lkmpg/examples/syscall.c

/*
 * syscall.c
 *
 * System call "stealing" sample.
 *
 * Disables page protection at a processor level by changing the 16th bit
 * in the cr0 register (could be Intel specific).
 *
 * Based on example by Peter Jay Salzman and
 * https://bbs.archlinux.org/viewtopic.php?id=139406
 */

#include <linux/delay.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/moduleparam.h> /* which will have params */
#include <linux/unistd.h>      /* The list of system calls */
#include <linux/version.h>

/* For the current (process) structure, we need this to know who the
 * current user is.
 */
#include <linux/sched.h>
#include <linux/uaccess.h>

/* The in-kernel calls to the ksys_close() syscall were removed in Linux v5.11+.
 */
#if (LINUX_VERSION_CODE < KERNEL_VERSION(5, 11, 0))
#include <linux/syscalls.h> /* ksys_close() wrapper for backward compatibility */
#define close_fd ksys_close
#else
#include <linux/fdtable.h> /* For close_fd */
#endif

unsigned long **sys_call_table;
unsigned long original_cr0;

/* UID we want to spy on - will be filled from the command line. */
static int uid;
module_param(uid, int, 0644);

/* A pointer to the original system call. The reason we keep this, rather
 * than call the original function (sys_open), is because somebody else
 * might have replaced the system call before us. Note that this is not
 * 100% safe, because if another module replaced sys_open before us,
 * then when we are inserted, we will call the function in that module -
 * and it might be removed before we are.
 *
 * Another reason for this is that we can not get sys_open.
 * It is a static variable, so it is not exported.
 */
asmlinkage int (*original_call)(const char *, int, int);

/* The function we will replace sys_open (the function called when you
 * call the open system call) with. To find the exact prototype, with
 * the number and type of arguments, we find the original function first
 * (it is at fs/open.c).
 *
 * In theory, this means that we are tied to the current version of the
 * kernel. In practice, the system calls almost never change (it would
 * wreck havoc and require programs to be recompiled, since the system
 * calls are the interface between the kernel and the processes).
 */
asmlinkage int our_sys_open(const char *filename, int flags, int mode)
{
    int i = 0;
    char ch;

    /* Report the file, if relevant */
    pr_info("Opened file by %d: ", uid);
    do {
        get_user(ch, filename + i);
        i++;
        pr_info("%c", ch);
    } while (ch != 0);
    pr_info("\n");

    /* Call the original sys_open - otherwise, we lose the ability to
     * open files.
     */
    return original_call(filename, flags, mode);
}

static unsigned long **aquire_sys_call_table(void)
{
    unsigned long int offset = PAGE_OFFSET;
    unsigned long **sct;

    while (offset < ULLONG_MAX) {
        sct = (unsigned long **) offset;

        if (sct[__NR_close] == (unsigned long *) close_fd)
            return sct;

        offset += sizeof(void *);
    }

    return NULL;
}

static int __init syscall_start(void)
{
    if (!(sys_call_table = aquire_sys_call_table()))
        return -1;

    original_cr0 = read_cr0();

    write_cr0(original_cr0 & ~0x00010000);

    /* keep track of the original open function */
    original_call = (void *) sys_call_table[__NR_open];

    /* use our open function instead */
    sys_call_table[__NR_open] = (unsigned long *) our_sys_open;

    write_cr0(original_cr0);

    pr_info("Spying on UID:%d\n", uid);

    return 0;
}

static void __exit syscall_end(void)
{
    if (!sys_call_table)
        return;

    /* Return the system call back to normal */
    if (sys_call_table[__NR_open] != (unsigned long *) our_sys_open) {
        pr_alert("Somebody else also played with the ");
        pr_alert("open system call\n");
        pr_alert("The system may be left in ");
        pr_alert("an unstable state.\n");
    }

    write_cr0(original_cr0 & ~0x00010000);
    sys_call_table[__NR_open] = (unsigned long *) original_call;
    write_cr0(original_cr0);

    msleep(2000);
}

module_init(syscall_start);
module_exit(syscall_end);

MODULE_LICENSE("GPL");