Null-packet protection

patch.c

// Let's not mangle these names
extern "C" {

#include "lua.h"
#include "lauxlib.h"
#include <sys/mman.h> 
#include <stdio.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#ifndef PAGESIZE
#define PAGESIZE 4096
#endif

#define w [_i++] = 
#define W(bb, x) bb[_i++] = (x) & 0xff; bb[_i++] = ((x) & 0xff00)>>8; \
  bb[_i++] = ((x) & 0xff0000)>>16; bb[_i++] = ((x) & 0xff000000)>>24

int mine(int fd, int eax) {
  static int first = 0;
  if (!first) {
    first = 1;
    int opt = 1;
    ioctl(fd, FIONBIO, &opt);
  }
  if (!eax) {
    // Let's go ahead and waste the fd
    char buf[1];
    int ret = recv(fd, buf, 1, MSG_PEEK);
    if (ret == 0) {
      recv(fd, buf, 0, 0);
      printf("Caught a null packet!\n");
    }
  }
  return 0;
}

extern void shellcode_asm() {
  asm("mov 0x8(%ebp), %edx\n"
      "push 0x8(%edx)\n"
      "mov $0x1234, %eax\n"
      "call *%eax\n"
      "pop %eax\n"
      "cmp $-1, %eax\n"
      "jnz next\n"
      "mov $-1, %eax\n"
      "mov $0x819ec7e, %edx\n"
      "jmp *%edx\n"
      "next:\n"
      "mov $0x819ec79, %edx\n"
      "jmp *%edx"
  );
}

extern int prepare(void* from) {
  void* page_start = (void*) (((unsigned long) from) & (~(PAGESIZE - 1)));
  
  if (mprotect(
        page_start, PAGESIZE, PROT_WRITE | PROT_READ | PROT_EXEC)) {
    printf("Couldn’t mprotect");
    return -1;
  }
  return 0;
}

int redirect(void* from, void* to) {
  char* bb = (char*) from;
  int _i = 0;
  unsigned long target = (unsigned long) to;

  bb w 0x8d; bb w 0x45; bb w 0xfc;
  bb w 0xb8; W(bb, target);
  bb w 0xff; bb w 0xe0; // jmp *%rax
  return 0;
}

int checksum() {
  unsigned long start = 0x0819ec3c;
  unsigned long end   = 0x0819ec80;
  unsigned long len   = (end - start)/4;
  unsigned int* buffer = (unsigned int*) start;
  unsigned int checksum = 0;
  for (int i = 0; i < len; i++) {
    checksum = checksum ^ buffer[i];
  }
  return checksum;
}


const char* shellcode = ((char*) shellcode_asm + 3);
//    "\x8b\x55\x08"
//    "\xff\x72\x08"
//    "\xb8\x00\x00\x00\x00\xff\xd0"
//    "\x58\x83\xf8\xff\x75\x0c\xb8\xff\xff\xff\xff\xba\x7e\xec"
//    "\x19\x08\xff\xe2\xba\x79\xec\x19\x08\xff\xe2";

int LUA_API luaopen_patch(lua_State *L) {
  int check = checksum();
  printf("Checking server integrity: %x... \n", check);
  if (check != 0x9cbfca47) {
    printf("\tBad CS2D Server, are you sure you are on 1.0.0.1?\n");
    return 1;
  }
  
  if (prepare((void*) shellcode)) {
    printf("\tCannot mutate code cave, aborting!\n");
    return 1;
  }
  
  char* call_loc = (char*) (shellcode + 7);
  int _i = 0;
  unsigned long target = (unsigned long) mine;
  W(call_loc, target);

  if (prepare((void*) 0x0819ec6d)) {
    printf("\tCannot mutate buggy code, aborting!\n");
    return 1;
  }

  if (redirect((void*) 0x0819ec6d, (void*) shellcode)) {
    printf("\tCannot redirect functions\n");
    return 1;
  }

  printf("Null-packet protection initialized successfully!\n");

  return 0;
}

int LUA_API luaopen_sys_lua_patch(lua_State *L) {
  return luaopen_patch(L);
}

}

Build this on ubuntu or some other linux system with

gcc -g -L %LUA_DIRECTORY%/lib/ -I%LUA_DIRECTORY%/includes/ -fpermissive -fPIC --shared -m32 -o patch.so patch.cpp -llua

and you can include it in sys/lua/server.lua with

require 'sys.lua.patch'