samples/landlock/sandboxer.c

   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
  27 static inline int
  28 landlock_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
  36 static 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
  47 static 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
  60 static 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
  88 static 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
 144 out_free_name:
 145         free(path_list);
 146         free(env_path_name);
 147         return ret;
 148 }
 149
 150 static 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
 178 out_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
 208 int 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
 393 err_close_ruleset:
 394         close(ruleset_fd);
 395         return 1;
 396 }