[PATCH v24 11/12] samples/landlock: Add a sandbox manager example

Mickaël Salaün
2020-11-12T20:52:34Z

From: Mickaël Salaün <mic@linux.microsoft.com>

Add a basic sandbox tool to launch a command which can only access a
whitelist of file hierarchies in a read-only or read-write way.

Cc: James Morris <jmorris@namei.org>
Cc: Jann Horn <jannh@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Serge E. Hallyn <serge@hallyn.com>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
---

Changes since v23:
* Re-add hints to help users understand the required kernel
configuration. This was removed with the removal of
landlock_get_features(2).

Changes since v21:
* Remove LANDLOCK_ACCESS_FS_CHROOT.
* Clean up help.

Changes since v20:
* Update with new syscalls and type names.
* Update errno check for EOPNOTSUPP.
* Use the full syscall interfaces: explicitely set the "flags" field to
zero.

Changes since v19:
* Update with the new Landlock syscalls.
* Comply with commit 5f2fb52fac15 ("kbuild: rename hostprogs-y/always to
hostprogs/always-y").

Changes since v16:
* Switch syscall attribute pointer and size arguments.

Changes since v15:
* Update access right names.
* Properly assign access right to files according to the new related
syscall restriction.
* Replace "select" with "depends on" HEADERS_INSTALL (suggested by Randy
Dunlap).

Changes since v14:
* Fix Kconfig dependency.
* Remove access rights that may be required for FD-only requests:
mmap, truncate, getattr, lock, chmod, chown, chgrp, ioctl.
* Fix useless hardcoded syscall number.
* Use execvpe().
* Follow symlinks.
* Extend help with common file paths.
* Constify variables.
* Clean up comments.
* Improve error message.

Changes since v11:
* Add back the filesystem sandbox manager and update it to work with the
new Landlock syscall.

Previous changes:
https://lore.kernel.org/lkml/20190721213116.23476-9-mic@digikod.net/
---
samples/Kconfig | 7 ++
samples/Makefile | 1 +
samples/landlock/.gitignore | 1 +
samples/landlock/Makefile | 15 +++
samples/landlock/sandboxer.c | 236 +++++++++++++++++++++++++++++++++++
5 files changed, 260 insertions(+)
create mode 100644 samples/landlock/.gitignore
create mode 100644 samples/landlock/Makefile
create mode 100644 samples/landlock/sandboxer.c

diff --git a/samples/Kconfig b/samples/Kconfig
index 0ed6e4d71d87..e6129496ced5 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -124,6 +124,13 @@ config SAMPLE_HIDRAW
bool "hidraw sample"
depends on CC_CAN_LINK && HEADERS_INSTALL

+config SAMPLE_LANDLOCK
+ bool "Build Landlock sample code"
+ depends on HEADERS_INSTALL
+ help
+ Build a simple Landlock sandbox manager able to launch a process
+ restricted by a user-defined filesystem access control.
+
config SAMPLE_PIDFD
bool "pidfd sample"
depends on CC_CAN_LINK && HEADERS_INSTALL
diff --git a/samples/Makefile b/samples/Makefile
index c3392a595e4b..087e0988ccc5 100644
--- a/samples/Makefile
+++ b/samples/Makefile
@@ -11,6 +11,7 @@ obj-$(CONFIG_SAMPLE_KDB) += kdb/
obj-$(CONFIG_SAMPLE_KFIFO) += kfifo/
obj-$(CONFIG_SAMPLE_KOBJECT) += kobject/
obj-$(CONFIG_SAMPLE_KPROBES) += kprobes/
+subdir-$(CONFIG_SAMPLE_LANDLOCK) += landlock
obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch/
subdir-$(CONFIG_SAMPLE_PIDFD) += pidfd
obj-$(CONFIG_SAMPLE_QMI_CLIENT) += qmi/
diff --git a/samples/landlock/.gitignore b/samples/landlock/.gitignore
new file mode 100644
index 000000000000..f43668b2d318
--- /dev/null
+++ b/samples/landlock/.gitignore
@@ -0,0 +1 @@
+/sandboxer
diff --git a/samples/landlock/Makefile b/samples/landlock/Makefile
new file mode 100644
index 000000000000..21eda5774948
--- /dev/null
+++ b/samples/landlock/Makefile
@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: BSD-3-Clause
+
+hostprogs := sandboxer
+
+always-y := $(hostprogs)
+
+KBUILD_HOSTCFLAGS += -I$(objtree)/usr/include
+
+.PHONY: all clean
+
+all:
+ $(MAKE) -C ../.. samples/landlock/
+
+clean:
+ $(MAKE) -C ../.. M=samples/landlock/ clean
diff --git a/samples/landlock/sandboxer.c b/samples/landlock/sandboxer.c
new file mode 100644
index 000000000000..127fb063c23a
--- /dev/null
+++ b/samples/landlock/sandboxer.c
@@ -0,0 +1,236 @@
+// SPDX-License-Identifier: BSD-3-Clause
+/*
+ * Simple Landlock sandbox manager able to launch a process restricted by a
+ * user-defined filesystem access control.
+ *
+ * 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)
+{
+ errno = 0;
+ 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)
+{
+ errno = 0;
+ return syscall(__NR_landlock_add_rule, ruleset_fd, rule_type,
+ rule_attr, flags);
+}
+#endif
+
+#ifndef landlock_enforce_ruleset_current
+static inline int landlock_enforce_ruleset_current(const int ruleset_fd,
+ const __u32 flags)
+{
+ errno = 0;
+ return syscall(__NR_landlock_enforce_ruleset_current, 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, path_nb = 0;
+
+ if (env_path) {
+ path_nb++;
+ for (i = 0; env_path[i]; i++) {
+ if (env_path[i] == ENV_PATH_TOKEN[0])
+ path_nb++;
+ }
+ }
+ *path_list = malloc(path_nb * sizeof(**path_list));
+ for (i = 0; i < path_nb; i++)
+ (*path_list)[i] = strsep(&env_path, ENV_PATH_TOKEN);
+
+ return path_nb;
+}
+
+#define ACCESS_FILE ( \
+ LANDLOCK_ACCESS_FS_EXECUTE | \
+ LANDLOCK_ACCESS_FS_WRITE_FILE | \
+ LANDLOCK_ACCESS_FS_READ_FILE)
+
+static int populate_ruleset(
+ const char *const env_var, const int ruleset_fd,
+ const __u64 allowed_access)
+{
+ int path_nb, i;
+ 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) {
+ fprintf(stderr, "Missing environment variable %s\n", env_var);
+ return 1;
+ }
+ env_path_name = strdup(env_path_name);
+ unsetenv(env_var);
+ path_nb = parse_path(env_path_name, &path_list);
+ if (path_nb == 1 && path_list[0][0] == '\0') {
+ fprintf(stderr, "Missing path in %s\n", env_var);
+ goto err_free_name;
+ }
+
+ for (i = 0; i < path_nb; 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 err_free_name;
+ }
+ if (fstat(path_beneath.parent_fd, &statbuf)) {
+ close(path_beneath.parent_fd);
+ goto err_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 err_free_name;
+ }
+ close(path_beneath.parent_fd);
+ }
+ free(env_path_name);
+ return 0;
+
+err_free_name:
+ free(env_path_name);
+ return 1;
+}
+
+#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)
+
+int main(const int argc, char *const argv[], char *const *const envp)
+{
+ const char *cmd_path;
+ char *const *cmd_argv;
+ int ruleset_fd;
+ struct landlock_ruleset_attr ruleset_attr = {
+ .handled_access_fs = ACCESS_FS_ROUGHLY_READ |
+ ACCESS_FS_ROUGHLY_WRITE,
+ };
+
+ 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_RO_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;
+ }
+
+ ruleset_fd = landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
+ if (ruleset_fd < 0) {
+ perror("Failed to create a ruleset");
+ switch (errno) {
+ 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;
+ }
+ if (populate_ruleset(ENV_FS_RO_NAME, ruleset_fd,
+ ACCESS_FS_ROUGHLY_READ)) {
+ goto err_close_ruleset;
+ }
+ if (populate_ruleset(ENV_FS_RW_NAME, ruleset_fd,
+ ACCESS_FS_ROUGHLY_READ | ACCESS_FS_ROUGHLY_WRITE)) {
+ 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_enforce_ruleset_current(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;
+}
--
2.29.2