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 landlock_create_ruleset(
  26                 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, const __u32 flags)
  37 {
  38         return syscall(__NR_landlock_add_rule, ruleset_fd, rule_type,
  39                         rule_attr, flags);
  40 }
  41 #endif
  42
  43 #ifndef landlock_restrict_self
  44 static inline int landlock_restrict_self(const int ruleset_fd,
  45                 const __u32 flags)
  46 {
  47         return syscall(__NR_landlock_restrict_self, ruleset_fd, flags);
  48 }
  49 #endif
  50
  51 #define ENV_FS_RO_NAME "LL_FS_RO"
  52 #define ENV_FS_RW_NAME "LL_FS_RW"
  53 #define ENV_PATH_TOKEN ":"
  54
  55 static int parse_path(char *env_path, const char ***const path_list)
  56 {
  57         int i, num_paths = 0;
  58
  59         if (env_path) {
  60                 num_paths++;
  61                 for (i = 0; env_path[i]; i++) {
  62                         if (env_path[i] == ENV_PATH_TOKEN[0])
  63                                 num_paths++;
  64                 }
  65         }
  66         *path_list = malloc(num_paths * sizeof(**path_list));
  67         for (i = 0; i < num_paths; i++)
  68                 (*path_list)[i] = strsep(&env_path, ENV_PATH_TOKEN);
  69
  70         return num_paths;
  71 }
  72
  73 #define ACCESS_FILE ( \
  74         LANDLOCK_ACCESS_FS_EXECUTE | \
  75         LANDLOCK_ACCESS_FS_WRITE_FILE | \
  76         LANDLOCK_ACCESS_FS_READ_FILE)
  77
  78 static int populate_ruleset(
  79                 const char *const env_var, const int ruleset_fd,
  80                 const __u64 allowed_access)
  81 {
  82         int num_paths, i, ret = 1;
  83         char *env_path_name;
  84         const char **path_list = NULL;
  85         struct landlock_path_beneath_attr path_beneath = {
  86                 .parent_fd = -1,
  87         };
  88
  89         env_path_name = getenv(env_var);
  90         if (!env_path_name) {
  91                 /* Prevents users to forget a setting. */
  92                 fprintf(stderr, "Missing environment variable %s\n", env_var);
  93                 return 1;
  94         }
  95         env_path_name = strdup(env_path_name);
  96         unsetenv(env_var);
  97         num_paths = parse_path(env_path_name, &path_list);
  98         if (num_paths == 1 && path_list[0][0] == '\0') {
  99                 /*
 100                  * Allows to not use all possible restrictions (e.g. use
 101                  * LL_FS_RO without LL_FS_RW).
 102                  */
 103                 ret = 0;
 104                 goto out_free_name;
 105         }
 106
 107         for (i = 0; i < num_paths; i++) {
 108                 struct stat statbuf;
 109
 110                 path_beneath.parent_fd = open(path_list[i], O_PATH |
 111                                 O_CLOEXEC);
 112                 if (path_beneath.parent_fd < 0) {
 113                         fprintf(stderr, "Failed to open \"%s\": %s\n",
 114                                         path_list[i],
 115                                         strerror(errno));
 116                         goto out_free_name;
 117                 }
 118                 if (fstat(path_beneath.parent_fd, &statbuf)) {
 119                         close(path_beneath.parent_fd);
 120                         goto out_free_name;
 121                 }
 122                 path_beneath.allowed_access = allowed_access;
 123                 if (!S_ISDIR(statbuf.st_mode))
 124                         path_beneath.allowed_access &= ACCESS_FILE;
 125                 if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
 126                                         &path_beneath, 0)) {
 127                         fprintf(stderr, "Failed to update the ruleset with \"%s\": %s\n",
 128                                         path_list[i], strerror(errno));
 129                         close(path_beneath.parent_fd);
 130                         goto out_free_name;
 131                 }
 132                 close(path_beneath.parent_fd);
 133         }
 134         ret = 0;
 135
 136 out_free_name:
 137         free(path_list);
 138         free(env_path_name);
 139         return ret;
 140 }
 141
 142 #define ACCESS_FS_ROUGHLY_READ ( \
 143         LANDLOCK_ACCESS_FS_EXECUTE | \
 144         LANDLOCK_ACCESS_FS_READ_FILE | \
 145         LANDLOCK_ACCESS_FS_READ_DIR)
 146
 147 #define ACCESS_FS_ROUGHLY_WRITE ( \
 148         LANDLOCK_ACCESS_FS_WRITE_FILE | \
 149         LANDLOCK_ACCESS_FS_REMOVE_DIR | \
 150         LANDLOCK_ACCESS_FS_REMOVE_FILE | \
 151         LANDLOCK_ACCESS_FS_MAKE_CHAR | \
 152         LANDLOCK_ACCESS_FS_MAKE_DIR | \
 153         LANDLOCK_ACCESS_FS_MAKE_REG | \
 154         LANDLOCK_ACCESS_FS_MAKE_SOCK | \
 155         LANDLOCK_ACCESS_FS_MAKE_FIFO | \
 156         LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
 157         LANDLOCK_ACCESS_FS_MAKE_SYM)
 158
 159 int main(const int argc, char *const argv[], char *const *const envp)
 160 {
 161         const char *cmd_path;
 162         char *const *cmd_argv;
 163         int ruleset_fd;
 164         struct landlock_ruleset_attr ruleset_attr = {
 165                 .handled_access_fs = ACCESS_FS_ROUGHLY_READ |
 166                         ACCESS_FS_ROUGHLY_WRITE,
 167         };
 168
 169         if (argc < 2) {
 170                 fprintf(stderr, "usage: %s=\"...\" %s=\"...\" %s <cmd> [args]...\n\n",
 171                                 ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
 172                 fprintf(stderr, "Launch a command in a restricted environment.\n\n");
 173                 fprintf(stderr, "Environment variables containing paths, "
 174                                 "each separated by a colon:\n");
 175                 fprintf(stderr, "* %s: list of paths allowed to be used in a read-only way.\n",
 176                                 ENV_FS_RO_NAME);
 177                 fprintf(stderr, "* %s: list of paths allowed to be used in a read-write way.\n",
 178                                 ENV_FS_RW_NAME);
 179                 fprintf(stderr, "\nexample:\n"
 180                                 "%s=\"/bin:/lib:/usr:/proc:/etc:/dev/urandom\" "
 181                                 "%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
 182                                 "%s bash -i\n",
 183                                 ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
 184                 return 1;
 185         }
 186
 187         ruleset_fd = landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
 188         if (ruleset_fd < 0) {
 189                 const int err = errno;
 190
 191                 perror("Failed to create a ruleset");
 192                 switch (err) {
 193                 case ENOSYS:
 194                         fprintf(stderr, "Hint: Landlock is not supported by the current kernel. "
 195                                         "To support it, build the kernel with "
 196                                         "CONFIG_SECURITY_LANDLOCK=y and prepend "
 197                                         "\"landlock,\" to the content of CONFIG_LSM.\n");
 198                         break;
 199                 case EOPNOTSUPP:
 200                         fprintf(stderr, "Hint: Landlock is currently disabled. "
 201                                         "It can be enabled in the kernel configuration by "
 202                                         "prepending \"landlock,\" to the content of CONFIG_LSM, "
 203                                         "or at boot time by setting the same content to the "
 204                                         "\"lsm\" kernel parameter.\n");
 205                         break;
 206                 }
 207                 return 1;
 208         }
 209         if (populate_ruleset(ENV_FS_RO_NAME, ruleset_fd,
 210                                 ACCESS_FS_ROUGHLY_READ)) {
 211                 goto err_close_ruleset;
 212         }
 213         if (populate_ruleset(ENV_FS_RW_NAME, ruleset_fd,
 214                                 ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE)) {
 215                 goto err_close_ruleset;
 216         }
 217         if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
 218                 perror("Failed to restrict privileges");
 219                 goto err_close_ruleset;
 220         }
 221         if (landlock_restrict_self(ruleset_fd, 0)) {
 222                 perror("Failed to enforce ruleset");
 223                 goto err_close_ruleset;
 224         }
 225         close(ruleset_fd);
 226
 227         cmd_path = argv[1];
 228         cmd_argv = argv + 1;
 229         execvpe(cmd_path, cmd_argv, envp);
 230         fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
 231                         strerror(errno));
 232         fprintf(stderr, "Hint: access to the binary, the interpreter or "
 233                         "shared libraries may be denied.\n");
 234         return 1;
 235
 236 err_close_ruleset:
 237         close(ruleset_fd);
 238         return 1;
 239 }