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// SPDX-License-Identifier: BSD-3-Clause
/*
* Simple Landlock sandbox manager able to launch a process restricted by a
* user-defined filesystem access control policy.
*
* Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
* Copyright © 2020 ANSSI
*/
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <linux/landlock.h>
#include <linux/prctl.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/prctl.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <unistd.h>
#ifndef landlock_create_ruleset
static inline int
landlock_create_ruleset(const struct landlock_ruleset_attr *const attr,
const size_t size, const __u32 flags)
{
return syscall(__NR_landlock_create_ruleset, attr, size, flags);
}
#endif
#ifndef landlock_add_rule
static inline int landlock_add_rule(const int ruleset_fd,
const enum landlock_rule_type rule_type,
const void *const rule_attr,
const __u32 flags)
{
return syscall(__NR_landlock_add_rule, ruleset_fd, rule_type, rule_attr,
flags);
}
#endif
#ifndef landlock_restrict_self
static inline int landlock_restrict_self(const int ruleset_fd,
const __u32 flags)
{
return syscall(__NR_landlock_restrict_self, ruleset_fd, flags);
}
#endif
#define ENV_FS_RO_NAME "LL_FS_RO"
#define ENV_FS_RW_NAME "LL_FS_RW"
#define ENV_PATH_TOKEN ":"
static int parse_path(char *env_path, const char ***const path_list)
{
int i, num_paths = 0;
if (env_path) {
num_paths++;
for (i = 0; env_path[i]; i++) {
if (env_path[i] == ENV_PATH_TOKEN[0])
num_paths++;
}
}
*path_list = malloc(num_paths * sizeof(**path_list));
for (i = 0; i < num_paths; i++)
(*path_list)[i] = strsep(&env_path, ENV_PATH_TOKEN);
return num_paths;
}
/* clang-format off */
#define ACCESS_FILE ( \
LANDLOCK_ACCESS_FS_EXECUTE | \
LANDLOCK_ACCESS_FS_WRITE_FILE | \
LANDLOCK_ACCESS_FS_READ_FILE)
/* clang-format on */
static int populate_ruleset(const char *const env_var, const int ruleset_fd,
const __u64 allowed_access)
{
int num_paths, i, ret = 1;
char *env_path_name;
const char **path_list = NULL;
struct landlock_path_beneath_attr path_beneath = {
.parent_fd = -1,
};
env_path_name = getenv(env_var);
if (!env_path_name) {
/* Prevents users to forget a setting. */
fprintf(stderr, "Missing environment variable %s\n", env_var);
return 1;
}
env_path_name = strdup(env_path_name);
unsetenv(env_var);
num_paths = parse_path(env_path_name, &path_list);
if (num_paths == 1 && path_list[0][0] == '\0') {
/*
* Allows to not use all possible restrictions (e.g. use
* LL_FS_RO without LL_FS_RW).
*/
ret = 0;
goto out_free_name;
}
for (i = 0; i < num_paths; i++) {
struct stat statbuf;
path_beneath.parent_fd = open(path_list[i], O_PATH | O_CLOEXEC);
if (path_beneath.parent_fd < 0) {
fprintf(stderr, "Failed to open \"%s\": %s\n",
path_list[i], strerror(errno));
goto out_free_name;
}
if (fstat(path_beneath.parent_fd, &statbuf)) {
close(path_beneath.parent_fd);
goto out_free_name;
}
path_beneath.allowed_access = allowed_access;
if (!S_ISDIR(statbuf.st_mode))
path_beneath.allowed_access &= ACCESS_FILE;
if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
&path_beneath, 0)) {
fprintf(stderr,
"Failed to update the ruleset with \"%s\": %s\n",
path_list[i], strerror(errno));
close(path_beneath.parent_fd);
goto out_free_name;
}
close(path_beneath.parent_fd);
}
ret = 0;
out_free_name:
free(path_list);
free(env_path_name);
return ret;
}
/* clang-format off */
#define ACCESS_FS_ROUGHLY_READ ( \
LANDLOCK_ACCESS_FS_EXECUTE | \
LANDLOCK_ACCESS_FS_READ_FILE | \
LANDLOCK_ACCESS_FS_READ_DIR)
#define ACCESS_FS_ROUGHLY_WRITE ( \
LANDLOCK_ACCESS_FS_WRITE_FILE | \
LANDLOCK_ACCESS_FS_REMOVE_DIR | \
LANDLOCK_ACCESS_FS_REMOVE_FILE | \
LANDLOCK_ACCESS_FS_MAKE_CHAR | \
LANDLOCK_ACCESS_FS_MAKE_DIR | \
LANDLOCK_ACCESS_FS_MAKE_REG | \
LANDLOCK_ACCESS_FS_MAKE_SOCK | \
LANDLOCK_ACCESS_FS_MAKE_FIFO | \
LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
LANDLOCK_ACCESS_FS_MAKE_SYM | \
LANDLOCK_ACCESS_FS_REFER)
#define ACCESS_ABI_2 ( \
LANDLOCK_ACCESS_FS_REFER)
/* clang-format on */
int main(const int argc, char *const argv[], char *const *const envp)
{
const char *cmd_path;
char *const *cmd_argv;
int ruleset_fd, abi;
__u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
struct landlock_ruleset_attr ruleset_attr = {
.handled_access_fs = access_fs_rw,
};
if (argc < 2) {
fprintf(stderr,
"usage: %s=\"...\" %s=\"...\" %s <cmd> [args]...\n\n",
ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
fprintf(stderr,
"Launch a command in a restricted environment.\n\n");
fprintf(stderr, "Environment variables containing paths, "
"each separated by a colon:\n");
fprintf(stderr,
"* %s: list of paths allowed to be used in a read-only way.\n",
ENV_FS_RO_NAME);
fprintf(stderr,
"* %s: list of paths allowed to be used in a read-write way.\n",
ENV_FS_RW_NAME);
fprintf(stderr,
"\nexample:\n"
"%s=\"/bin:/lib:/usr:/proc:/etc:/dev/urandom\" "
"%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
"%s bash -i\n",
ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
return 1;
}
abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
if (abi < 0) {
const int err = errno;
perror("Failed to check Landlock compatibility");
switch (err) {
case ENOSYS:
fprintf(stderr,
"Hint: Landlock is not supported by the current kernel. "
"To support it, build the kernel with "
"CONFIG_SECURITY_LANDLOCK=y and prepend "
"\"landlock,\" to the content of CONFIG_LSM.\n");
break;
case EOPNOTSUPP:
fprintf(stderr,
"Hint: Landlock is currently disabled. "
"It can be enabled in the kernel configuration by "
"prepending \"landlock,\" to the content of CONFIG_LSM, "
"or at boot time by setting the same content to the "
"\"lsm\" kernel parameter.\n");
break;
}
return 1;
}
/* Best-effort security. */
if (abi < 2) {
ruleset_attr.handled_access_fs &= ~ACCESS_ABI_2;
access_fs_ro &= ~ACCESS_ABI_2;
access_fs_rw &= ~ACCESS_ABI_2;
}
ruleset_fd =
landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
if (ruleset_fd < 0) {
perror("Failed to create a ruleset");
return 1;
}
if (populate_ruleset(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
goto err_close_ruleset;
}
if (populate_ruleset(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
goto err_close_ruleset;
}
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
perror("Failed to restrict privileges");
goto err_close_ruleset;
}
if (landlock_restrict_self(ruleset_fd, 0)) {
perror("Failed to enforce ruleset");
goto err_close_ruleset;
}
close(ruleset_fd);
cmd_path = argv[1];
cmd_argv = argv + 1;
execvpe(cmd_path, cmd_argv, envp);
fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
strerror(errno));
fprintf(stderr, "Hint: access to the binary, the interpreter or "
"shared libraries may be denied.\n");
return 1;
err_close_ruleset:
close(ruleset_fd);
return 1;
}
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