// 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 <mic@digikod.net>
 * 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 8

#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;
                __attribute__((fallthrough));
        case 7:
                /* 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;
}