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 <[email protected]>
   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         for (i = 0; i < num_paths; i++)
  69                 (*path_list)[i] = strsep(&env_path, ENV_PATH_TOKEN);
  70
  71         return num_paths;
  72 }
  73
  74 /* clang-format off */
  75
  76 #define ACCESS_FILE ( \
  77         LANDLOCK_ACCESS_FS_EXECUTE | \
  78         LANDLOCK_ACCESS_FS_WRITE_FILE | \
  79         LANDLOCK_ACCESS_FS_READ_FILE | \
  80         LANDLOCK_ACCESS_FS_TRUNCATE)
  81
  82 /* clang-format on */
  83
  84 static int populate_ruleset(const char *const env_var, const int ruleset_fd,
  85                             const __u64 allowed_access)
  86 {
  87         int num_paths, i, ret = 1;
  88         char *env_path_name;
  89         const char **path_list = NULL;
  90         struct landlock_path_beneath_attr path_beneath = {
  91                 .parent_fd = -1,
  92         };
  93
  94         env_path_name = getenv(env_var);
  95         if (!env_path_name) {
  96                 /* Prevents users to forget a setting. */
  97                 fprintf(stderr, "Missing environment variable %s\n", env_var);
  98                 return 1;
  99         }
 100         env_path_name = strdup(env_path_name);
 101         unsetenv(env_var);
 102         num_paths = parse_path(env_path_name, &path_list);
 103         if (num_paths == 1 && path_list[0][0] == '\0') {
 104                 /*
 105                  * Allows to not use all possible restrictions (e.g. use
 106                  * LL_FS_RO without LL_FS_RW).
 107                  */
 108                 ret = 0;
 109                 goto out_free_name;
 110         }
 111
 112         for (i = 0; i < num_paths; i++) {
 113                 struct stat statbuf;
 114
 115                 path_beneath.parent_fd = open(path_list[i], O_PATH | O_CLOEXEC);
 116                 if (path_beneath.parent_fd < 0) {
 117                         fprintf(stderr, "Failed to open \"%s\": %s\n",
 118                                 path_list[i], strerror(errno));
 119                         goto out_free_name;
 120                 }
 121                 if (fstat(path_beneath.parent_fd, &statbuf)) {
 122                         close(path_beneath.parent_fd);
 123                         goto out_free_name;
 124                 }
 125                 path_beneath.allowed_access = allowed_access;
 126                 if (!S_ISDIR(statbuf.st_mode))
 127                         path_beneath.allowed_access &= ACCESS_FILE;
 128                 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
 129                                       &path_beneath, 0)) {
 130                         fprintf(stderr,
 131                                 "Failed to update the ruleset with \"%s\": %s\n",
 132                                 path_list[i], strerror(errno));
 133                         close(path_beneath.parent_fd);
 134                         goto out_free_name;
 135                 }
 136                 close(path_beneath.parent_fd);
 137         }
 138         ret = 0;
 139
 140 out_free_name:
 141         free(path_list);
 142         free(env_path_name);
 143         return ret;
 144 }
 145
 146 /* clang-format off */
 147
 148 #define ACCESS_FS_ROUGHLY_READ ( \
 149         LANDLOCK_ACCESS_FS_EXECUTE | \
 150         LANDLOCK_ACCESS_FS_READ_FILE | \
 151         LANDLOCK_ACCESS_FS_READ_DIR)
 152
 153 #define ACCESS_FS_ROUGHLY_WRITE ( \
 154         LANDLOCK_ACCESS_FS_WRITE_FILE | \
 155         LANDLOCK_ACCESS_FS_REMOVE_DIR | \
 156         LANDLOCK_ACCESS_FS_REMOVE_FILE | \
 157         LANDLOCK_ACCESS_FS_MAKE_CHAR | \
 158         LANDLOCK_ACCESS_FS_MAKE_DIR | \
 159         LANDLOCK_ACCESS_FS_MAKE_REG | \
 160         LANDLOCK_ACCESS_FS_MAKE_SOCK | \
 161         LANDLOCK_ACCESS_FS_MAKE_FIFO | \
 162         LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
 163         LANDLOCK_ACCESS_FS_MAKE_SYM | \
 164         LANDLOCK_ACCESS_FS_REFER | \
 165         LANDLOCK_ACCESS_FS_TRUNCATE)
 166
 167 /* clang-format on */
 168
 169 #define LANDLOCK_ABI_LAST 3
 170
 171 int main(const int argc, char *const argv[], char *const *const envp)
 172 {
 173         const char *cmd_path;
 174         char *const *cmd_argv;
 175         int ruleset_fd, abi;
 176         __u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
 177               access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
 178         struct landlock_ruleset_attr ruleset_attr = {
 179                 .handled_access_fs = access_fs_rw,
 180         };
 181
 182         if (argc < 2) {
 183                 fprintf(stderr,
 184                         "usage: %s=\"...\" %s=\"...\" %s <cmd> [args]...\n\n",
 185                         ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
 186                 fprintf(stderr,
 187                         "Launch a command in a restricted environment.\n\n");
 188                 fprintf(stderr, "Environment variables containing paths, "
 189                                 "each separated by a colon:\n");
 190                 fprintf(stderr,
 191                         "* %s: list of paths allowed to be used in a read-only way.\n",
 192                         ENV_FS_RO_NAME);
 193                 fprintf(stderr,
 194                         "* %s: list of paths allowed to be used in a read-write way.\n",
 195                         ENV_FS_RW_NAME);
 196                 fprintf(stderr,
 197                         "\nexample:\n"
 198                         "%s=\"/bin:/lib:/usr:/proc:/etc:/dev/urandom\" "
 199                         "%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
 200                         "%s bash -i\n\n",
 201                         ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
 202                 fprintf(stderr,
 203                         "This sandboxer can use Landlock features "
 204                         "up to ABI version %d.\n",
 205                         LANDLOCK_ABI_LAST);
 206                 return 1;
 207         }
 208
 209         abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
 210         if (abi < 0) {
 211                 const int err = errno;
 212
 213                 perror("Failed to check Landlock compatibility");
 214                 switch (err) {
 215                 case ENOSYS:
 216                         fprintf(stderr,
 217                                 "Hint: Landlock is not supported by the current kernel. "
 218                                 "To support it, build the kernel with "
 219                                 "CONFIG_SECURITY_LANDLOCK=y and prepend "
 220                                 "\"landlock,\" to the content of CONFIG_LSM.\n");
 221                         break;
 222                 case EOPNOTSUPP:
 223                         fprintf(stderr,
 224                                 "Hint: Landlock is currently disabled. "
 225                                 "It can be enabled in the kernel configuration by "
 226                                 "prepending \"landlock,\" to the content of CONFIG_LSM, "
 227                                 "or at boot time by setting the same content to the "
 228                                 "\"lsm\" kernel parameter.\n");
 229                         break;
 230                 }
 231                 return 1;
 232         }
 233
 234         /* Best-effort security. */
 235         switch (abi) {
 236         case 1:
 237                 /*
 238                  * Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
 239                  *
 240                  * Note: The "refer" operations (file renaming and linking
 241                  * across different directories) are always forbidden when using
 242                  * Landlock with ABI 1.
 243                  *
 244                  * If only ABI 1 is available, this sandboxer knowingly forbids
 245                  * refer operations.
 246                  *
 247                  * If a program *needs* to do refer operations after enabling
 248                  * Landlock, it can not use Landlock at ABI level 1.  To be
 249                  * compatible with different kernel versions, such programs
 250                  * should then fall back to not restrict themselves at all if
 251                  * the running kernel only supports ABI 1.
 252                  */
 253                 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
 254                 __attribute__((fallthrough));
 255         case 2:
 256                 /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
 257                 ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
 258
 259                 fprintf(stderr,
 260                         "Hint: You should update the running kernel "
 261                         "to leverage Landlock features "
 262                         "provided by ABI version %d (instead of %d).\n",
 263                         LANDLOCK_ABI_LAST, abi);
 264                 __attribute__((fallthrough));
 265         case LANDLOCK_ABI_LAST:
 266                 break;
 267         default:
 268                 fprintf(stderr,
 269                         "Hint: You should update this sandboxer "
 270                         "to leverage Landlock features "
 271                         "provided by ABI version %d (instead of %d).\n",
 272                         abi, LANDLOCK_ABI_LAST);
 273         }
 274         access_fs_ro &= ruleset_attr.handled_access_fs;
 275         access_fs_rw &= ruleset_attr.handled_access_fs;
 276
 277         ruleset_fd =
 278                 landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
 279         if (ruleset_fd < 0) {
 280                 perror("Failed to create a ruleset");
 281                 return 1;
 282         }
 283         if (populate_ruleset(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
 284                 goto err_close_ruleset;
 285         }
 286         if (populate_ruleset(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
 287                 goto err_close_ruleset;
 288         }
 289         if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
 290                 perror("Failed to restrict privileges");
 291                 goto err_close_ruleset;
 292         }
 293         if (landlock_restrict_self(ruleset_fd, 0)) {
 294                 perror("Failed to enforce ruleset");
 295                 goto err_close_ruleset;
 296         }
 297         close(ruleset_fd);
 298
 299         cmd_path = argv[1];
 300         cmd_argv = argv + 1;
 301         execvpe(cmd_path, cmd_argv, envp);
 302         fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
 303                 strerror(errno));
 304         fprintf(stderr, "Hint: access to the binary, the interpreter or "
 305                         "shared libraries may be denied.\n");
 306         return 1;
 307
 308 err_close_ruleset:
 309         close(ruleset_fd);
 310         return 1;
 311 }