/* Copyright (C) 2017 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
 *
 * This module is nonsense. There is no good reason to use it, unless you really
 * understand for whom it is useful and for what it's good. Chances are that it
 * spectacularly fails to accomplish what you were hoping it would. In short, it's
 * only useful if you don't know how to subvert it, and if all you're trying to
 * accomplish is taking away your own ability to do something, so that if somebody
 * asks you, "do you know how to do it?" you can say "no." It's kind of like
 * when you forget where you put your car keys.
 *
 * written by zx2c4 in the year 2017
 * greetz to pipacs for remembering to barrier all cr0 modifications
 */

#define pr_fmt(fmt) "blind operator mode: " fmt

#include <linux/kconfig.h>
#include <linux/version.h>
#include <linux/types.h>
#include <generated/utsrelease.h>
#include <linux/module.h>
#include <linux/kallsyms.h>
#include <linux/security.h>
#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 1, 0)
#include <linux/lsm_hooks.h>
#endif
#include <linux/skbuff.h>
#include <linux/netlink.h>
#include <linux/genetlink.h>
#include <linux/workqueue.h>
#include <net/sock.h>
#include <net/net_namespace.h>

#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 12, 0)
#define nlmsg_parse(a, b, c, d, e, f) nlmsg_parse(a, b, c, d, e)
#define nla_parse_nested(a, b, c, d, e) nla_parse_nested(a, b, c, d)
#endif

#include "wireguard.h"

static int *modules_disabled_sysctl;
static const struct proto_ops *netlink_ops;
#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 2, 0)
static struct security_operations *security_ops;
#else
static struct security_hook_heads *security_hooks;
static struct security_hook_list socket_sock_rcv_skb_entry;
static struct security_hook_list socket_create_entry;
static struct security_hook_list inode_permission_entry;
static struct security_hook_list ptrace_access_check_entry;
static struct security_hook_list ptrace_traceme_entry;
#endif
static void install_delayed_hooks(struct work_struct *work);
static DECLARE_DELAYED_WORK(install_delayed_hooks_work, install_delayed_hooks);

#define modify_ro_page(STATEMENTS) do { \
	unsigned long cr0; \
	\
	preempt_disable(); \
	barrier(); \
	cr0 = read_cr0(); \
	write_cr0(cr0 & ~X86_CR0_WP); \
	barrier(); \
	\
	STATEMENTS \
	\
	barrier(); \
	write_cr0(cr0); \
	barrier(); \
	preempt_enable(); \
} while (0)

#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 2, 0)
#define install_lsm_hook(__hook)
#define init_lsm_hook(__hook) security_ops->__hook = (__hook##_callback);
#else
#define install_lsm_hook(__hook) list_add_tail_rcu(&(__hook##_entry).list, (__hook##_entry).head);
#define init_lsm_hook(__hook) do { \
	(__hook##_entry).head = &security_hooks->__hook; \
	(__hook##_entry).hook.__hook = (__hook##_callback); \
} while (0)
#endif

static int socket_sock_rcv_skb_callback(struct sock *sk, struct sk_buff *skb)
{
	static u16 wg_id;
	size_t hdrlen;
	struct nlmsghdr *nlh;
	int ret;

	if (!sk || !sk->sk_socket || sk->sk_socket->ops != netlink_ops || sk->sk_protocol != NETLINK_GENERIC)
		return 0;

	nlh = nlmsg_hdr(skb);
	if (nlh->nlmsg_type == GENL_ID_CTRL) {
		struct nlattr *attrs[CTRL_ATTR_MAX + 1];

		hdrlen = GENL_HDRLEN;
		if (nlh->nlmsg_len < nlmsg_msg_size(hdrlen))
			return 0;

		ret = nlmsg_parse(nlh, hdrlen, attrs, CTRL_ATTR_MAX, NULL, NULL);
		if (ret < 0)
			return 0;

		if (!attrs[CTRL_ATTR_FAMILY_NAME] || !attrs[CTRL_ATTR_VERSION] || !attrs[CTRL_ATTR_FAMILY_ID])
			return 0;

		if (strcmp(nla_data(attrs[CTRL_ATTR_FAMILY_NAME]), WG_GENL_NAME) || nla_get_u32(attrs[CTRL_ATTR_VERSION]) != WG_GENL_VERSION)
			return 0;

		wg_id = nla_get_u16(attrs[CTRL_ATTR_FAMILY_ID]);
	} else if (wg_id && nlh->nlmsg_type == wg_id) {
		struct nlattr *attr, *device[WGDEVICE_A_MAX + 1], *peer[WGPEER_A_MAX + 1];
		int rem;

		hdrlen = GENL_HDRLEN;
		if (nlh->nlmsg_len < nlmsg_msg_size(hdrlen))
			return -EINVAL;

		ret = nlmsg_parse(nlh, hdrlen, device, WGDEVICE_A_MAX, NULL, NULL);
		if (ret < 0)
			return -EINVAL;

		if (!device[WGDEVICE_A_PEERS])
			return 0;

		nla_for_each_nested(attr, device[WGDEVICE_A_PEERS], rem) {
			ret = nla_parse_nested(peer, WGPEER_A_MAX, attr, NULL, NULL);
			if (ret < 0)
				return -EINVAL;
			if (peer[WGPEER_A_ENDPOINT])
				memset(nla_data(peer[WGPEER_A_ENDPOINT]), 0, nla_len(peer[WGPEER_A_ENDPOINT]));
			if (peer[WGPEER_A_ALLOWEDIPS])
				memset(nla_data(peer[WGPEER_A_ALLOWEDIPS]), 0, nla_len(peer[WGPEER_A_ALLOWEDIPS]));
		}
	}

	return 0;
}

static int socket_create_callback(int family, int type, int protocol, int kern)
{
	if (family == AF_PACKET)
		return -EPERM;
	if (family == AF_INET && type == SOCK_RAW)
		return -EPERM;
	if (family == AF_INET6 && type == SOCK_RAW)
		return -EPERM;
	return 0;
}

static int inode_permission_callback(struct inode *inode, int mask)
{
	if ((inode->i_mode & S_IFCHR) && imajor(inode) == 1) {
		switch (iminor(inode)) {
		case 2: /* /dev/kmem */
		case 1: /* /dev/mem */
		case 4: /* /dev/port */
			return -EPERM;
		}
	}

	if (!strcmp(inode->i_sb->s_type->name, "proc")) {
		struct dentry *dentry;
		char *buffer, *path;
		int ret;

		/* Maybe not the most iron clad way of doing this... A better
		 * way would be to just traverse proc's structures and nop out
		 * the kcore fops, but who has the patience for that? */
		dentry = d_find_alias(inode);
		if (!dentry)
			return -EPERM;

		buffer = (char *)__get_free_page(GFP_KERNEL);
		ret = -ENOMEM;
		if (!buffer)
			goto err_dentry;

		path = dentry_path_raw(dentry, buffer, PAGE_SIZE);
		ret = PTR_ERR(path);
		if (IS_ERR(path))
			goto err_page;

		ret = 0;
		if (!strcmp(path, "/kcore"))
			ret = -EPERM;
		if (!strcmp(path + strlen(path) - 4, "/mem"))
			ret = -EPERM;

err_page:
		free_page((unsigned long)buffer);
err_dentry:
		dput(dentry);
		return ret;
	}

	return 0;
}

static int ptrace_access_check_callback(struct task_struct *child, unsigned int mode)
{
	return -EPERM;
}

static int ptrace_traceme_callback(struct task_struct *parent)
{
	return -EPERM;
}

static void install_delayed_hooks(struct work_struct *work)
{
	init_lsm_hook(socket_create);
	modify_ro_page({
		install_lsm_hook(socket_create);
	});
	pr_info("hooked raw socket creation\n");

	if (modules_disabled_sysctl)
		*modules_disabled_sysctl = 1;
	else {
		ssize_t ret;
		loff_t off = 0;
		struct file *file;

		file = filp_open("/proc/sys/kernel/modules_disabled", O_WRONLY, 0);
		if (IS_ERR(file)) {
			pr_err("unable to disallow module loading/unloading: %ld\n", PTR_ERR(file));
			return;
		}
#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 14, 0)
		ret = kernel_write(file, "1\n", 2, off);
#else
		ret = kernel_write(file, "1\n", 2, &off);
#endif
		filp_close(file, 0);
		if (ret != 2) {
			pr_err("unable to disallow module loading/unloading: %ld\n", ret);
			return;
		}

	}
	pr_info("disabled module loading/unloading\n");

	pr_warn("enabled\n");
}

static int __init mod_init(void)
{
	u8 *do_coredump;

#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 2, 0)
	u8 *search;
	void(*reset_security_ops)(void) = (void(*)(void))kallsyms_lookup_name("reset_security_ops");
	if (!reset_security_ops) {
		pr_err("unable to lookup reset_security_ops\n");
		goto err;
	}
	reset_security_ops();
	for (search = (u8 *)reset_security_ops; search < (u8 *)reset_security_ops + 0x1ff; ++search) {
		/* REX.W + MOV... this is the worst thing ever. */
		if (search[0] == 0x48 && search[1] == 0xc7) {
			security_ops = (struct security_operations *)((~0UL << 32) | *(unsigned int *)(search + 7));
			break;
		}
	}
	if (!security_ops) {
		pr_err("unable to lookup security_ops\n");
		goto err;
	}
#else
	security_hooks = (struct security_hook_heads *)kallsyms_lookup_name("security_hook_heads");
	if (!security_hooks) {
		pr_err("unable to lookup security_hook_heads\n");
		goto err;
	}
#endif

	if (!init_net.genl_sock || !init_net.genl_sock->sk_socket || !init_net.genl_sock->sk_socket->ops) {
		pr_err("unable to lookup netlink proto ops\n");
		goto err;
	}
	netlink_ops = init_net.genl_sock->sk_socket->ops;

	do_coredump = (u8 *)kallsyms_lookup_name("do_coredump");
	if (!do_coredump) {
		pr_err("unable to lookup do_coredump\n");
		goto err;
	}

	modules_disabled_sysctl = (int *)kallsyms_lookup_name("modules_disabled");

	init_lsm_hook(socket_sock_rcv_skb);
	init_lsm_hook(inode_permission);
	init_lsm_hook(ptrace_access_check);
	init_lsm_hook(ptrace_traceme);
	modify_ro_page({
		install_lsm_hook(socket_sock_rcv_skb);
		install_lsm_hook(inode_permission);
		install_lsm_hook(ptrace_access_check);
		install_lsm_hook(ptrace_traceme);
		do_coredump[0] = 0xc3; /* RET */
	});
	pr_info("hooked wireguard netlink responses\n");
	pr_info("hooked kernel memory permissions\n");
	pr_info("hooked ptrace\n");
	pr_info("disabled coredumps\n");

	schedule_delayed_work(&install_delayed_hooks_work, HZ * 60);
	pr_info("other mechanisms set to deploy in 60 seconds\n");
	return 0;

err:
	pr_err("disabled\n");
	return -EOPNOTSUPP;
}

module_init(mod_init);
MODULE_LICENSE("GPL v2");
MODULE_AUTHOR("Jason A. Donenfeld");
MODULE_DESCRIPTION("Rancid monkey-patcher for snake-oil blind operator mode");