312 lines
8.4 KiB
C
312 lines
8.4 KiB
C
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// SPDX-License-Identifier: BSD-3-Clause
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/*
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* Simple Landlock sandbox manager able to launch a process restricted by a
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* user-defined filesystem access control policy.
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*
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* Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
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* Copyright © 2020 ANSSI
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*/
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#define _GNU_SOURCE
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#include <errno.h>
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#include <fcntl.h>
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#include <linux/landlock.h>
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#include <linux/prctl.h>
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#include <stddef.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <sys/prctl.h>
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#include <sys/stat.h>
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#include <sys/syscall.h>
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#include <unistd.h>
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#ifndef landlock_create_ruleset
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static inline int
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landlock_create_ruleset(const struct landlock_ruleset_attr *const attr,
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const size_t size, const __u32 flags)
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{
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return syscall(__NR_landlock_create_ruleset, attr, size, flags);
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}
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#endif
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#ifndef landlock_add_rule
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static inline int landlock_add_rule(const int ruleset_fd,
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const enum landlock_rule_type rule_type,
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const void *const rule_attr,
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const __u32 flags)
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{
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return syscall(__NR_landlock_add_rule, ruleset_fd, rule_type, rule_attr,
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flags);
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}
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#endif
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#ifndef landlock_restrict_self
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static inline int landlock_restrict_self(const int ruleset_fd,
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const __u32 flags)
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{
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return syscall(__NR_landlock_restrict_self, ruleset_fd, flags);
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}
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#endif
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#define ENV_FS_RO_NAME "LL_FS_RO"
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#define ENV_FS_RW_NAME "LL_FS_RW"
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#define ENV_PATH_TOKEN ":"
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static int parse_path(char *env_path, const char ***const path_list)
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{
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int i, num_paths = 0;
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if (env_path) {
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num_paths++;
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for (i = 0; env_path[i]; i++) {
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if (env_path[i] == ENV_PATH_TOKEN[0])
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num_paths++;
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}
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}
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*path_list = malloc(num_paths * sizeof(**path_list));
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for (i = 0; i < num_paths; i++)
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(*path_list)[i] = strsep(&env_path, ENV_PATH_TOKEN);
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return num_paths;
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}
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/* clang-format off */
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#define ACCESS_FILE ( \
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LANDLOCK_ACCESS_FS_EXECUTE | \
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LANDLOCK_ACCESS_FS_WRITE_FILE | \
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LANDLOCK_ACCESS_FS_READ_FILE | \
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LANDLOCK_ACCESS_FS_TRUNCATE)
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/* clang-format on */
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static int populate_ruleset(const char *const env_var, const int ruleset_fd,
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const __u64 allowed_access)
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{
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int num_paths, i, ret = 1;
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char *env_path_name;
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const char **path_list = NULL;
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struct landlock_path_beneath_attr path_beneath = {
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.parent_fd = -1,
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};
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env_path_name = getenv(env_var);
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if (!env_path_name) {
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/* Prevents users to forget a setting. */
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fprintf(stderr, "Missing environment variable %s\n", env_var);
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return 1;
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}
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env_path_name = strdup(env_path_name);
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unsetenv(env_var);
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num_paths = parse_path(env_path_name, &path_list);
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if (num_paths == 1 && path_list[0][0] == '\0') {
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/*
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* Allows to not use all possible restrictions (e.g. use
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* LL_FS_RO without LL_FS_RW).
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*/
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ret = 0;
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goto out_free_name;
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}
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for (i = 0; i < num_paths; i++) {
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struct stat statbuf;
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path_beneath.parent_fd = open(path_list[i], O_PATH | O_CLOEXEC);
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if (path_beneath.parent_fd < 0) {
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fprintf(stderr, "Failed to open \"%s\": %s\n",
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path_list[i], strerror(errno));
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goto out_free_name;
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}
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if (fstat(path_beneath.parent_fd, &statbuf)) {
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close(path_beneath.parent_fd);
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goto out_free_name;
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}
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path_beneath.allowed_access = allowed_access;
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if (!S_ISDIR(statbuf.st_mode))
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path_beneath.allowed_access &= ACCESS_FILE;
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if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
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&path_beneath, 0)) {
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fprintf(stderr,
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"Failed to update the ruleset with \"%s\": %s\n",
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path_list[i], strerror(errno));
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close(path_beneath.parent_fd);
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goto out_free_name;
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}
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close(path_beneath.parent_fd);
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}
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ret = 0;
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out_free_name:
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free(path_list);
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free(env_path_name);
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return ret;
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}
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/* clang-format off */
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#define ACCESS_FS_ROUGHLY_READ ( \
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LANDLOCK_ACCESS_FS_EXECUTE | \
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LANDLOCK_ACCESS_FS_READ_FILE | \
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LANDLOCK_ACCESS_FS_READ_DIR)
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#define ACCESS_FS_ROUGHLY_WRITE ( \
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LANDLOCK_ACCESS_FS_WRITE_FILE | \
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LANDLOCK_ACCESS_FS_REMOVE_DIR | \
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LANDLOCK_ACCESS_FS_REMOVE_FILE | \
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LANDLOCK_ACCESS_FS_MAKE_CHAR | \
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LANDLOCK_ACCESS_FS_MAKE_DIR | \
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LANDLOCK_ACCESS_FS_MAKE_REG | \
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LANDLOCK_ACCESS_FS_MAKE_SOCK | \
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LANDLOCK_ACCESS_FS_MAKE_FIFO | \
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LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
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LANDLOCK_ACCESS_FS_MAKE_SYM | \
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LANDLOCK_ACCESS_FS_REFER | \
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LANDLOCK_ACCESS_FS_TRUNCATE)
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/* clang-format on */
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#define LANDLOCK_ABI_LAST 3
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int main(const int argc, char *const argv[], char *const *const envp)
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{
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const char *cmd_path;
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char *const *cmd_argv;
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int ruleset_fd, abi;
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__u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
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access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
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struct landlock_ruleset_attr ruleset_attr = {
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.handled_access_fs = access_fs_rw,
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};
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if (argc < 2) {
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fprintf(stderr,
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"usage: %s=\"...\" %s=\"...\" %s <cmd> [args]...\n\n",
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ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
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fprintf(stderr,
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"Launch a command in a restricted environment.\n\n");
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fprintf(stderr, "Environment variables containing paths, "
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"each separated by a colon:\n");
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fprintf(stderr,
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"* %s: list of paths allowed to be used in a read-only way.\n",
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ENV_FS_RO_NAME);
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fprintf(stderr,
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"* %s: list of paths allowed to be used in a read-write way.\n",
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ENV_FS_RW_NAME);
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fprintf(stderr,
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"\nexample:\n"
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"%s=\"/bin:/lib:/usr:/proc:/etc:/dev/urandom\" "
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"%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
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"%s bash -i\n\n",
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ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
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fprintf(stderr,
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"This sandboxer can use Landlock features "
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"up to ABI version %d.\n",
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LANDLOCK_ABI_LAST);
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return 1;
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}
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abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
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if (abi < 0) {
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const int err = errno;
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perror("Failed to check Landlock compatibility");
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switch (err) {
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case ENOSYS:
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fprintf(stderr,
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"Hint: Landlock is not supported by the current kernel. "
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"To support it, build the kernel with "
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"CONFIG_SECURITY_LANDLOCK=y and prepend "
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"\"landlock,\" to the content of CONFIG_LSM.\n");
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break;
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case EOPNOTSUPP:
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fprintf(stderr,
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"Hint: Landlock is currently disabled. "
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"It can be enabled in the kernel configuration by "
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"prepending \"landlock,\" to the content of CONFIG_LSM, "
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"or at boot time by setting the same content to the "
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"\"lsm\" kernel parameter.\n");
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break;
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}
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return 1;
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}
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/* Best-effort security. */
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switch (abi) {
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case 1:
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/*
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* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
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*
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* Note: The "refer" operations (file renaming and linking
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* across different directories) are always forbidden when using
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* Landlock with ABI 1.
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*
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* If only ABI 1 is available, this sandboxer knowingly forbids
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* refer operations.
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*
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* If a program *needs* to do refer operations after enabling
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* Landlock, it can not use Landlock at ABI level 1. To be
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* compatible with different kernel versions, such programs
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* should then fall back to not restrict themselves at all if
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* the running kernel only supports ABI 1.
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*/
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ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
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__attribute__((fallthrough));
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case 2:
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/* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
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ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
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fprintf(stderr,
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"Hint: You should update the running kernel "
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"to leverage Landlock features "
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"provided by ABI version %d (instead of %d).\n",
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LANDLOCK_ABI_LAST, abi);
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__attribute__((fallthrough));
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case LANDLOCK_ABI_LAST:
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break;
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default:
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fprintf(stderr,
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"Hint: You should update this sandboxer "
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"to leverage Landlock features "
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"provided by ABI version %d (instead of %d).\n",
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abi, LANDLOCK_ABI_LAST);
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}
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access_fs_ro &= ruleset_attr.handled_access_fs;
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access_fs_rw &= ruleset_attr.handled_access_fs;
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ruleset_fd =
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landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
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if (ruleset_fd < 0) {
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perror("Failed to create a ruleset");
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return 1;
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}
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if (populate_ruleset(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
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goto err_close_ruleset;
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}
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if (populate_ruleset(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
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goto err_close_ruleset;
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}
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if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
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perror("Failed to restrict privileges");
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goto err_close_ruleset;
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}
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if (landlock_restrict_self(ruleset_fd, 0)) {
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perror("Failed to enforce ruleset");
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goto err_close_ruleset;
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}
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close(ruleset_fd);
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cmd_path = argv[1];
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cmd_argv = argv + 1;
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execvpe(cmd_path, cmd_argv, envp);
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fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
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strerror(errno));
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fprintf(stderr, "Hint: access to the binary, the interpreter or "
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"shared libraries may be denied.\n");
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return 1;
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err_close_ruleset:
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close(ruleset_fd);
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return 1;
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}
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