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         if (!*path_list)
  95                 return -1;
  96
  97         for (i = 0; i < num_paths; i++)
  98                 (*path_list)[i] = strsep(&env_path, ENV_DELIMITER);
  99
 100         return num_paths;
 101 }
 102
 103 /* clang-format off */
 104
 105 #define ACCESS_FILE ( \
 106         LANDLOCK_ACCESS_FS_EXECUTE | \
 107         LANDLOCK_ACCESS_FS_WRITE_FILE | \
 108         LANDLOCK_ACCESS_FS_READ_FILE | \
 109         LANDLOCK_ACCESS_FS_TRUNCATE | \
 110         LANDLOCK_ACCESS_FS_IOCTL_DEV)
 111
 112 /* clang-format on */
 113
 114 static int populate_ruleset_fs(const char *const env_var, const int ruleset_fd,
 115                                const __u64 allowed_access)
 116 {
 117         int num_paths, i, ret = 1;
 118         char *env_path_name;
 119         const char **path_list = NULL;
 120         struct landlock_path_beneath_attr path_beneath = {
 121                 .parent_fd = -1,
 122         };
 123
 124         env_path_name = getenv(env_var);
 125         if (!env_path_name) {
 126                 /* Prevents users to forget a setting. */
 127                 fprintf(stderr, "Missing environment variable %s\n", env_var);
 128                 return 1;
 129         }
 130         env_path_name = strdup(env_path_name);
 131         unsetenv(env_var);
 132         num_paths = parse_path(env_path_name, &path_list);
 133         if (num_paths < 0) {
 134                 fprintf(stderr, "Failed to allocate memory\n");
 135                 goto out_free_name;
 136         }
 137         if (num_paths == 1 && path_list[0][0] == '\0') {
 138                 /*
 139                  * Allows to not use all possible restrictions (e.g. use
 140                  * LL_FS_RO without LL_FS_RW).
 141                  */
 142                 ret = 0;
 143                 goto out_free_name;
 144         }
 145
 146         for (i = 0; i < num_paths; i++) {
 147                 struct stat statbuf;
 148
 149                 path_beneath.parent_fd = open(path_list[i], O_PATH | O_CLOEXEC);
 150                 if (path_beneath.parent_fd < 0) {
 151                         fprintf(stderr, "Failed to open \"%s\": %s\n",
 152                                 path_list[i], strerror(errno));
 153                         continue;
 154                 }
 155                 if (fstat(path_beneath.parent_fd, &statbuf)) {
 156                         fprintf(stderr, "Failed to stat \"%s\": %s\n",
 157                                 path_list[i], strerror(errno));
 158                         close(path_beneath.parent_fd);
 159                         goto out_free_name;
 160                 }
 161                 path_beneath.allowed_access = allowed_access;
 162                 if (!S_ISDIR(statbuf.st_mode))
 163                         path_beneath.allowed_access &= ACCESS_FILE;
 164                 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
 165                                       &path_beneath, 0)) {
 166                         fprintf(stderr,
 167                                 "Failed to update the ruleset with \"%s\": %s\n",
 168                                 path_list[i], strerror(errno));
 169                         close(path_beneath.parent_fd);
 170                         goto out_free_name;
 171                 }
 172                 close(path_beneath.parent_fd);
 173         }
 174         ret = 0;
 175
 176 out_free_name:
 177         free(path_list);
 178         free(env_path_name);
 179         return ret;
 180 }
 181
 182 static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
 183                                 const __u64 allowed_access)
 184 {
 185         int ret = 1;
 186         char *env_port_name, *env_port_name_next, *strport;
 187         struct landlock_net_port_attr net_port = {
 188                 .allowed_access = allowed_access,
 189         };
 190
 191         env_port_name = getenv(env_var);
 192         if (!env_port_name)
 193                 return 0;
 194         env_port_name = strdup(env_port_name);
 195         unsetenv(env_var);
 196
 197         env_port_name_next = env_port_name;
 198         while ((strport = strsep(&env_port_name_next, ENV_DELIMITER))) {
 199                 __u64 port;
 200
 201                 if (strcmp(strport, "") == 0)
 202                         continue;
 203
 204                 if (str2num(strport, &port)) {
 205                         fprintf(stderr, "Failed to parse port at \"%s\"\n",
 206                                 strport);
 207                         goto out_free_name;
 208                 }
 209                 net_port.port = port;
 210                 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT,
 211                                       &net_port, 0)) {
 212                         fprintf(stderr,
 213                                 "Failed to update the ruleset with port \"%llu\": %s\n",
 214                                 net_port.port, strerror(errno));
 215                         goto out_free_name;
 216                 }
 217         }
 218         ret = 0;
 219
 220 out_free_name:
 221         free(env_port_name);
 222         return ret;
 223 }
 224
 225 /* Returns true on error, false otherwise. */
 226 static bool check_ruleset_scope(const char *const env_var,
 227                                 struct landlock_ruleset_attr *ruleset_attr)
 228 {
 229         char *env_type_scope, *env_type_scope_next, *ipc_scoping_name;
 230         bool error = false;
 231         bool abstract_scoping = false;
 232         bool signal_scoping = false;
 233
 234         /* Scoping is not supported by Landlock ABI */
 235         if (!(ruleset_attr->scoped &
 236               (LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET | LANDLOCK_SCOPE_SIGNAL)))
 237                 goto out_unset;
 238
 239         env_type_scope = getenv(env_var);
 240         /* Scoping is not supported by the user */
 241         if (!env_type_scope || strcmp("", env_type_scope) == 0)
 242                 goto out_unset;
 243
 244         env_type_scope = strdup(env_type_scope);
 245         env_type_scope_next = env_type_scope;
 246         while ((ipc_scoping_name =
 247                         strsep(&env_type_scope_next, ENV_DELIMITER))) {
 248                 if (strcmp("a", ipc_scoping_name) == 0 && !abstract_scoping) {
 249                         abstract_scoping = true;
 250                 } else if (strcmp("s", ipc_scoping_name) == 0 &&
 251                            !signal_scoping) {
 252                         signal_scoping = true;
 253                 } else {
 254                         fprintf(stderr, "Unknown or duplicate scope \"%s\"\n",
 255                                 ipc_scoping_name);
 256                         error = true;
 257                         goto out_free_name;
 258                 }
 259         }
 260
 261 out_free_name:
 262         free(env_type_scope);
 263
 264 out_unset:
 265         if (!abstract_scoping)
 266                 ruleset_attr->scoped &= ~LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET;
 267         if (!signal_scoping)
 268                 ruleset_attr->scoped &= ~LANDLOCK_SCOPE_SIGNAL;
 269
 270         unsetenv(env_var);
 271         return error;
 272 }
 273
 274 /* clang-format off */
 275
 276 #define ACCESS_FS_ROUGHLY_READ ( \
 277         LANDLOCK_ACCESS_FS_EXECUTE | \
 278         LANDLOCK_ACCESS_FS_READ_FILE | \
 279         LANDLOCK_ACCESS_FS_READ_DIR)
 280
 281 #define ACCESS_FS_ROUGHLY_WRITE ( \
 282         LANDLOCK_ACCESS_FS_WRITE_FILE | \
 283         LANDLOCK_ACCESS_FS_REMOVE_DIR | \
 284         LANDLOCK_ACCESS_FS_REMOVE_FILE | \
 285         LANDLOCK_ACCESS_FS_MAKE_CHAR | \
 286         LANDLOCK_ACCESS_FS_MAKE_DIR | \
 287         LANDLOCK_ACCESS_FS_MAKE_REG | \
 288         LANDLOCK_ACCESS_FS_MAKE_SOCK | \
 289         LANDLOCK_ACCESS_FS_MAKE_FIFO | \
 290         LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
 291         LANDLOCK_ACCESS_FS_MAKE_SYM | \
 292         LANDLOCK_ACCESS_FS_REFER | \
 293         LANDLOCK_ACCESS_FS_TRUNCATE | \
 294         LANDLOCK_ACCESS_FS_IOCTL_DEV)
 295
 296 /* clang-format on */
 297
 298 #define LANDLOCK_ABI_LAST 6
 299
 300 #define XSTR(s) #s
 301 #define STR(s) XSTR(s)
 302
 303 /* clang-format off */
 304
 305 static const char help[] =
 306         "usage: " ENV_FS_RO_NAME "=\"...\" " ENV_FS_RW_NAME "=\"...\" "
 307         "[other environment variables] %1$s <cmd> [args]...\n"
 308         "\n"
 309         "Execute the given command in a restricted environment.\n"
 310         "Multi-valued settings (lists of ports, paths, scopes) are colon-delimited.\n"
 311         "\n"
 312         "Mandatory settings:\n"
 313         "* " ENV_FS_RO_NAME ": paths allowed to be used in a read-only way\n"
 314         "* " ENV_FS_RW_NAME ": paths allowed to be used in a read-write way\n"
 315         "\n"
 316         "Optional settings (when not set, their associated access check "
 317         "is always allowed, which is different from an empty string which "
 318         "means an empty list):\n"
 319         "* " ENV_TCP_BIND_NAME ": ports allowed to bind (server)\n"
 320         "* " ENV_TCP_CONNECT_NAME ": ports allowed to connect (client)\n"
 321         "* " ENV_SCOPED_NAME ": actions denied on the outside of the landlock domain\n"
 322         "  - \"a\" to restrict opening abstract unix sockets\n"
 323         "  - \"s\" to restrict sending signals\n"
 324         "\n"
 325         "Example:\n"
 326         ENV_FS_RO_NAME "=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
 327         ENV_FS_RW_NAME "=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
 328         ENV_TCP_BIND_NAME "=\"9418\" "
 329         ENV_TCP_CONNECT_NAME "=\"80:443\" "
 330         ENV_SCOPED_NAME "=\"a:s\" "
 331         "%1$s bash -i\n"
 332         "\n"
 333         "This sandboxer can use Landlock features up to ABI version "
 334         STR(LANDLOCK_ABI_LAST) ".\n";
 335
 336 /* clang-format on */
 337
 338 int main(const int argc, char *const argv[], char *const *const envp)
 339 {
 340         const char *cmd_path;
 341         char *const *cmd_argv;
 342         int ruleset_fd, abi;
 343         char *env_port_name;
 344         __u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
 345               access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
 346
 347         struct landlock_ruleset_attr ruleset_attr = {
 348                 .handled_access_fs = access_fs_rw,
 349                 .handled_access_net = LANDLOCK_ACCESS_NET_BIND_TCP |
 350                                       LANDLOCK_ACCESS_NET_CONNECT_TCP,
 351                 .scoped = LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
 352                           LANDLOCK_SCOPE_SIGNAL,
 353         };
 354
 355         if (argc < 2) {
 356                 fprintf(stderr, help, argv[0]);
 357                 return 1;
 358         }
 359
 360         abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
 361         if (abi < 0) {
 362                 const int err = errno;
 363
 364                 perror("Failed to check Landlock compatibility");
 365                 switch (err) {
 366                 case ENOSYS:
 367                         fprintf(stderr,
 368                                 "Hint: Landlock is not supported by the current kernel. "
 369                                 "To support it, build the kernel with "
 370                                 "CONFIG_SECURITY_LANDLOCK=y and prepend "
 371                                 "\"landlock,\" to the content of CONFIG_LSM.\n");
 372                         break;
 373                 case EOPNOTSUPP:
 374                         fprintf(stderr,
 375                                 "Hint: Landlock is currently disabled. "
 376                                 "It can be enabled in the kernel configuration by "
 377                                 "prepending \"landlock,\" to the content of CONFIG_LSM, "
 378                                 "or at boot time by setting the same content to the "
 379                                 "\"lsm\" kernel parameter.\n");
 380                         break;
 381                 }
 382                 return 1;
 383         }
 384
 385         /* Best-effort security. */
 386         switch (abi) {
 387         case 1:
 388                 /*
 389                  * Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
 390                  *
 391                  * Note: The "refer" operations (file renaming and linking
 392                  * across different directories) are always forbidden when using
 393                  * Landlock with ABI 1.
 394                  *
 395                  * If only ABI 1 is available, this sandboxer knowingly forbids
 396                  * refer operations.
 397                  *
 398                  * If a program *needs* to do refer operations after enabling
 399                  * Landlock, it can not use Landlock at ABI level 1.  To be
 400                  * compatible with different kernel versions, such programs
 401                  * should then fall back to not restrict themselves at all if
 402                  * the running kernel only supports ABI 1.
 403                  */
 404                 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
 405                 __attribute__((fallthrough));
 406         case 2:
 407                 /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
 408                 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
 409                 __attribute__((fallthrough));
 410         case 3:
 411                 /* Removes network support for ABI < 4 */
 412                 ruleset_attr.handled_access_net &=
 413                         ~(LANDLOCK_ACCESS_NET_BIND_TCP |
 414                           LANDLOCK_ACCESS_NET_CONNECT_TCP);
 415                 __attribute__((fallthrough));
 416         case 4:
 417                 /* Removes LANDLOCK_ACCESS_FS_IOCTL_DEV for ABI < 5 */
 418                 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_IOCTL_DEV;
 419
 420                 __attribute__((fallthrough));
 421         case 5:
 422                 /* Removes LANDLOCK_SCOPE_* for ABI < 6 */
 423                 ruleset_attr.scoped &= ~(LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
 424                                          LANDLOCK_SCOPE_SIGNAL);
 425                 fprintf(stderr,
 426                         "Hint: You should update the running kernel "
 427                         "to leverage Landlock features "
 428                         "provided by ABI version %d (instead of %d).\n",
 429                         LANDLOCK_ABI_LAST, abi);
 430                 __attribute__((fallthrough));
 431         case LANDLOCK_ABI_LAST:
 432                 break;
 433         default:
 434                 fprintf(stderr,
 435                         "Hint: You should update this sandboxer "
 436                         "to leverage Landlock features "
 437                         "provided by ABI version %d (instead of %d).\n",
 438                         abi, LANDLOCK_ABI_LAST);
 439         }
 440         access_fs_ro &= ruleset_attr.handled_access_fs;
 441         access_fs_rw &= ruleset_attr.handled_access_fs;
 442
 443         /* Removes bind access attribute if not supported by a user. */
 444         env_port_name = getenv(ENV_TCP_BIND_NAME);
 445         if (!env_port_name) {
 446                 ruleset_attr.handled_access_net &=
 447                         ~LANDLOCK_ACCESS_NET_BIND_TCP;
 448         }
 449         /* Removes connect access attribute if not supported by a user. */
 450         env_port_name = getenv(ENV_TCP_CONNECT_NAME);
 451         if (!env_port_name) {
 452                 ruleset_attr.handled_access_net &=
 453                         ~LANDLOCK_ACCESS_NET_CONNECT_TCP;
 454         }
 455
 456         if (check_ruleset_scope(ENV_SCOPED_NAME, &ruleset_attr))
 457                 return 1;
 458
 459         ruleset_fd =
 460                 landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
 461         if (ruleset_fd < 0) {
 462                 perror("Failed to create a ruleset");
 463                 return 1;
 464         }
 465
 466         if (populate_ruleset_fs(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
 467                 goto err_close_ruleset;
 468         }
 469         if (populate_ruleset_fs(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
 470                 goto err_close_ruleset;
 471         }
 472
 473         if (populate_ruleset_net(ENV_TCP_BIND_NAME, ruleset_fd,
 474                                  LANDLOCK_ACCESS_NET_BIND_TCP)) {
 475                 goto err_close_ruleset;
 476         }
 477         if (populate_ruleset_net(ENV_TCP_CONNECT_NAME, ruleset_fd,
 478                                  LANDLOCK_ACCESS_NET_CONNECT_TCP)) {
 479                 goto err_close_ruleset;
 480         }
 481
 482         if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
 483                 perror("Failed to restrict privileges");
 484                 goto err_close_ruleset;
 485         }
 486         if (landlock_restrict_self(ruleset_fd, 0)) {
 487                 perror("Failed to enforce ruleset");
 488                 goto err_close_ruleset;
 489         }
 490         close(ruleset_fd);
 491
 492         cmd_path = argv[1];
 493         cmd_argv = argv + 1;
 494         fprintf(stderr, "Executing the sandboxed command...\n");
 495         execvpe(cmd_path, cmd_argv, envp);
 496         fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
 497                 strerror(errno));
 498         fprintf(stderr, "Hint: access to the binary, the interpreter or "
 499                         "shared libraries may be denied.\n");
 500         return 1;
 501
 502 err_close_ruleset:
 503         close(ruleset_fd);
 504         return 1;
 505 }