samples/landlock/sandboxer.c

0001 // SPDX-License-Identifier: BSD-3-Clause
0002 /*
0003  * Simple Landlock sandbox manager able to launch a process restricted by a
0004  * user-defined filesystem access control policy.
0005  *
0006  * Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
0007  * Copyright © 2020 ANSSI
0008  */
0009
0010 #define _GNU_SOURCE
0011 #include <errno.h>
0012 #include <fcntl.h>
0013 #include <linux/landlock.h>
0014 #include <linux/prctl.h>
0015 #include <stddef.h>
0016 #include <stdio.h>
0017 #include <stdlib.h>
0018 #include <string.h>
0019 #include <sys/prctl.h>
0020 #include <sys/stat.h>
0021 #include <sys/syscall.h>
0022 #include <unistd.h>
0023
0024 #ifndef landlock_create_ruleset
0025 static inline int
0026 landlock_create_ruleset(const struct landlock_ruleset_attr *const attr,
0027             const size_t size, const __u32 flags)
0028 {
0029     return syscall(__NR_landlock_create_ruleset, attr, size, flags);
0030 }
0031 #endif
0032
0033 #ifndef landlock_add_rule
0034 static inline int landlock_add_rule(const int ruleset_fd,
0035                     const enum landlock_rule_type rule_type,
0036                     const void *const rule_attr,
0037                     const __u32 flags)
0038 {
0039     return syscall(__NR_landlock_add_rule, ruleset_fd, rule_type, rule_attr,
0040                flags);
0041 }
0042 #endif
0043
0044 #ifndef landlock_restrict_self
0045 static inline int landlock_restrict_self(const int ruleset_fd,
0046                      const __u32 flags)
0047 {
0048     return syscall(__NR_landlock_restrict_self, ruleset_fd, flags);
0049 }
0050 #endif
0051
0052 #define ENV_FS_RO_NAME "LL_FS_RO"
0053 #define ENV_FS_RW_NAME "LL_FS_RW"
0054 #define ENV_PATH_TOKEN ":"
0055
0056 static int parse_path(char *env_path, const char ***const path_list)
0057 {
0058     int i, num_paths = 0;
0059
0060     if (env_path) {
0061         num_paths++;
0062         for (i = 0; env_path[i]; i++) {
0063             if (env_path[i] == ENV_PATH_TOKEN[0])
0064                 num_paths++;
0065         }
0066     }
0067     *path_list = malloc(num_paths * sizeof(**path_list));
0068     for (i = 0; i < num_paths; i++)
0069         (*path_list)[i] = strsep(&env_path, ENV_PATH_TOKEN);
0070
0071     return num_paths;
0072 }
0073
0074 /* clang-format off */
0075
0076 #define ACCESS_FILE ( \
0077     LANDLOCK_ACCESS_FS_EXECUTE | \
0078     LANDLOCK_ACCESS_FS_WRITE_FILE | \
0079     LANDLOCK_ACCESS_FS_READ_FILE)
0080
0081 /* clang-format on */
0082
0083 static int populate_ruleset(const char *const env_var, const int ruleset_fd,
0084                 const __u64 allowed_access)
0085 {
0086     int num_paths, i, ret = 1;
0087     char *env_path_name;
0088     const char **path_list = NULL;
0089     struct landlock_path_beneath_attr path_beneath = {
0090         .parent_fd = -1,
0091     };
0092
0093     env_path_name = getenv(env_var);
0094     if (!env_path_name) {
0095         /* Prevents users to forget a setting. */
0096         fprintf(stderr, "Missing environment variable %s\n", env_var);
0097         return 1;
0098     }
0099     env_path_name = strdup(env_path_name);
0100     unsetenv(env_var);
0101     num_paths = parse_path(env_path_name, &path_list);
0102     if (num_paths == 1 && path_list[0][0] == '\0') {
0103         /*
0104          * Allows to not use all possible restrictions (e.g. use
0105          * LL_FS_RO without LL_FS_RW).
0106          */
0107         ret = 0;
0108         goto out_free_name;
0109     }
0110
0111     for (i = 0; i < num_paths; i++) {
0112         struct stat statbuf;
0113
0114         path_beneath.parent_fd = open(path_list[i], O_PATH | O_CLOEXEC);
0115         if (path_beneath.parent_fd < 0) {
0116             fprintf(stderr, "Failed to open \"%s\": %s\n",
0117                 path_list[i], strerror(errno));
0118             goto out_free_name;
0119         }
0120         if (fstat(path_beneath.parent_fd, &statbuf)) {
0121             close(path_beneath.parent_fd);
0122             goto out_free_name;
0123         }
0124         path_beneath.allowed_access = allowed_access;
0125         if (!S_ISDIR(statbuf.st_mode))
0126             path_beneath.allowed_access &= ACCESS_FILE;
0127         if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
0128                       &path_beneath, 0)) {
0129             fprintf(stderr,
0130                 "Failed to update the ruleset with \"%s\": %s\n",
0131                 path_list[i], strerror(errno));
0132             close(path_beneath.parent_fd);
0133             goto out_free_name;
0134         }
0135         close(path_beneath.parent_fd);
0136     }
0137     ret = 0;
0138
0139 out_free_name:
0140     free(path_list);
0141     free(env_path_name);
0142     return ret;
0143 }
0144
0145 /* clang-format off */
0146
0147 #define ACCESS_FS_ROUGHLY_READ ( \
0148     LANDLOCK_ACCESS_FS_EXECUTE | \
0149     LANDLOCK_ACCESS_FS_READ_FILE | \
0150     LANDLOCK_ACCESS_FS_READ_DIR)
0151
0152 #define ACCESS_FS_ROUGHLY_WRITE ( \
0153     LANDLOCK_ACCESS_FS_WRITE_FILE | \
0154     LANDLOCK_ACCESS_FS_REMOVE_DIR | \
0155     LANDLOCK_ACCESS_FS_REMOVE_FILE | \
0156     LANDLOCK_ACCESS_FS_MAKE_CHAR | \
0157     LANDLOCK_ACCESS_FS_MAKE_DIR | \
0158     LANDLOCK_ACCESS_FS_MAKE_REG | \
0159     LANDLOCK_ACCESS_FS_MAKE_SOCK | \
0160     LANDLOCK_ACCESS_FS_MAKE_FIFO | \
0161     LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
0162     LANDLOCK_ACCESS_FS_MAKE_SYM | \
0163     LANDLOCK_ACCESS_FS_REFER)
0164
0165 #define ACCESS_ABI_2 ( \
0166     LANDLOCK_ACCESS_FS_REFER)
0167
0168 /* clang-format on */
0169
0170 int main(const int argc, char *const argv[], char *const *const envp)
0171 {
0172     const char *cmd_path;
0173     char *const *cmd_argv;
0174     int ruleset_fd, abi;
0175     __u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
0176           access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
0177     struct landlock_ruleset_attr ruleset_attr = {
0178         .handled_access_fs = access_fs_rw,
0179     };
0180
0181     if (argc < 2) {
0182         fprintf(stderr,
0183             "usage: %s=\"...\" %s=\"...\" %s <cmd> [args]...\n\n",
0184             ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
0185         fprintf(stderr,
0186             "Launch a command in a restricted environment.\n\n");
0187         fprintf(stderr, "Environment variables containing paths, "
0188                 "each separated by a colon:\n");
0189         fprintf(stderr,
0190             "* %s: list of paths allowed to be used in a read-only way.\n",
0191             ENV_FS_RO_NAME);
0192         fprintf(stderr,
0193             "* %s: list of paths allowed to be used in a read-write way.\n",
0194             ENV_FS_RW_NAME);
0195         fprintf(stderr,
0196             "\nexample:\n"
0197             "%s=\"/bin:/lib:/usr:/proc:/etc:/dev/urandom\" "
0198             "%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
0199             "%s bash -i\n",
0200             ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
0201         return 1;
0202     }
0203
0204     abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
0205     if (abi < 0) {
0206         const int err = errno;
0207
0208         perror("Failed to check Landlock compatibility");
0209         switch (err) {
0210         case ENOSYS:
0211             fprintf(stderr,
0212                 "Hint: Landlock is not supported by the current kernel. "
0213                 "To support it, build the kernel with "
0214                 "CONFIG_SECURITY_LANDLOCK=y and prepend "
0215                 "\"landlock,\" to the content of CONFIG_LSM.\n");
0216             break;
0217         case EOPNOTSUPP:
0218             fprintf(stderr,
0219                 "Hint: Landlock is currently disabled. "
0220                 "It can be enabled in the kernel configuration by "
0221                 "prepending \"landlock,\" to the content of CONFIG_LSM, "
0222                 "or at boot time by setting the same content to the "
0223                 "\"lsm\" kernel parameter.\n");
0224             break;
0225         }
0226         return 1;
0227     }
0228     /* Best-effort security. */
0229     if (abi < 2) {
0230         ruleset_attr.handled_access_fs &= ~ACCESS_ABI_2;
0231         access_fs_ro &= ~ACCESS_ABI_2;
0232         access_fs_rw &= ~ACCESS_ABI_2;
0233     }
0234
0235     ruleset_fd =
0236         landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
0237     if (ruleset_fd < 0) {
0238         perror("Failed to create a ruleset");
0239         return 1;
0240     }
0241     if (populate_ruleset(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
0242         goto err_close_ruleset;
0243     }
0244     if (populate_ruleset(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
0245         goto err_close_ruleset;
0246     }
0247     if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
0248         perror("Failed to restrict privileges");
0249         goto err_close_ruleset;
0250     }
0251     if (landlock_restrict_self(ruleset_fd, 0)) {
0252         perror("Failed to enforce ruleset");
0253         goto err_close_ruleset;
0254     }
0255     close(ruleset_fd);
0256
0257     cmd_path = argv[1];
0258     cmd_argv = argv + 1;
0259     execvpe(cmd_path, cmd_argv, envp);
0260     fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
0261         strerror(errno));
0262     fprintf(stderr, "Hint: access to the binary, the interpreter or "
0263             "shared libraries may be denied.\n");
0264     return 1;
0265
0266 err_close_ruleset:
0267     close(ruleset_fd);
0268     return 1;
0269 }