1// SPDX-License-Identifier: BSD-3-Clause
  2/*
  3 * Simple Landlock sandbox manager able to launch a process restricted by a
  4 * user-defined filesystem access control policy.
  5 *
  6 * Copyright © 2017-2020 Mickaël Salaün <mic@digikod.net>
  7 * Copyright © 2020 ANSSI
  8 */
  9
 10#define _GNU_SOURCE
 11#include <errno.h>
 12#include <fcntl.h>
 13#include <linux/landlock.h>
 14#include <linux/prctl.h>
 15#include <stddef.h>
 16#include <stdio.h>
 17#include <stdlib.h>
 18#include <string.h>
 19#include <sys/prctl.h>
 20#include <sys/stat.h>
 21#include <sys/syscall.h>
 22#include <unistd.h>
 23
 24#ifndef landlock_create_ruleset
 25static inline int landlock_create_ruleset(
 26		const struct landlock_ruleset_attr *const attr,
 27		const size_t size, const __u32 flags)
 28{
 29	return syscall(__NR_landlock_create_ruleset, attr, size, flags);
 30}
 31#endif
 32
 33#ifndef landlock_add_rule
 34static inline int landlock_add_rule(const int ruleset_fd,
 35		const enum landlock_rule_type rule_type,
 36		const void *const rule_attr, const __u32 flags)
 37{
 38	return syscall(__NR_landlock_add_rule, ruleset_fd, rule_type,
 39			rule_attr, flags);
 40}
 41#endif
 42
 43#ifndef landlock_restrict_self
 44static inline int landlock_restrict_self(const int ruleset_fd,
 45		const __u32 flags)
 46{
 47	return syscall(__NR_landlock_restrict_self, ruleset_fd, flags);
 48}
 49#endif
 50
 51#define ENV_FS_RO_NAME "LL_FS_RO"
 52#define ENV_FS_RW_NAME "LL_FS_RW"
 53#define ENV_PATH_TOKEN ":"
 54
 55static int parse_path(char *env_path, const char ***const path_list)
 56{
 57	int i, num_paths = 0;
 58
 59	if (env_path) {
 60		num_paths++;
 61		for (i = 0; env_path[i]; i++) {
 62			if (env_path[i] == ENV_PATH_TOKEN[0])
 63				num_paths++;
 64		}
 65	}
 66	*path_list = malloc(num_paths * sizeof(**path_list));
 67	for (i = 0; i < num_paths; i++)
 68		(*path_list)[i] = strsep(&env_path, ENV_PATH_TOKEN);
 69
 70	return num_paths;
 71}
 72
 73#define ACCESS_FILE ( \
 74	LANDLOCK_ACCESS_FS_EXECUTE | \
 75	LANDLOCK_ACCESS_FS_WRITE_FILE | \
 76	LANDLOCK_ACCESS_FS_READ_FILE)
 77
 78static int populate_ruleset(
 79		const char *const env_var, const int ruleset_fd,
 80		const __u64 allowed_access)
 81{
 82	int num_paths, i, ret = 1;
 83	char *env_path_name;
 84	const char **path_list = NULL;
 85	struct landlock_path_beneath_attr path_beneath = {
 86		.parent_fd = -1,
 87	};
 88
 89	env_path_name = getenv(env_var);
 90	if (!env_path_name) {
 91		/* Prevents users to forget a setting. */
 92		fprintf(stderr, "Missing environment variable %s\n", env_var);
 93		return 1;
 94	}
 95	env_path_name = strdup(env_path_name);
 96	unsetenv(env_var);
 97	num_paths = parse_path(env_path_name, &path_list);
 98	if (num_paths == 1 && path_list[0][0] == '\0') {
 99		/*
100		 * Allows to not use all possible restrictions (e.g. use
101		 * LL_FS_RO without LL_FS_RW).
102		 */
103		ret = 0;
104		goto out_free_name;
105	}
106
107	for (i = 0; i < num_paths; i++) {
108		struct stat statbuf;
109
110		path_beneath.parent_fd = open(path_list[i], O_PATH |
111				O_CLOEXEC);
112		if (path_beneath.parent_fd < 0) {
113			fprintf(stderr, "Failed to open \"%s\": %s\n",
114					path_list[i],
115					strerror(errno));
116			goto out_free_name;
117		}
118		if (fstat(path_beneath.parent_fd, &statbuf)) {
119			close(path_beneath.parent_fd);
120			goto out_free_name;
121		}
122		path_beneath.allowed_access = allowed_access;
123		if (!S_ISDIR(statbuf.st_mode))
124			path_beneath.allowed_access &= ACCESS_FILE;
125		if (landlock_add_rule(ruleset_fd, LANDLOCK_RULE_PATH_BENEATH,
126					&path_beneath, 0)) {
127			fprintf(stderr, "Failed to update the ruleset with \"%s\": %s\n",
128					path_list[i], strerror(errno));
129			close(path_beneath.parent_fd);
130			goto out_free_name;
131		}
132		close(path_beneath.parent_fd);
133	}
134	ret = 0;
135
136out_free_name:
137	free(env_path_name);
138	return ret;
139}
140
141#define ACCESS_FS_ROUGHLY_READ ( \
142	LANDLOCK_ACCESS_FS_EXECUTE | \
143	LANDLOCK_ACCESS_FS_READ_FILE | \
144	LANDLOCK_ACCESS_FS_READ_DIR)
145
146#define ACCESS_FS_ROUGHLY_WRITE ( \
147	LANDLOCK_ACCESS_FS_WRITE_FILE | \
148	LANDLOCK_ACCESS_FS_REMOVE_DIR | \
149	LANDLOCK_ACCESS_FS_REMOVE_FILE | \
150	LANDLOCK_ACCESS_FS_MAKE_CHAR | \
151	LANDLOCK_ACCESS_FS_MAKE_DIR | \
152	LANDLOCK_ACCESS_FS_MAKE_REG | \
153	LANDLOCK_ACCESS_FS_MAKE_SOCK | \
154	LANDLOCK_ACCESS_FS_MAKE_FIFO | \
155	LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
156	LANDLOCK_ACCESS_FS_MAKE_SYM)
157
158int main(const int argc, char *const argv[], char *const *const envp)
159{
160	const char *cmd_path;
161	char *const *cmd_argv;
162	int ruleset_fd;
163	struct landlock_ruleset_attr ruleset_attr = {
164		.handled_access_fs = ACCESS_FS_ROUGHLY_READ |
165			ACCESS_FS_ROUGHLY_WRITE,
166	};
167
168	if (argc < 2) {
169		fprintf(stderr, "usage: %s=\"...\" %s=\"...\" %s <cmd> [args]...\n\n",
170				ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
171		fprintf(stderr, "Launch a command in a restricted environment.\n\n");
172		fprintf(stderr, "Environment variables containing paths, "
173				"each separated by a colon:\n");
174		fprintf(stderr, "* %s: list of paths allowed to be used in a read-only way.\n",
175				ENV_FS_RO_NAME);
176		fprintf(stderr, "* %s: list of paths allowed to be used in a read-write way.\n",
177				ENV_FS_RW_NAME);
178		fprintf(stderr, "\nexample:\n"
179				"%s=\"/bin:/lib:/usr:/proc:/etc:/dev/urandom\" "
180				"%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
181				"%s bash -i\n",
182				ENV_FS_RO_NAME, ENV_FS_RW_NAME, argv[0]);
183		return 1;
184	}
185
186	ruleset_fd = landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
187	if (ruleset_fd < 0) {
188		const int err = errno;
189
190		perror("Failed to create a ruleset");
191		switch (err) {
192		case ENOSYS:
193			fprintf(stderr, "Hint: Landlock is not supported by the current kernel. "
194					"To support it, build the kernel with "
195					"CONFIG_SECURITY_LANDLOCK=y and prepend "
196					"\"landlock,\" to the content of CONFIG_LSM.\n");
197			break;
198		case EOPNOTSUPP:
199			fprintf(stderr, "Hint: Landlock is currently disabled. "
200					"It can be enabled in the kernel configuration by "
201					"prepending \"landlock,\" to the content of CONFIG_LSM, "
202					"or at boot time by setting the same content to the "
203					"\"lsm\" kernel parameter.\n");
204			break;
205		}
206		return 1;
207	}
208	if (populate_ruleset(ENV_FS_RO_NAME, ruleset_fd,
209				ACCESS_FS_ROUGHLY_READ)) {
210		goto err_close_ruleset;
211	}
212	if (populate_ruleset(ENV_FS_RW_NAME, ruleset_fd,
213				ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE)) {
214		goto err_close_ruleset;
215	}
216	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
217		perror("Failed to restrict privileges");
218		goto err_close_ruleset;
219	}
220	if (landlock_restrict_self(ruleset_fd, 0)) {
221		perror("Failed to enforce ruleset");
222		goto err_close_ruleset;
223	}
224	close(ruleset_fd);
225
226	cmd_path = argv[1];
227	cmd_argv = argv + 1;
228	execvpe(cmd_path, cmd_argv, envp);
229	fprintf(stderr, "Failed to execute \"%s\": %s\n", cmd_path,
230			strerror(errno));
231	fprintf(stderr, "Hint: access to the binary, the interpreter or "
232			"shared libraries may be denied.\n");
233	return 1;
234
235err_close_ruleset:
236	close(ruleset_fd);
237	return 1;
238}