// SPDX-License-Identifier: BSD-3-Clause
/*
* Simple Landlock sandbox manager able to execute a process restricted by
* user-defined file system and network access control policies.
*
* Copyright © 2017-2020 Mickaël Salaün <[email protected]>
* Copyright © 2020 ANSSI
*/
#define _GNU_SOURCE
#define __SANE_USERSPACE_TYPES__
#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>
#include <linux/landlock.h>
#include <linux/prctl.h>
#include <linux/socket.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>
#include <stdbool.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_TCP_BIND_NAME "LL_TCP_BIND"
#define ENV_TCP_CONNECT_NAME "LL_TCP_CONNECT"
#define ENV_SCOPED_NAME "LL_SCOPED"
#define ENV_DELIMITER ":"
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_DELIMITER[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_DELIMITER);
return num_paths;
}
/* clang-format off */
#define ACCESS_FILE ( \
LANDLOCK_ACCESS_FS_EXECUTE | \
LANDLOCK_ACCESS_FS_WRITE_FILE | \
LANDLOCK_ACCESS_FS_READ_FILE | \
LANDLOCK_ACCESS_FS_TRUNCATE | \
LANDLOCK_ACCESS_FS_IOCTL_DEV)
/* clang-format on */
static int populate_ruleset_fs(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));
continue;
}
if (fstat(path_beneath.parent_fd, &statbuf)) {
fprintf(stderr, "Failed to stat \"%s\": %s\n",
path_list[i], strerror(errno));
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;
}
static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
const __u64 allowed_access)
{
int ret = 1;
char *env_port_name, *env_port_name_next, *strport;
struct landlock_net_port_attr net_port = {
.allowed_access = allowed_access,
.port = 0,
};
env_port_name = getenv(env_var);
if (!env_port_name)
return 0;
env_port_name = strdup(env_port_name);
unsetenv(env_var);
env_port_name_next = env_port_name;
while ((strport = strsep(&env_port_name_next, ENV_DELIMITER))) {
net_port.port = atoi(strport);
if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT,
&net_port, 0)) {
fprintf(stderr,
"Failed to update the ruleset with port \"%llu\": %s\n",
net_port.port, strerror(errno));
goto out_free_name;
}
}
ret = 0;
out_free_name:
free(env_port_name);
return ret;
}
/* Returns true on error, false otherwise. */
static bool check_ruleset_scope(const char *const env_var,
struct landlock_ruleset_attr *ruleset_attr)
{
char *env_type_scope, *env_type_scope_next, *ipc_scoping_name;
bool error = false;
bool abstract_scoping = false;
bool signal_scoping = false;
/* Scoping is not supported by Landlock ABI */
if (!(ruleset_attr->scoped &
(LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET | LANDLOCK_SCOPE_SIGNAL)))
goto out_unset;
env_type_scope = getenv(env_var);
/* Scoping is not supported by the user */
if (!env_type_scope || strcmp("", env_type_scope) == 0)
goto out_unset;
env_type_scope = strdup(env_type_scope);
env_type_scope_next = env_type_scope;
while ((ipc_scoping_name =
strsep(&env_type_scope_next, ENV_DELIMITER))) {
if (strcmp("a", ipc_scoping_name) == 0 && !abstract_scoping) {
abstract_scoping = true;
} else if (strcmp("s", ipc_scoping_name) == 0 &&
!signal_scoping) {
signal_scoping = true;
} else {
fprintf(stderr, "Unknown or duplicate scope \"%s\"\n",
ipc_scoping_name);
error = true;
goto out_free_name;
}
}
out_free_name:
free(env_type_scope);
out_unset:
if (!abstract_scoping)
ruleset_attr->scoped &= ~LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET;
if (!signal_scoping)
ruleset_attr->scoped &= ~LANDLOCK_SCOPE_SIGNAL;
unsetenv(env_var);
return error;
}
/* 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 | \
LANDLOCK_ACCESS_FS_TRUNCATE | \
LANDLOCK_ACCESS_FS_IOCTL_DEV)
/* clang-format on */
#define LANDLOCK_ABI_LAST 6
int main(const int argc, char *const argv[], char *const *const envp)
{
const char *cmd_path;
char *const *cmd_argv;
int ruleset_fd, abi;
char *env_port_name;
__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,
.handled_access_net = LANDLOCK_ACCESS_NET_BIND_TCP |
LANDLOCK_ACCESS_NET_CONNECT_TCP,
.scoped = LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
LANDLOCK_SCOPE_SIGNAL,
};
if (argc < 2) {
fprintf(stderr,
"usage: %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\" %s "
"<cmd> [args]...\n\n",
ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
ENV_TCP_CONNECT_NAME, ENV_SCOPED_NAME, argv[0]);
fprintf(stderr,
"Execute a command in a restricted environment.\n\n");
fprintf(stderr,
"Environment variables containing paths and ports "
"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\n",
ENV_FS_RW_NAME);
fprintf(stderr,
"Environment variables containing ports are optional "
"and could be skipped.\n");
fprintf(stderr,
"* %s: list of ports allowed to bind (server).\n",
ENV_TCP_BIND_NAME);
fprintf(stderr,
"* %s: list of ports allowed to connect (client).\n",
ENV_TCP_CONNECT_NAME);
fprintf(stderr, "* %s: list of scoped IPCs.\n",
ENV_SCOPED_NAME);
fprintf(stderr,
"\nexample:\n"
"%s=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
"%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
"%s=\"9418\" "
"%s=\"80:443\" "
"%s=\"a:s\" "
"%s bash -i\n\n",
ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
ENV_TCP_CONNECT_NAME, ENV_SCOPED_NAME, argv[0]);
fprintf(stderr,
"This sandboxer can use Landlock features "
"up to ABI version %d.\n",
LANDLOCK_ABI_LAST);
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. */
switch (abi) {
case 1:
/*
* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
*
* Note: The "refer" operations (file renaming and linking
* across different directories) are always forbidden when using
* Landlock with ABI 1.
*
* If only ABI 1 is available, this sandboxer knowingly forbids
* refer operations.
*
* If a program *needs* to do refer operations after enabling
* Landlock, it can not use Landlock at ABI level 1. To be
* compatible with different kernel versions, such programs
* should then fall back to not restrict themselves at all if
* the running kernel only supports ABI 1.
*/
ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
__attribute__((fallthrough));
case 2:
/* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
__attribute__((fallthrough));
case 3:
/* Removes network support for ABI < 4 */
ruleset_attr.handled_access_net &=
~(LANDLOCK_ACCESS_NET_BIND_TCP |
LANDLOCK_ACCESS_NET_CONNECT_TCP);
__attribute__((fallthrough));
case 4:
/* Removes LANDLOCK_ACCESS_FS_IOCTL_DEV for ABI < 5 */
ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_IOCTL_DEV;
__attribute__((fallthrough));
case 5:
/* Removes LANDLOCK_SCOPE_* for ABI < 6 */
ruleset_attr.scoped &= ~(LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
LANDLOCK_SCOPE_SIGNAL);
fprintf(stderr,
"Hint: You should update the running kernel "
"to leverage Landlock features "
"provided by ABI version %d (instead of %d).\n",
LANDLOCK_ABI_LAST, abi);
__attribute__((fallthrough));
case LANDLOCK_ABI_LAST:
break;
default:
fprintf(stderr,
"Hint: You should update this sandboxer "
"to leverage Landlock features "
"provided by ABI version %d (instead of %d).\n",
abi, LANDLOCK_ABI_LAST);
}
access_fs_ro &= ruleset_attr.handled_access_fs;
access_fs_rw &= ruleset_attr.handled_access_fs;
/* Removes bind access attribute if not supported by a user. */
env_port_name = getenv(ENV_TCP_BIND_NAME);
if (!env_port_name) {
ruleset_attr.handled_access_net &=
~LANDLOCK_ACCESS_NET_BIND_TCP;
}
/* Removes connect access attribute if not supported by a user. */
env_port_name = getenv(ENV_TCP_CONNECT_NAME);
if (!env_port_name) {
ruleset_attr.handled_access_net &=
~LANDLOCK_ACCESS_NET_CONNECT_TCP;
}
if (check_ruleset_scope(ENV_SCOPED_NAME, &ruleset_attr))
return 1;
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_fs(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
goto err_close_ruleset;
}
if (populate_ruleset_fs(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
goto err_close_ruleset;
}
if (populate_ruleset_net(ENV_TCP_BIND_NAME, ruleset_fd,
LANDLOCK_ACCESS_NET_BIND_TCP)) {
goto err_close_ruleset;
}
if (populate_ruleset_net(ENV_TCP_CONNECT_NAME, ruleset_fd,
LANDLOCK_ACCESS_NET_CONNECT_TCP)) {
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;
fprintf(stderr, "Executing the sandboxed command...\n");
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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