samples/landlock/sandboxer.c

   1 // SPDX-License-Identifier: BSD-3-Clause
   2 /*
   3  * Simple Landlock sandbox manager able to execute a process restricted by
   4  * user-defined file system and network access control policies.
   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 <linux/socket.h>
  18 #include <stddef.h>
  19 #include <stdio.h>
  20 #include <stdlib.h>
  21 #include <string.h>
  22 #include <sys/prctl.h>
  23 #include <sys/stat.h>
  24 #include <sys/syscall.h>
  25 #include <unistd.h>
  26 #include <stdbool.h>
  27
  28 #ifndef landlock_create_ruleset
  29 static inline int
  30 landlock_create_ruleset(const struct landlock_ruleset_attr *const attr,
  31                         const size_t size, const __u32 flags)
  32 {
  33         return syscall(__NR_landlock_create_ruleset, attr, size, flags);
  34 }
  35 #endif
  36
  37 #ifndef landlock_add_rule
  38 static inline int landlock_add_rule(const int ruleset_fd,
  39                                     const enum landlock_rule_type rule_type,
  40                                     const void *const rule_attr,
  41                                     const __u32 flags)
  42 {
  43         return syscall(__NR_landlock_add_rule, ruleset_fd, rule_type, rule_attr,
  44                        flags);
  45 }
  46 #endif
  47
  48 #ifndef landlock_restrict_self
  49 static inline int landlock_restrict_self(const int ruleset_fd,
  50                                          const __u32 flags)
  51 {
  52         return syscall(__NR_landlock_restrict_self, ruleset_fd, flags);
  53 }
  54 #endif
  55
  56 #define ENV_FS_RO_NAME "LL_FS_RO"
  57 #define ENV_FS_RW_NAME "LL_FS_RW"
  58 #define ENV_TCP_BIND_NAME "LL_TCP_BIND"
  59 #define ENV_TCP_CONNECT_NAME "LL_TCP_CONNECT"
  60 #define ENV_SCOPED_NAME "LL_SCOPED"
  61 #define ENV_DELIMITER ":"
  62
  63 static int parse_path(char *env_path, const char ***const path_list)
  64 {
  65         int i, num_paths = 0;
  66
  67         if (env_path) {
  68                 num_paths++;
  69                 for (i = 0; env_path[i]; i++) {
  70                         if (env_path[i] == ENV_DELIMITER[0])
  71                                 num_paths++;
  72                 }
  73         }
  74         *path_list = malloc(num_paths * sizeof(**path_list));
  75         for (i = 0; i < num_paths; i++)
  76                 (*path_list)[i] = strsep(&env_path, ENV_DELIMITER);
  77
  78         return num_paths;
  79 }
  80
  81 /* clang-format off */
  82
  83 #define ACCESS_FILE ( \
  84         LANDLOCK_ACCESS_FS_EXECUTE | \
  85         LANDLOCK_ACCESS_FS_WRITE_FILE | \
  86         LANDLOCK_ACCESS_FS_READ_FILE | \
  87         LANDLOCK_ACCESS_FS_TRUNCATE | \
  88         LANDLOCK_ACCESS_FS_IOCTL_DEV)
  89
  90 /* clang-format on */
  91
  92 static int populate_ruleset_fs(const char *const env_var, const int ruleset_fd,
  93                                const __u64 allowed_access)
  94 {
  95         int num_paths, i, ret = 1;
  96         char *env_path_name;
  97         const char **path_list = NULL;
  98         struct landlock_path_beneath_attr path_beneath = {
  99                 .parent_fd = -1,
 100         };
 101
 102         env_path_name = getenv(env_var);
 103         if (!env_path_name) {
 104                 /* Prevents users to forget a setting. */
 105                 fprintf(stderr, "Missing environment variable %s\n", env_var);
 106                 return 1;
 107         }
 108         env_path_name = strdup(env_path_name);
 109         unsetenv(env_var);
 110         num_paths = parse_path(env_path_name, &path_list);
 111         if (num_paths == 1 && path_list[0][0] == '\0') {
 112                 /*
 113                  * Allows to not use all possible restrictions (e.g. use
 114                  * LL_FS_RO without LL_FS_RW).
 115                  */
 116                 ret = 0;
 117                 goto out_free_name;
 118         }
 119
 120         for (i = 0; i < num_paths; i++) {
 121                 struct stat statbuf;
 122
 123                 path_beneath.parent_fd = open(path_list[i], O_PATH | O_CLOEXEC);
 124                 if (path_beneath.parent_fd < 0) {
 125                         fprintf(stderr, "Failed to open \"%s\": %s\n",
 126                                 path_list[i], strerror(errno));
 127                         continue;
 128                 }
 129                 if (fstat(path_beneath.parent_fd, &statbuf)) {
 130                         fprintf(stderr, "Failed to stat \"%s\": %s\n",
 131                                 path_list[i], strerror(errno));
 132                         close(path_beneath.parent_fd);
 133                         goto out_free_name;
 134                 }
 135                 path_beneath.allowed_access = allowed_access;
 136                 if (!S_ISDIR(statbuf.st_mode))
 137                         path_beneath.allowed_access &= ACCESS_FILE;
 138                 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
 139                                       &path_beneath, 0)) {
 140                         fprintf(stderr,
 141                                 "Failed to update the ruleset with \"%s\": %s\n",
 142                                 path_list[i], strerror(errno));
 143                         close(path_beneath.parent_fd);
 144                         goto out_free_name;
 145                 }
 146                 close(path_beneath.parent_fd);
 147         }
 148         ret = 0;
 149
 150 out_free_name:
 151         free(path_list);
 152         free(env_path_name);
 153         return ret;
 154 }
 155
 156 static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
 157                                 const __u64 allowed_access)
 158 {
 159         int ret = 1;
 160         char *env_port_name, *env_port_name_next, *strport;
 161         struct landlock_net_port_attr net_port = {
 162                 .allowed_access = allowed_access,
 163                 .port = 0,
 164         };
 165
 166         env_port_name = getenv(env_var);
 167         if (!env_port_name)
 168                 return 0;
 169         env_port_name = strdup(env_port_name);
 170         unsetenv(env_var);
 171
 172         env_port_name_next = env_port_name;
 173         while ((strport = strsep(&env_port_name_next, ENV_DELIMITER))) {
 174                 net_port.port = atoi(strport);
 175                 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT,
 176                                       &net_port, 0)) {
 177                         fprintf(stderr,
 178                                 "Failed to update the ruleset with port \"%llu\": %s\n",
 179                                 net_port.port, strerror(errno));
 180                         goto out_free_name;
 181                 }
 182         }
 183         ret = 0;
 184
 185 out_free_name:
 186         free(env_port_name);
 187         return ret;
 188 }
 189
 190 /* Returns true on error, false otherwise. */
 191 static bool check_ruleset_scope(const char *const env_var,
 192                                 struct landlock_ruleset_attr *ruleset_attr)
 193 {
 194         char *env_type_scope, *env_type_scope_next, *ipc_scoping_name;
 195         bool error = false;
 196         bool abstract_scoping = false;
 197         bool signal_scoping = false;
 198
 199         /* Scoping is not supported by Landlock ABI */
 200         if (!(ruleset_attr->scoped &
 201               (LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET | LANDLOCK_SCOPE_SIGNAL)))
 202                 goto out_unset;
 203
 204         env_type_scope = getenv(env_var);
 205         /* Scoping is not supported by the user */
 206         if (!env_type_scope || strcmp("", env_type_scope) == 0)
 207                 goto out_unset;
 208
 209         env_type_scope = strdup(env_type_scope);
 210         env_type_scope_next = env_type_scope;
 211         while ((ipc_scoping_name =
 212                         strsep(&env_type_scope_next, ENV_DELIMITER))) {
 213                 if (strcmp("a", ipc_scoping_name) == 0 && !abstract_scoping) {
 214                         abstract_scoping = true;
 215                 } else if (strcmp("s", ipc_scoping_name) == 0 &&
 216                            !signal_scoping) {
 217                         signal_scoping = true;
 218                 } else {
 219                         fprintf(stderr, "Unknown or duplicate scope \"%s\"\n",
 220                                 ipc_scoping_name);
 221                         error = true;
 222                         goto out_free_name;
 223                 }
 224         }
 225
 226 out_free_name:
 227         free(env_type_scope);
 228
 229 out_unset:
 230         if (!abstract_scoping)
 231                 ruleset_attr->scoped &= ~LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET;
 232         if (!signal_scoping)
 233                 ruleset_attr->scoped &= ~LANDLOCK_SCOPE_SIGNAL;
 234
 235         unsetenv(env_var);
 236         return error;
 237 }
 238
 239 /* clang-format off */
 240
 241 #define ACCESS_FS_ROUGHLY_READ ( \
 242         LANDLOCK_ACCESS_FS_EXECUTE | \
 243         LANDLOCK_ACCESS_FS_READ_FILE | \
 244         LANDLOCK_ACCESS_FS_READ_DIR)
 245
 246 #define ACCESS_FS_ROUGHLY_WRITE ( \
 247         LANDLOCK_ACCESS_FS_WRITE_FILE | \
 248         LANDLOCK_ACCESS_FS_REMOVE_DIR | \
 249         LANDLOCK_ACCESS_FS_REMOVE_FILE | \
 250         LANDLOCK_ACCESS_FS_MAKE_CHAR | \
 251         LANDLOCK_ACCESS_FS_MAKE_DIR | \
 252         LANDLOCK_ACCESS_FS_MAKE_REG | \
 253         LANDLOCK_ACCESS_FS_MAKE_SOCK | \
 254         LANDLOCK_ACCESS_FS_MAKE_FIFO | \
 255         LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
 256         LANDLOCK_ACCESS_FS_MAKE_SYM | \
 257         LANDLOCK_ACCESS_FS_REFER | \
 258         LANDLOCK_ACCESS_FS_TRUNCATE | \
 259         LANDLOCK_ACCESS_FS_IOCTL_DEV)
 260
 261 /* clang-format on */
 262
 263 #define LANDLOCK_ABI_LAST 6
 264
 265 int main(const int argc, char *const argv[], char *const *const envp)
 266 {
 267         const char *cmd_path;
 268         char *const *cmd_argv;
 269         int ruleset_fd, abi;
 270         char *env_port_name;
 271         __u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
 272               access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
 273
 274         struct landlock_ruleset_attr ruleset_attr = {
 275                 .handled_access_fs = access_fs_rw,
 276                 .handled_access_net = LANDLOCK_ACCESS_NET_BIND_TCP |
 277                                       LANDLOCK_ACCESS_NET_CONNECT_TCP,
 278                 .scoped = LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
 279                           LANDLOCK_SCOPE_SIGNAL,
 280         };
 281
 282         if (argc < 2) {
 283                 fprintf(stderr,
 284                         "usage: %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\" %s "
 285                         "<cmd> [args]...\n\n",
 286                         ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
 287                         ENV_TCP_CONNECT_NAME, ENV_SCOPED_NAME, argv[0]);
 288                 fprintf(stderr,
 289                         "Execute a command in a restricted environment.\n\n");
 290                 fprintf(stderr,
 291                         "Environment variables containing paths and ports "
 292                         "each separated by a colon:\n");
 293                 fprintf(stderr,
 294                         "* %s: list of paths allowed to be used in a read-only way.\n",
 295                         ENV_FS_RO_NAME);
 296                 fprintf(stderr,
 297                         "* %s: list of paths allowed to be used in a read-write way.\n\n",
 298                         ENV_FS_RW_NAME);
 299                 fprintf(stderr,
 300                         "Environment variables containing ports are optional "
 301                         "and could be skipped.\n");
 302                 fprintf(stderr,
 303                         "* %s: list of ports allowed to bind (server).\n",
 304                         ENV_TCP_BIND_NAME);
 305                 fprintf(stderr,
 306                         "* %s: list of ports allowed to connect (client).\n",
 307                         ENV_TCP_CONNECT_NAME);
 308                 fprintf(stderr, "* %s: list of scoped IPCs.\n",
 309                         ENV_SCOPED_NAME);
 310                 fprintf(stderr,
 311                         "\nexample:\n"
 312                         "%s=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
 313                         "%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
 314                         "%s=\"9418\" "
 315                         "%s=\"80:443\" "
 316                         "%s=\"a:s\" "
 317                         "%s bash -i\n\n",
 318                         ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
 319                         ENV_TCP_CONNECT_NAME, ENV_SCOPED_NAME, argv[0]);
 320                 fprintf(stderr,
 321                         "This sandboxer can use Landlock features "
 322                         "up to ABI version %d.\n",
 323                         LANDLOCK_ABI_LAST);
 324                 return 1;
 325         }
 326
 327         abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
 328         if (abi < 0) {
 329                 const int err = errno;
 330
 331                 perror("Failed to check Landlock compatibility");
 332                 switch (err) {
 333                 case ENOSYS:
 334                         fprintf(stderr,
 335                                 "Hint: Landlock is not supported by the current kernel. "
 336                                 "To support it, build the kernel with "
 337                                 "CONFIG_SECURITY_LANDLOCK=y and prepend "
 338                                 "\"landlock,\" to the content of CONFIG_LSM.\n");
 339                         break;
 340                 case EOPNOTSUPP:
 341                         fprintf(stderr,
 342                                 "Hint: Landlock is currently disabled. "
 343                                 "It can be enabled in the kernel configuration by "
 344                                 "prepending \"landlock,\" to the content of CONFIG_LSM, "
 345                                 "or at boot time by setting the same content to the "
 346                                 "\"lsm\" kernel parameter.\n");
 347                         break;
 348                 }
 349                 return 1;
 350         }
 351
 352         /* Best-effort security. */
 353         switch (abi) {
 354         case 1:
 355                 /*
 356                  * Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
 357                  *
 358                  * Note: The "refer" operations (file renaming and linking
 359                  * across different directories) are always forbidden when using
 360                  * Landlock with ABI 1.
 361                  *
 362                  * If only ABI 1 is available, this sandboxer knowingly forbids
 363                  * refer operations.
 364                  *
 365                  * If a program *needs* to do refer operations after enabling
 366                  * Landlock, it can not use Landlock at ABI level 1.  To be
 367                  * compatible with different kernel versions, such programs
 368                  * should then fall back to not restrict themselves at all if
 369                  * the running kernel only supports ABI 1.
 370                  */
 371                 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
 372                 __attribute__((fallthrough));
 373         case 2:
 374                 /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
 375                 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
 376                 __attribute__((fallthrough));
 377         case 3:
 378                 /* Removes network support for ABI < 4 */
 379                 ruleset_attr.handled_access_net &=
 380                         ~(LANDLOCK_ACCESS_NET_BIND_TCP |
 381                           LANDLOCK_ACCESS_NET_CONNECT_TCP);
 382                 __attribute__((fallthrough));
 383         case 4:
 384                 /* Removes LANDLOCK_ACCESS_FS_IOCTL_DEV for ABI < 5 */
 385                 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_IOCTL_DEV;
 386
 387                 __attribute__((fallthrough));
 388         case 5:
 389                 /* Removes LANDLOCK_SCOPE_* for ABI < 6 */
 390                 ruleset_attr.scoped &= ~(LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
 391                                          LANDLOCK_SCOPE_SIGNAL);
 392                 fprintf(stderr,
 393                         "Hint: You should update the running kernel "
 394                         "to leverage Landlock features "
 395                         "provided by ABI version %d (instead of %d).\n",
 396                         LANDLOCK_ABI_LAST, abi);
 397                 __attribute__((fallthrough));
 398         case LANDLOCK_ABI_LAST:
 399                 break;
 400         default:
 401                 fprintf(stderr,
 402                         "Hint: You should update this sandboxer "
 403                         "to leverage Landlock features "
 404                         "provided by ABI version %d (instead of %d).\n",
 405                         abi, LANDLOCK_ABI_LAST);
 406         }
 407         access_fs_ro &= ruleset_attr.handled_access_fs;
 408         access_fs_rw &= ruleset_attr.handled_access_fs;
 409
 410         /* Removes bind access attribute if not supported by a user. */
 411         env_port_name = getenv(ENV_TCP_BIND_NAME);
 412         if (!env_port_name) {
 413                 ruleset_attr.handled_access_net &=
 414                         ~LANDLOCK_ACCESS_NET_BIND_TCP;
 415         }
 416         /* Removes connect access attribute if not supported by a user. */
 417         env_port_name = getenv(ENV_TCP_CONNECT_NAME);
 418         if (!env_port_name) {
 419                 ruleset_attr.handled_access_net &=
 420                         ~LANDLOCK_ACCESS_NET_CONNECT_TCP;
 421         }
 422
 423         if (check_ruleset_scope(ENV_SCOPED_NAME, &ruleset_attr))
 424                 return 1;
 425
 426         ruleset_fd =
 427                 landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
 428         if (ruleset_fd < 0) {
 429                 perror("Failed to create a ruleset");
 430                 return 1;
 431         }
 432
 433         if (populate_ruleset_fs(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
 434                 goto err_close_ruleset;
 435         }
 436         if (populate_ruleset_fs(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
 437                 goto err_close_ruleset;
 438         }
 439
 440         if (populate_ruleset_net(ENV_TCP_BIND_NAME, ruleset_fd,
 441                                  LANDLOCK_ACCESS_NET_BIND_TCP)) {
 442                 goto err_close_ruleset;
 443         }
 444         if (populate_ruleset_net(ENV_TCP_CONNECT_NAME, ruleset_fd,
 445                                  LANDLOCK_ACCESS_NET_CONNECT_TCP)) {
 446                 goto err_close_ruleset;
 447         }
 448
 449         if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
 450                 perror("Failed to restrict privileges");
 451                 goto err_close_ruleset;
 452         }
 453         if (landlock_restrict_self(ruleset_fd, 0)) {
 454                 perror("Failed to enforce ruleset");
 455                 goto err_close_ruleset;
 456         }
 457         close(ruleset_fd);
 458
 459         cmd_path = argv[1];
 460         cmd_argv = argv + 1;
 461         fprintf(stderr, "Executing the sandboxed command...\n");
 462         execvpe(cmd_path, cmd_argv, envp);
 463         fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
 464                 strerror(errno));
 465         fprintf(stderr, "Hint: access to the binary, the interpreter or "
 466                         "shared libraries may be denied.\n");
 467         return 1;
 468
 469 err_close_ruleset:
 470         close(ruleset_fd);
 471         return 1;
 472 }