sandboxer.c

  1// SPDX-License-Identifier: BSD-3-Clause
  2/*
  3 * Simple Landlock sandbox manager able to execute a process restricted by
  4 * user-defined file system and network access control policies.
  5 *
  6 * Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
  7 * Copyright © 2020 ANSSI
  8 */
  9
 10#define _GNU_SOURCE
 11#define __SANE_USERSPACE_TYPES__
 12#include <arpa/inet.h>
 13#include <errno.h>
 14#include <fcntl.h>
 15#include <linux/landlock.h>
 16#include <linux/socket.h>
 17#include <stddef.h>
 18#include <stdio.h>
 19#include <stdlib.h>
 20#include <string.h>
 21#include <sys/prctl.h>
 22#include <sys/stat.h>
 23#include <sys/syscall.h>
 24#include <unistd.h>
 25#include <stdbool.h>
 26
 27#if defined(__GLIBC__)
 28#include <linux/prctl.h>
 29#endif
 30
 31#ifndef landlock_create_ruleset
 32static inline int
 33landlock_create_ruleset(const struct landlock_ruleset_attr *const attr,
 34			const size_t size, const __u32 flags)
 35{
 36	return syscall(__NR_landlock_create_ruleset, attr, size, flags);
 37}
 38#endif
 39
 40#ifndef landlock_add_rule
 41static inline int landlock_add_rule(const int ruleset_fd,
 42				    const enum landlock_rule_type rule_type,
 43				    const void *const rule_attr,
 44				    const __u32 flags)
 45{
 46	return syscall(__NR_landlock_add_rule, ruleset_fd, rule_type, rule_attr,
 47		       flags);
 48}
 49#endif
 50
 51#ifndef landlock_restrict_self
 52static inline int landlock_restrict_self(const int ruleset_fd,
 53					 const __u32 flags)
 54{
 55	return syscall(__NR_landlock_restrict_self, ruleset_fd, flags);
 56}
 57#endif
 58
 59#define ENV_FS_RO_NAME "LL_FS_RO"
 60#define ENV_FS_RW_NAME "LL_FS_RW"
 61#define ENV_TCP_BIND_NAME "LL_TCP_BIND"
 62#define ENV_TCP_CONNECT_NAME "LL_TCP_CONNECT"
 63#define ENV_SCOPED_NAME "LL_SCOPED"
 64#define ENV_FORCE_LOG_NAME "LL_FORCE_LOG"
 65#define ENV_DELIMITER ":"
 66
 67static int str2num(const char *numstr, __u64 *num_dst)
 68{
 69	char *endptr = NULL;
 70	int err = 0;
 71	__u64 num;
 72
 73	errno = 0;
 74	num = strtoull(numstr, &endptr, 10);
 75	if (errno != 0)
 76		err = errno;
 77	/* Was the string empty, or not entirely parsed successfully? */
 78	else if ((*numstr == '\0') || (*endptr != '\0'))
 79		err = EINVAL;
 80	else
 81		*num_dst = num;
 82
 83	return err;
 84}
 85
 86static int parse_path(char *env_path, const char ***const path_list)
 87{
 88	int i, num_paths = 0;
 89
 90	if (env_path) {
 91		num_paths++;
 92		for (i = 0; env_path[i]; i++) {
 93			if (env_path[i] == ENV_DELIMITER[0])
 94				num_paths++;
 95		}
 96	}
 97	*path_list = malloc(num_paths * sizeof(**path_list));
 98	if (!*path_list)
 99		return -1;
100
101	for (i = 0; i < num_paths; i++)
102		(*path_list)[i] = strsep(&env_path, ENV_DELIMITER);
103
104	return num_paths;
105}
106
107/* clang-format off */
108
109#define ACCESS_FILE ( \
110	LANDLOCK_ACCESS_FS_EXECUTE | \
111	LANDLOCK_ACCESS_FS_WRITE_FILE | \
112	LANDLOCK_ACCESS_FS_READ_FILE | \
113	LANDLOCK_ACCESS_FS_TRUNCATE | \
114	LANDLOCK_ACCESS_FS_IOCTL_DEV)
115
116/* clang-format on */
117
118static int populate_ruleset_fs(const char *const env_var, const int ruleset_fd,
119			       const __u64 allowed_access)
120{
121	int num_paths, i, ret = 1;
122	char *env_path_name;
123	const char **path_list = NULL;
124	struct landlock_path_beneath_attr path_beneath = {
125		.parent_fd = -1,
126	};
127
128	env_path_name = getenv(env_var);
129	if (!env_path_name) {
130		/* Prevents users to forget a setting. */
131		fprintf(stderr, "Missing environment variable %s\n", env_var);
132		return 1;
133	}
134	env_path_name = strdup(env_path_name);
135	unsetenv(env_var);
136	num_paths = parse_path(env_path_name, &path_list);
137	if (num_paths < 0) {
138		fprintf(stderr, "Failed to allocate memory\n");
139		goto out_free_name;
140	}
141	if (num_paths == 1 && path_list[0][0] == '\0') {
142		/*
143		 * Allows to not use all possible restrictions (e.g. use
144		 * LL_FS_RO without LL_FS_RW).
145		 */
146		ret = 0;
147		goto out_free_name;
148	}
149
150	for (i = 0; i < num_paths; i++) {
151		struct stat statbuf;
152
153		path_beneath.parent_fd = open(path_list[i], O_PATH | O_CLOEXEC);
154		if (path_beneath.parent_fd < 0) {
155			fprintf(stderr, "Failed to open \"%s\": %s\n",
156				path_list[i], strerror(errno));
157			continue;
158		}
159		if (fstat(path_beneath.parent_fd, &statbuf)) {
160			fprintf(stderr, "Failed to stat \"%s\": %s\n",
161				path_list[i], strerror(errno));
162			close(path_beneath.parent_fd);
163			goto out_free_name;
164		}
165		path_beneath.allowed_access = allowed_access;
166		if (!S_ISDIR(statbuf.st_mode))
167			path_beneath.allowed_access &= ACCESS_FILE;
168		if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
169				      &path_beneath, 0)) {
170			fprintf(stderr,
171				"Failed to update the ruleset with \"%s\": %s\n",
172				path_list[i], strerror(errno));
173			close(path_beneath.parent_fd);
174			goto out_free_name;
175		}
176		close(path_beneath.parent_fd);
177	}
178	ret = 0;
179
180out_free_name:
181	free(path_list);
182	free(env_path_name);
183	return ret;
184}
185
186static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
187				const __u64 allowed_access)
188{
189	int ret = 1;
190	char *env_port_name, *env_port_name_next, *strport;
191	struct landlock_net_port_attr net_port = {
192		.allowed_access = allowed_access,
193	};
194
195	env_port_name = getenv(env_var);
196	if (!env_port_name)
197		return 0;
198	env_port_name = strdup(env_port_name);
199	unsetenv(env_var);
200
201	env_port_name_next = env_port_name;
202	while ((strport = strsep(&env_port_name_next, ENV_DELIMITER))) {
203		__u64 port;
204
205		if (strcmp(strport, "") == 0)
206			continue;
207
208		if (str2num(strport, &port)) {
209			fprintf(stderr, "Failed to parse port at \"%s\"\n",
210				strport);
211			goto out_free_name;
212		}
213		net_port.port = port;
214		if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_PORT,
215				      &net_port, 0)) {
216			fprintf(stderr,
217				"Failed to update the ruleset with port \"%llu\": %s\n",
218				net_port.port, strerror(errno));
219			goto out_free_name;
220		}
221	}
222	ret = 0;
223
224out_free_name:
225	free(env_port_name);
226	return ret;
227}
228
229/* Returns true on error, false otherwise. */
230static bool check_ruleset_scope(const char *const env_var,
231				struct landlock_ruleset_attr *ruleset_attr)
232{
233	char *env_type_scope, *env_type_scope_next, *ipc_scoping_name;
234	bool error = false;
235	bool abstract_scoping = false;
236	bool signal_scoping = false;
237
238	/* Scoping is not supported by Landlock ABI */
239	if (!(ruleset_attr->scoped &
240	      (LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET | LANDLOCK_SCOPE_SIGNAL)))
241		goto out_unset;
242
243	env_type_scope = getenv(env_var);
244	/* Scoping is not supported by the user */
245	if (!env_type_scope || strcmp("", env_type_scope) == 0)
246		goto out_unset;
247
248	env_type_scope = strdup(env_type_scope);
249	env_type_scope_next = env_type_scope;
250	while ((ipc_scoping_name =
251			strsep(&env_type_scope_next, ENV_DELIMITER))) {
252		if (strcmp("a", ipc_scoping_name) == 0 && !abstract_scoping) {
253			abstract_scoping = true;
254		} else if (strcmp("s", ipc_scoping_name) == 0 &&
255			   !signal_scoping) {
256			signal_scoping = true;
257		} else {
258			fprintf(stderr, "Unknown or duplicate scope \"%s\"\n",
259				ipc_scoping_name);
260			error = true;
261			goto out_free_name;
262		}
263	}
264
265out_free_name:
266	free(env_type_scope);
267
268out_unset:
269	if (!abstract_scoping)
270		ruleset_attr->scoped &= ~LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET;
271	if (!signal_scoping)
272		ruleset_attr->scoped &= ~LANDLOCK_SCOPE_SIGNAL;
273
274	unsetenv(env_var);
275	return error;
276}
277
278/* clang-format off */
279
280#define ACCESS_FS_ROUGHLY_READ ( \
281	LANDLOCK_ACCESS_FS_EXECUTE | \
282	LANDLOCK_ACCESS_FS_READ_FILE | \
283	LANDLOCK_ACCESS_FS_READ_DIR)
284
285#define ACCESS_FS_ROUGHLY_WRITE ( \
286	LANDLOCK_ACCESS_FS_WRITE_FILE | \
287	LANDLOCK_ACCESS_FS_REMOVE_DIR | \
288	LANDLOCK_ACCESS_FS_REMOVE_FILE | \
289	LANDLOCK_ACCESS_FS_MAKE_CHAR | \
290	LANDLOCK_ACCESS_FS_MAKE_DIR | \
291	LANDLOCK_ACCESS_FS_MAKE_REG | \
292	LANDLOCK_ACCESS_FS_MAKE_SOCK | \
293	LANDLOCK_ACCESS_FS_MAKE_FIFO | \
294	LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
295	LANDLOCK_ACCESS_FS_MAKE_SYM | \
296	LANDLOCK_ACCESS_FS_REFER | \
297	LANDLOCK_ACCESS_FS_TRUNCATE | \
298	LANDLOCK_ACCESS_FS_IOCTL_DEV)
299
300/* clang-format on */
301
302#define LANDLOCK_ABI_LAST 7
303
304#define XSTR(s) #s
305#define STR(s) XSTR(s)
306
307/* clang-format off */
308
309static const char help[] =
310	"usage: " ENV_FS_RO_NAME "=\"...\" " ENV_FS_RW_NAME "=\"...\" "
311	"[other environment variables] %1$s <cmd> [args]...\n"
312	"\n"
313	"Execute the given command in a restricted environment.\n"
314	"Multi-valued settings (lists of ports, paths, scopes) are colon-delimited.\n"
315	"\n"
316	"Mandatory settings:\n"
317	"* " ENV_FS_RO_NAME ": paths allowed to be used in a read-only way\n"
318	"* " ENV_FS_RW_NAME ": paths allowed to be used in a read-write way\n"
319	"\n"
320	"Optional settings (when not set, their associated access check "
321	"is always allowed, which is different from an empty string which "
322	"means an empty list):\n"
323	"* " ENV_TCP_BIND_NAME ": ports allowed to bind (server)\n"
324	"* " ENV_TCP_CONNECT_NAME ": ports allowed to connect (client)\n"
325	"* " ENV_SCOPED_NAME ": actions denied on the outside of the landlock domain\n"
326	"  - \"a\" to restrict opening abstract unix sockets\n"
327	"  - \"s\" to restrict sending signals\n"
328	"\n"
329	"A sandboxer should not log denied access requests to avoid spamming logs, "
330	"but to test audit we can set " ENV_FORCE_LOG_NAME "=1\n"
331	"\n"
332	"Example:\n"
333	ENV_FS_RO_NAME "=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
334	ENV_FS_RW_NAME "=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
335	ENV_TCP_BIND_NAME "=\"9418\" "
336	ENV_TCP_CONNECT_NAME "=\"80:443\" "
337	ENV_SCOPED_NAME "=\"a:s\" "
338	"%1$s bash -i\n"
339	"\n"
340	"This sandboxer can use Landlock features up to ABI version "
341	STR(LANDLOCK_ABI_LAST) ".\n";
342
343/* clang-format on */
344
345int main(const int argc, char *const argv[], char *const *const envp)
346{
347	const char *cmd_path;
348	char *const *cmd_argv;
349	int ruleset_fd, abi;
350	char *env_port_name, *env_force_log;
351	__u64 access_fs_ro = ACCESS_FS_ROUGHLY_READ,
352	      access_fs_rw = ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE;
353
354	struct landlock_ruleset_attr ruleset_attr = {
355		.handled_access_fs = access_fs_rw,
356		.handled_access_net = LANDLOCK_ACCESS_NET_BIND_TCP |
357				      LANDLOCK_ACCESS_NET_CONNECT_TCP,
358		.scoped = LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
359			  LANDLOCK_SCOPE_SIGNAL,
360	};
361	int supported_restrict_flags = LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
362	int set_restrict_flags = 0;
363
364	if (argc < 2) {
365		fprintf(stderr, help, argv[0]);
366		return 1;
367	}
368
369	abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION);
370	if (abi < 0) {
371		const int err = errno;
372
373		perror("Failed to check Landlock compatibility");
374		switch (err) {
375		case ENOSYS:
376			fprintf(stderr,
377				"Hint: Landlock is not supported by the current kernel. "
378				"To support it, build the kernel with "
379				"CONFIG_SECURITY_LANDLOCK=y and prepend "
380				"\"landlock,\" to the content of CONFIG_LSM.\n");
381			break;
382		case EOPNOTSUPP:
383			fprintf(stderr,
384				"Hint: Landlock is currently disabled. "
385				"It can be enabled in the kernel configuration by "
386				"prepending \"landlock,\" to the content of CONFIG_LSM, "
387				"or at boot time by setting the same content to the "
388				"\"lsm\" kernel parameter.\n");
389			break;
390		}
391		return 1;
392	}
393
394	/* Best-effort security. */
395	switch (abi) {
396	case 1:
397		/*
398		 * Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2
399		 *
400		 * Note: The "refer" operations (file renaming and linking
401		 * across different directories) are always forbidden when using
402		 * Landlock with ABI 1.
403		 *
404		 * If only ABI 1 is available, this sandboxer knowingly forbids
405		 * refer operations.
406		 *
407		 * If a program *needs* to do refer operations after enabling
408		 * Landlock, it can not use Landlock at ABI level 1.  To be
409		 * compatible with different kernel versions, such programs
410		 * should then fall back to not restrict themselves at all if
411		 * the running kernel only supports ABI 1.
412		 */
413		ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER;
414		__attribute__((fallthrough));
415	case 2:
416		/* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */
417		ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE;
418		__attribute__((fallthrough));
419	case 3:
420		/* Removes network support for ABI < 4 */
421		ruleset_attr.handled_access_net &=
422			~(LANDLOCK_ACCESS_NET_BIND_TCP |
423			  LANDLOCK_ACCESS_NET_CONNECT_TCP);
424		__attribute__((fallthrough));
425	case 4:
426		/* Removes LANDLOCK_ACCESS_FS_IOCTL_DEV for ABI < 5 */
427		ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_IOCTL_DEV;
428
429		__attribute__((fallthrough));
430	case 5:
431		/* Removes LANDLOCK_SCOPE_* for ABI < 6 */
432		ruleset_attr.scoped &= ~(LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET |
433					 LANDLOCK_SCOPE_SIGNAL);
434		__attribute__((fallthrough));
435	case 6:
436		/* Removes LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON for ABI < 7 */
437		supported_restrict_flags &=
438			~LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
439
440		/* Must be printed for any ABI < LANDLOCK_ABI_LAST. */
441		fprintf(stderr,
442			"Hint: You should update the running kernel "
443			"to leverage Landlock features "
444			"provided by ABI version %d (instead of %d).\n",
445			LANDLOCK_ABI_LAST, abi);
446		__attribute__((fallthrough));
447	case LANDLOCK_ABI_LAST:
448		break;
449	default:
450		fprintf(stderr,
451			"Hint: You should update this sandboxer "
452			"to leverage Landlock features "
453			"provided by ABI version %d (instead of %d).\n",
454			abi, LANDLOCK_ABI_LAST);
455	}
456	access_fs_ro &= ruleset_attr.handled_access_fs;
457	access_fs_rw &= ruleset_attr.handled_access_fs;
458
459	/* Removes bind access attribute if not supported by a user. */
460	env_port_name = getenv(ENV_TCP_BIND_NAME);
461	if (!env_port_name) {
462		ruleset_attr.handled_access_net &=
463			~LANDLOCK_ACCESS_NET_BIND_TCP;
464	}
465	/* Removes connect access attribute if not supported by a user. */
466	env_port_name = getenv(ENV_TCP_CONNECT_NAME);
467	if (!env_port_name) {
468		ruleset_attr.handled_access_net &=
469			~LANDLOCK_ACCESS_NET_CONNECT_TCP;
470	}
471
472	if (check_ruleset_scope(ENV_SCOPED_NAME, &ruleset_attr))
473		return 1;
474
475	/* Enables optional logs. */
476	env_force_log = getenv(ENV_FORCE_LOG_NAME);
477	if (env_force_log) {
478		if (strcmp(env_force_log, "1") != 0) {
479			fprintf(stderr, "Unknown value for " ENV_FORCE_LOG_NAME
480					" (only \"1\" is handled)\n");
481			return 1;
482		}
483		if (!(supported_restrict_flags &
484		      LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON)) {
485			fprintf(stderr,
486				"Audit logs not supported by current kernel\n");
487			return 1;
488		}
489		set_restrict_flags |= LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON;
490		unsetenv(ENV_FORCE_LOG_NAME);
491	}
492
493	ruleset_fd =
494		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
495	if (ruleset_fd < 0) {
496		perror("Failed to create a ruleset");
497		return 1;
498	}
499
500	if (populate_ruleset_fs(ENV_FS_RO_NAME, ruleset_fd, access_fs_ro)) {
501		goto err_close_ruleset;
502	}
503	if (populate_ruleset_fs(ENV_FS_RW_NAME, ruleset_fd, access_fs_rw)) {
504		goto err_close_ruleset;
505	}
506
507	if (populate_ruleset_net(ENV_TCP_BIND_NAME, ruleset_fd,
508				 LANDLOCK_ACCESS_NET_BIND_TCP)) {
509		goto err_close_ruleset;
510	}
511	if (populate_ruleset_net(ENV_TCP_CONNECT_NAME, ruleset_fd,
512				 LANDLOCK_ACCESS_NET_CONNECT_TCP)) {
513		goto err_close_ruleset;
514	}
515
516	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
517		perror("Failed to restrict privileges");
518		goto err_close_ruleset;
519	}
520	if (landlock_restrict_self(ruleset_fd, set_restrict_flags)) {
521		perror("Failed to enforce ruleset");
522		goto err_close_ruleset;
523	}
524	close(ruleset_fd);
525
526	cmd_path = argv[1];
527	cmd_argv = argv + 1;
528	fprintf(stderr, "Executing the sandboxed command...\n");
529	execvpe(cmd_path, cmd_argv, envp);
530	fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
531		strerror(errno));
532	fprintf(stderr, "Hint: access to the binary, the interpreter or "
533			"shared libraries may be denied.\n");
534	return 1;
535
536err_close_ruleset:
537	close(ruleset_fd);
538	return 1;
539}