sandboxer.c

// 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/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>

#if defined(__GLIBC__)
#include <linux/prctl.h>
#endif

#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_FORCE_LOG_NAME "LL_FORCE_LOG"
#define ENV_DELIMITER ":"

static int str2num(const char *numstr, __u64 *num_dst)
{
	char *endptr = NULL;
	int err = 0;
	__u64 num;

	errno = 0;
	num = strtoull(numstr, &endptr, 10);
	if (errno != 0)
		err = errno;
	/* Was the string empty, or not entirely parsed successfully? */
	else if ((*numstr == '\0') || (*endptr != '\0'))
		err = EINVAL;
	else
		*num_dst = num;

	return err;
}

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));
	if (!*path_list)
		return -1;

	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 < 0) {
		fprintf(stderr, "Failed to allocate memory\n");
		goto out_free_name;
	}
	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,
	};

	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))) {
		__u64 port;

		if (strcmp(strport, "") == 0)
			continue;

		if (str2num(strport, &port)) {
			fprintf(stderr, "Failed to parse port at \"%s\"\n",
				strport);
			goto out_free_name;
		}
		net_port.port = port;
		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 7

#define XSTR(s) #s
#define STR(s) XSTR(s)

/* clang-format off */

static const char help[] =
	"usage: " ENV_FS_RO_NAME "=\"...\" " ENV_FS_RW_NAME "=\"...\" "
	"[other environment variables] %1$s <cmd> [args]...\n"
	"\n"
	"Execute the given command in a restricted environment.\n"
	"Multi-valued settings (lists of ports, paths, scopes) are colon-delimited.\n"
	"\n"
	"Mandatory settings:\n"
	"* " ENV_FS_RO_NAME ": paths allowed to be used in a read-only way\n"
	"* " ENV_FS_RW_NAME ": paths allowed to be used in a read-write way\n"
	"\n"
	"Optional settings (when not set, their associated access check "
	"is always allowed, which is different from an empty string which "
	"means an empty list):\n"
	"* " ENV_TCP_BIND_NAME ": ports allowed to bind (server)\n"
	"* " ENV_TCP_CONNECT_NAME ": ports allowed to connect (client)\n"
	"* " ENV_SCOPED_NAME ": actions denied on the outside of the landlock domain\n"
	"  - \"a\" to restrict opening abstract unix sockets\n"
	"  - \"s\" to restrict sending signals\n"
	"\n"
	"A sandboxer should not log denied access requests to avoid spamming logs, "
	"but to test audit we can set " ENV_FORCE_LOG_NAME "=1\n"
	"\n"
	"Example:\n"
	ENV_FS_RO_NAME "=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
	ENV_FS_RW_NAME "=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
	ENV_TCP_BIND_NAME "=\"9418\" "
	ENV_TCP_CONNECT_NAME "=\"80:443\" "
	ENV_SCOPED_NAME "=\"a:s\" "
	"%1$s bash -i\n"
	"\n"
	"This sandboxer can use Landlock features up to ABI version "
	STR(LANDLOCK_ABI_LAST) ".\n";

/* 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;
	char *env_port_name, *env_force_log;
	__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,
	};
	int supported_restrict_flags = LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
	int set_restrict_flags = 0;

	if (argc < 2) {
		fprintf(stderr, help, 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. */
	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);
		__attribute__((fallthrough));
	case 6:
		/* Removes LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON for ABI < 7 */
		supported_restrict_flags &=
			~LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;

		/* Must be printed for any ABI < LANDLOCK_ABI_LAST. */
		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;

	/* Enables optional logs. */
	env_force_log = getenv(ENV_FORCE_LOG_NAME);
	if (env_force_log) {
		if (strcmp(env_force_log, "1") != 0) {
			fprintf(stderr, "Unknown value for " ENV_FORCE_LOG_NAME
					" (only \"1\" is handled)\n");
			return 1;
		}
		if (!(supported_restrict_flags &
		      LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON)) {
			fprintf(stderr,
				"Audit logs not supported by current kernel\n");
			return 1;
		}
		set_restrict_flags |= LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
		unsetenv(ENV_FORCE_LOG_NAME);
	}

	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, set_restrict_flags)) {
		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;
}