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 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 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 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 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 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 3 177 178 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