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 <[email protected]>
   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 str2num(const char *numstr, __u64 *num_dst)
  64 {
  65         char *endptr = NULL;
  66         int err = 0;
  67         __u64 num;
  68
  69         errno = 0;
  70         num = strtoull(numstr, &endptr, 10);
  71         if (errno != 0)
  72                 err = errno;
  73         /* Was the string empty, or not entirely parsed successfully? */
  74         else if ((*numstr == '\0') || (*endptr != '\0'))
  75                 err = EINVAL;
  76         else
  77                 *num_dst = num;
  78
  79         return err;
  80 }
  81
  82 static int parse_path(char *env_path, const char ***const path_list)
  83 {
  84         int i, num_paths = 0;
  85
  86         if (env_path) {
  87                 num_paths++;
  88                 for (i = 0; env_path[i]; i++) {
  89                         if (env_path[i] == ENV_DELIMITER[0])
  90                                 num_paths++;
  91                 }
  92         }
  93         *path_list = malloc(num_paths * sizeof(**path_list));
  94         for (i = 0; i < num_paths; i++)
  95                 (*path_list)[i] = strsep(&env_path, ENV_DELIMITER);
  96
  97         return num_paths;
  98 }
  99
 100 /* clang-format off */
 101
 102 #define ACCESS_FILE ( \
 103         LANDLOCK_ACCESS_FS_EXECUTE | \
 104         LANDLOCK_ACCESS_FS_WRITE_FILE | \
 105         LANDLOCK_ACCESS_FS_READ_FILE | \
 106         LANDLOCK_ACCESS_FS_TRUNCATE | \
 107         LANDLOCK_ACCESS_FS_IOCTL_DEV)
 108
 109 /* clang-format on */
 110
 111 static int populate_ruleset_fs(const char *const env_var, const int ruleset_fd,
 112                                const __u64 allowed_access)
 113 {
 114         int num_paths, i, ret = 1;
 115         char *env_path_name;
 116         const char **path_list = NULL;
 117         struct landlock_path_beneath_attr path_beneath = {
 118                 .parent_fd = -1,
 119         };
 120
 121         env_path_name = getenv(env_var);
 122         if (!env_path_name) {
 123                 /* Prevents users to forget a setting. */
 124                 fprintf(stderr, "Missing environment variable %s\n", env_var);
 125                 return 1;
 126         }
 127         env_path_name = strdup(env_path_name);
 128         unsetenv(env_var);
 129         num_paths = parse_path(env_path_name, &path_list);
 130         if (num_paths == 1 && path_list[0][0] == '\0') {
 131                 /*
 132                  * Allows to not use all possible restrictions (e.g. use
 133                  * LL_FS_RO without LL_FS_RW).
 134                  */
 135                 ret = 0;
 136                 goto out_free_name;
 137         }
 138
 139         for (i = 0; i < num_paths; i++) {
 140                 struct stat statbuf;
 141
 142                 path_beneath.parent_fd = open(path_list[i], O_PATH | O_CLOEXEC);
 143                 if (path_beneath.parent_fd < 0) {
 144                         fprintf(stderr, "Failed to open \"%s\": %s\n",
 145                                 path_list[i], strerror(errno));
 146                         continue;
 147                 }
 148                 if (fstat(path_beneath.parent_fd, &statbuf)) {
 149                         fprintf(stderr, "Failed to stat \"%s\": %s\n",
 150                                 path_list[i], strerror(errno));
 151                         close(path_beneath.parent_fd);
 152                         goto out_free_name;
 153                 }
 154                 path_beneath.allowed_access = allowed_access;
 155                 if (!S_ISDIR(statbuf.st_mode))
 156                         path_beneath.allowed_access &= ACCESS_FILE;
 157                 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
 158                                       &path_beneath, 0)) {
 159                         fprintf(stderr,
 160                                 "Failed to update the ruleset with \"%s\": %s\n",
 161                                 path_list[i], strerror(errno));
 162                         close(path_beneath.parent_fd);
 163                         goto out_free_name;
 164                 }
 165                 close(path_beneath.parent_fd);
 166         }
 167         ret = 0;
 168
 169 out_free_name:
 170         free(path_list);
 171         free(env_path_name);
 172         return ret;
 173 }
 174
 175 static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
 176                                 const __u64 allowed_access)
 177 {
 178         int ret = 1;
 179         char *env_port_name, *env_port_name_next, *strport;
 180         struct landlock_net_port_attr net_port = {
 181                 .allowed_access = allowed_access,
 182         };
 183
 184         env_port_name = getenv(env_var);
 185         if (!env_port_name)
 186                 return 0;
 187         env_port_name = strdup(env_port_name);
 188         unsetenv(env_var);
 189
 190         env_port_name_next = env_port_name;
 191         while ((strport = strsep(&env_port_name_next, ENV_DELIMITER))) {
 192                 __u64 port;
 193
 194                 if (strcmp(strport, "") == 0)
 195                         continue;
 196
 197                 if (str2num(strport, &port)) {
 198                         fprintf(stderr, "Failed to parse port at \"%s\"\n",
 199                                 strport);
 200                         goto out_free_name;
 201                 }
 202                 net_port.port = port;
 203                 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT,
 204                                       &net_port, 0)) {
 205                         fprintf(stderr,
 206                                 "Failed to update the ruleset with port \"%llu\": %s\n",
 207                                 net_port.port, strerror(errno));
 208                         goto out_free_name;
 209                 }
 210         }
 211         ret = 0;
 212
 213 out_free_name:
 214         free(env_port_name);
 215         return ret;
 216 }
 217
 218 /* Returns true on error, false otherwise. */
 219 static bool check_ruleset_scope(const char *const env_var,
 220                                 struct landlock_ruleset_attr *ruleset_attr)
 221 {
 222         char *env_type_scope, *env_type_scope_next, *ipc_scoping_name;
 223         bool error = false;
 224         bool abstract_scoping = false;
 225         bool signal_scoping = false;
 226
 227         /* Scoping is not supported by Landlock ABI */
 228         if (!(ruleset_attr->scoped &
 229               (LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET | LANDLOCK_SCOPE_SIGNAL)))
 230                 goto out_unset;
 231
 232         env_type_scope = getenv(env_var);
 233         /* Scoping is not supported by the user */
 234         if (!env_type_scope || strcmp("", env_type_scope) == 0)
 235                 goto out_unset;
 236
 237         env_type_scope = strdup(env_type_scope);
 238         env_type_scope_next = env_type_scope;
 239         while ((ipc_scoping_name =
 240                         strsep(&env_type_scope_next, ENV_DELIMITER))) {
 241                 if (strcmp("a", ipc_scoping_name) == 0 && !abstract_scoping) {
 242                         abstract_scoping = true;
 243                 } else if (strcmp("s", ipc_scoping_name) == 0 &&
 244                            !signal_scoping) {
 245                         signal_scoping = true;
 246                 } else {
 247                         fprintf(stderr, "Unknown or duplicate scope \"%s\"\n",
 248                                 ipc_scoping_name);
 249                         error = true;
 250                         goto out_free_name;
 251                 }
 252         }
 253
 254 out_free_name:
 255         free(env_type_scope);
 256
 257 out_unset:
 258         if (!abstract_scoping)
 259                 ruleset_attr->scoped &= ~LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET;
 260         if (!signal_scoping)
 261                 ruleset_attr->scoped &= ~LANDLOCK_SCOPE_SIGNAL;
 262
 263         unsetenv(env_var);
 264         return error;
 265 }
 266
 267 /* clang-format off */
 268
 269 #define ACCESS_FS_ROUGHLY_READ ( \
 270         LANDLOCK_ACCESS_FS_EXECUTE | \
 271         LANDLOCK_ACCESS_FS_READ_FILE | \
 272         LANDLOCK_ACCESS_FS_READ_DIR)
 273
 274 #define ACCESS_FS_ROUGHLY_WRITE ( \
 275         LANDLOCK_ACCESS_FS_WRITE_FILE | \
 276         LANDLOCK_ACCESS_FS_REMOVE_DIR | \
 277         LANDLOCK_ACCESS_FS_REMOVE_FILE | \
 278         LANDLOCK_ACCESS_FS_MAKE_CHAR | \
 279         LANDLOCK_ACCESS_FS_MAKE_DIR | \
 280         LANDLOCK_ACCESS_FS_MAKE_REG | \
 281         LANDLOCK_ACCESS_FS_MAKE_SOCK | \
 282         LANDLOCK_ACCESS_FS_MAKE_FIFO | \
 283         LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
 284         LANDLOCK_ACCESS_FS_MAKE_SYM | \
 285         LANDLOCK_ACCESS_FS_REFER | \
 286         LANDLOCK_ACCESS_FS_TRUNCATE | \
 287         LANDLOCK_ACCESS_FS_IOCTL_DEV)
 288
 289 /* clang-format on */
 290
 291 #define LANDLOCK_ABI_LAST 6
 292
 293 #define XSTR(s) #s
 294 #define STR(s) XSTR(s)
 295
 296 /* clang-format off */
 297
 298 static const char help[] =
 299         "usage: " ENV_FS_RO_NAME "=\"...\" " ENV_FS_RW_NAME "=\"...\" "
 300         "[other environment variables] %1$s <cmd> [args]...\n"
 301         "\n"
 302         "Execute the given command in a restricted environment.\n"
 303         "Multi-valued settings (lists of ports, paths, scopes) are colon-delimited.\n"
 304         "\n"
 305         "Mandatory settings:\n"
 306         "* " ENV_FS_RO_NAME ": paths allowed to be used in a read-only way\n"
 307         "* " ENV_FS_RW_NAME ": paths allowed to be used in a read-write way\n"
 308         "\n"
 309         "Optional settings (when not set, their associated access check "
 310         "is always allowed, which is different from an empty string which "
 311         "means an empty list):\n"
 312         "* " ENV_TCP_BIND_NAME ": ports allowed to bind (server)\n"
 313         "* " ENV_TCP_CONNECT_NAME ": ports allowed to connect (client)\n"
 314         "* " ENV_SCOPED_NAME ": actions denied on the outside of the landlock domain\n"
 315         "  - \"a\" to restrict opening abstract unix sockets\n"
 316         "  - \"s\" to restrict sending signals\n"
 317         "\n"
 318         "Example:\n"
 319         ENV_FS_RO_NAME "=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
 320         ENV_FS_RW_NAME "=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
 321         ENV_TCP_BIND_NAME "=\"9418\" "
 322         ENV_TCP_CONNECT_NAME "=\"80:443\" "
 323         ENV_SCOPED_NAME "=\"a:s\" "
 324         "%1$s bash -i\n"
 325         "\n"
 326         "This sandboxer can use Landlock features up to ABI version "
 327         STR(LANDLOCK_ABI_LAST) ".\n";
 328
 329 /* clang-format on */
 330
 331 int main(const int argc, char *const argv[], char *const *const envp)
 332 {
 333         const char *cmd_path;
 334         char *const *cmd_argv;
 335         int ruleset_fd, abi;
 336         char *env_port_name;
 337         __u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
 338               access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
 339
 340         struct landlock_ruleset_attr ruleset_attr = {
 341                 .handled_access_fs = access_fs_rw,
 342                 .handled_access_net = LANDLOCK_ACCESS_NET_BIND_TCP |
 343                                       LANDLOCK_ACCESS_NET_CONNECT_TCP,
 344                 .scoped = LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
 345                           LANDLOCK_SCOPE_SIGNAL,
 346         };
 347
 348         if (argc < 2) {
 349                 fprintf(stderr, help, argv[0]);
 350                 return 1;
 351         }
 352
 353         abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
 354         if (abi < 0) {
 355                 const int err = errno;
 356
 357                 perror("Failed to check Landlock compatibility");
 358                 switch (err) {
 359                 case ENOSYS:
 360                         fprintf(stderr,
 361                                 "Hint: Landlock is not supported by the current kernel. "
 362                                 "To support it, build the kernel with "
 363                                 "CONFIG_SECURITY_LANDLOCK=y and prepend "
 364                                 "\"landlock,\" to the content of CONFIG_LSM.\n");
 365                         break;
 366                 case EOPNOTSUPP:
 367                         fprintf(stderr,
 368                                 "Hint: Landlock is currently disabled. "
 369                                 "It can be enabled in the kernel configuration by "
 370                                 "prepending \"landlock,\" to the content of CONFIG_LSM, "
 371                                 "or at boot time by setting the same content to the "
 372                                 "\"lsm\" kernel parameter.\n");
 373                         break;
 374                 }
 375                 return 1;
 376         }
 377
 378         /* Best-effort security. */
 379         switch (abi) {
 380         case 1:
 381                 /*
 382                  * Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
 383                  *
 384                  * Note: The "refer" operations (file renaming and linking
 385                  * across different directories) are always forbidden when using
 386                  * Landlock with ABI 1.
 387                  *
 388                  * If only ABI 1 is available, this sandboxer knowingly forbids
 389                  * refer operations.
 390                  *
 391                  * If a program *needs* to do refer operations after enabling
 392                  * Landlock, it can not use Landlock at ABI level 1.  To be
 393                  * compatible with different kernel versions, such programs
 394                  * should then fall back to not restrict themselves at all if
 395                  * the running kernel only supports ABI 1.
 396                  */
 397                 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
 398                 __attribute__((fallthrough));
 399         case 2:
 400                 /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
 401                 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
 402                 __attribute__((fallthrough));
 403         case 3:
 404                 /* Removes network support for ABI < 4 */
 405                 ruleset_attr.handled_access_net &=
 406                         ~(LANDLOCK_ACCESS_NET_BIND_TCP |
 407                           LANDLOCK_ACCESS_NET_CONNECT_TCP);
 408                 __attribute__((fallthrough));
 409         case 4:
 410                 /* Removes LANDLOCK_ACCESS_FS_IOCTL_DEV for ABI < 5 */
 411                 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_IOCTL_DEV;
 412
 413                 __attribute__((fallthrough));
 414         case 5:
 415                 /* Removes LANDLOCK_SCOPE_* for ABI < 6 */
 416                 ruleset_attr.scoped &= ~(LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
 417                                          LANDLOCK_SCOPE_SIGNAL);
 418                 fprintf(stderr,
 419                         "Hint: You should update the running kernel "
 420                         "to leverage Landlock features "
 421                         "provided by ABI version %d (instead of %d).\n",
 422                         LANDLOCK_ABI_LAST, abi);
 423                 __attribute__((fallthrough));
 424         case LANDLOCK_ABI_LAST:
 425                 break;
 426         default:
 427                 fprintf(stderr,
 428                         "Hint: You should update this sandboxer "
 429                         "to leverage Landlock features "
 430                         "provided by ABI version %d (instead of %d).\n",
 431                         abi, LANDLOCK_ABI_LAST);
 432         }
 433         access_fs_ro &= ruleset_attr.handled_access_fs;
 434         access_fs_rw &= ruleset_attr.handled_access_fs;
 435
 436         /* Removes bind access attribute if not supported by a user. */
 437         env_port_name = getenv(ENV_TCP_BIND_NAME);
 438         if (!env_port_name) {
 439                 ruleset_attr.handled_access_net &=
 440                         ~LANDLOCK_ACCESS_NET_BIND_TCP;
 441         }
 442         /* Removes connect access attribute if not supported by a user. */
 443         env_port_name = getenv(ENV_TCP_CONNECT_NAME);
 444         if (!env_port_name) {
 445                 ruleset_attr.handled_access_net &=
 446                         ~LANDLOCK_ACCESS_NET_CONNECT_TCP;
 447         }
 448
 449         if (check_ruleset_scope(ENV_SCOPED_NAME, &ruleset_attr))
 450                 return 1;
 451
 452         ruleset_fd =
 453                 landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
 454         if (ruleset_fd < 0) {
 455                 perror("Failed to create a ruleset");
 456                 return 1;
 457         }
 458
 459         if (populate_ruleset_fs(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
 460                 goto err_close_ruleset;
 461         }
 462         if (populate_ruleset_fs(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
 463                 goto err_close_ruleset;
 464         }
 465
 466         if (populate_ruleset_net(ENV_TCP_BIND_NAME, ruleset_fd,
 467                                  LANDLOCK_ACCESS_NET_BIND_TCP)) {
 468                 goto err_close_ruleset;
 469         }
 470         if (populate_ruleset_net(ENV_TCP_CONNECT_NAME, ruleset_fd,
 471                                  LANDLOCK_ACCESS_NET_CONNECT_TCP)) {
 472                 goto err_close_ruleset;
 473         }
 474
 475         if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
 476                 perror("Failed to restrict privileges");
 477                 goto err_close_ruleset;
 478         }
 479         if (landlock_restrict_self(ruleset_fd, 0)) {
 480                 perror("Failed to enforce ruleset");
 481                 goto err_close_ruleset;
 482         }
 483         close(ruleset_fd);
 484
 485         cmd_path = argv[1];
 486         cmd_argv = argv + 1;
 487         fprintf(stderr, "Executing the sandboxed command...\n");
 488         execvpe(cmd_path, cmd_argv, envp);
 489         fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
 490                 strerror(errno));
 491         fprintf(stderr, "Hint: access to the binary, the interpreter or "
 492                         "shared libraries may be denied.\n");
 493         return 1;
 494
 495 err_close_ruleset:
 496         close(ruleset_fd);
 497         return 1;
 498 }