (populate_ruleset(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
294 goto err_close_ruleset;
295 }
296 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
297 perror("Failed to restrict privileges");
298 goto err_close_ruleset;
299 }
300 if (landlock_restrict_self(ruleset_fd, 0)) {
301 perror("Failed to enforce ruleset");
302 goto err_close_ruleset;
303 }
304 close(ruleset_fd);
305
306 cmd_path = argv[1];
307 cmd_argv = argv + 1;
308 execvpe(cmd_path, cmd_argv, envp);
309 fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
310 strerror(errno));
311 fprintf(stderr, "Hint: access to the binary, the interpreter or "
312 "shared libraries may be denied.\n");
313 return 1;
314
315 err_close_ruleset:
316 close(ruleset_fd);
317 return 1;
318 }
319