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elf.c
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elf.c
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/*
* CreateRemoteThread for Linux
*
* Copyright (c) 2018, ilammy
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* version 2 as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public
* License along with this program; if not, write to the Free
* Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
* Boston, MA 02110-1301 USA.
*/
#include "elf.h"
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <elf.h>
#include "procfs.h"
typedef struct {
Elf64_Word nbucket;
Elf64_Word nchain;
/* an array of buckets followed by an array of chains */
Elf64_Word entries[];
} Elf64_Hash;
struct dynamic_section {
unsigned long vaddr;
unsigned long size;
const Elf64_Dyn *entries;
unsigned long count;
};
struct dynstr_section {
unsigned long vaddr;
unsigned long size;
const char *strings;
};
struct dynsym_section {
unsigned long vaddr;
unsigned long syment;
const Elf64_Sym *symbols;
unsigned long count;
};
struct hash_section {
unsigned long vaddr;
const Elf64_Hash *table;
};
struct symbol_table {
unsigned long base_vaddr;
struct dynstr_section dynstr;
struct dynsym_section dynsym;
struct hash_section hash;
};
static const void* resolve_vaddr(unsigned long vaddr, struct library *mapping)
{
for (size_t i = 0; i < mapping->region_count; i++) {
struct memory_region *region = &mapping->regions[i];
if (region->vaddr_low <= vaddr && vaddr < region->vaddr_high)
return region->content + (vaddr - region->vaddr_low);
}
return NULL;
}
static bool valid_elf_header(const Elf64_Ehdr *ehdr)
{
if (!(ehdr->e_ident[EI_MAG0] == ELFMAG0 &&
ehdr->e_ident[EI_MAG1] == ELFMAG1 &&
ehdr->e_ident[EI_MAG2] == ELFMAG2 &&
ehdr->e_ident[EI_MAG3] == ELFMAG3))
{
fprintf(stderr, "[!] invalid ELF magic: '%c%c%c%c'\n",
ehdr->e_ident[EI_MAG0], ehdr->e_ident[EI_MAG1],
ehdr->e_ident[EI_MAG2], ehdr->e_ident[EI_MAG3]);
return false;
}
if (ehdr->e_ident[EI_CLASS] != ELFCLASS64) {
fprintf(stderr, "[!] unsupported ELF class: %d\n",
ehdr->e_ident[EI_CLASS]);
return false;
}
if (ehdr->e_ident[EI_DATA] != ELFDATA2LSB) {
fprintf(stderr, "[!] unsupported ELF byte order: %d\n",
ehdr->e_ident[EI_DATA]);
return false;
}
if (ehdr->e_ident[EI_VERSION] != EV_CURRENT) {
fprintf(stderr, "[!] invalid ELF version: %d\n",
ehdr->e_ident[EI_VERSION]);
return false;
}
if ((ehdr->e_ident[EI_OSABI] != ELFOSABI_LINUX) &&
(ehdr->e_ident[EI_OSABI] != ELFOSABI_NONE))
{
fprintf(stderr, "[!] unsupported OS ABI: %d\n",
ehdr->e_ident[EI_OSABI]);
return false;
}
if (ehdr->e_type != ET_DYN) {
fprintf(stderr, "[!] ELF is not a shared object: %d\n",
ehdr->e_type);
return false;
}
if (ehdr->e_machine != EM_X86_64) {
fprintf(stderr, "[!] ELF image is not x86_64: %d\n",
ehdr->e_machine);
return false;
}
return true;
}
struct program_headers {
const Elf64_Phdr *headers;
uint16_t count;
};
static int locate_program_headers(struct library *mapping,
struct program_headers *ph)
{
unsigned long base_vaddr = mapping->regions[0].vaddr_low;
const Elf64_Ehdr *ehdr = mapping->regions[0].content;
if (ehdr->e_phoff == 0 || ehdr->e_phnum == 0) {
fprintf(stderr, "[!] missing ELF program headers\n");
return -1;
}
ph->count = ehdr->e_phnum;
ph->headers = resolve_vaddr(base_vaddr + ehdr->e_phoff, mapping);
if (!ph->headers) {
fprintf(stderr, "[!] invalid program header vaddr: %lx\n",
base_vaddr + ehdr->e_phoff);
return -1;
}
return 0;
}
static const Elf64_Phdr *find_pt_dynamic_header(struct program_headers *ph)
{
/*
* Actually, ELF also has a wacky special case for when there are
* more than PN_XNUM program headers, but we do not handle it here
* as this will require access to section headers.
*/
for (uint16_t i = 0; i < ph->count; i++) {
if (ph->headers[i].p_type == PT_DYNAMIC) {
return &ph->headers[i];
}
}
return NULL;
}
static int locate_dynamic_section(struct library *mapping,
struct dynamic_section *dynamic)
{
struct program_headers ph;
/*
* In order to locate the ".dynamic" section we have to use program
* headers, not section headers. Usually the section headers are not
* mapped into memory as they are not necessary for library loading.
*/
if (locate_program_headers(mapping, &ph) < 0)
return -1;
/*
* Scan through the headers to find the PT_DYNAMIC one. This is the
* program header that describes where exactly the .dynamic section
* is located when loaded.
*/
const Elf64_Phdr *pt_dynamic = find_pt_dynamic_header(&ph);
if (!pt_dynamic) {
fprintf(stderr, "[!] missing PT_DYNAMIC header\n");
return -1;
}
/*
* Now we can locate the section data in our memory mapping.
* The 'p_vaddr' field actually contains virtual address _offset_
* of the ".dynamic" section, not its absolute virtual address.
*/
unsigned long base_vaddr = mapping->regions[0].vaddr_low;
dynamic->vaddr = base_vaddr + pt_dynamic->p_vaddr;
dynamic->size = pt_dynamic->p_memsz;
dynamic->entries = resolve_vaddr(dynamic->vaddr, mapping);
dynamic->count = dynamic->size / sizeof(Elf64_Dyn);
if (!dynamic->entries) {
fprintf(stderr, "[!] invalid .dynamic section vaddr: %lx\n",
dynamic->vaddr);
return -1;
}
return 0;
}
static int locate_dynstr_section(struct dynamic_section *dynamic,
struct library *mapping, struct dynstr_section *dynstr)
{
dynstr->vaddr = 0;
dynstr->size = 0;
for (unsigned long i = 0; i < dynamic->count; i++) {
if (dynamic->entries[i].d_tag == DT_STRTAB)
dynstr->vaddr = dynamic->entries[i].d_un.d_ptr;
if (dynamic->entries[i].d_tag == DT_STRSZ)
dynstr->size = dynamic->entries[i].d_un.d_val;
if (dynamic->entries[i].d_tag == DT_NULL)
break;
}
if (!dynstr->vaddr) {
fprintf(stderr, "[!] missing DT_STRTAB entry\n");
return -1;
}
if (!dynstr->size) {
fprintf(stderr, "[!] missing DT_STRSZ entry\n");
return -1;
}
dynstr->strings = resolve_vaddr(dynstr->vaddr, mapping);
if (!dynstr->strings) {
fprintf(stderr, "[!] invalid .dynstr section vaddr: %lx\n",
dynstr->vaddr);
return -1;
}
return 0;
}
static int locate_dynsym_section(struct dynamic_section *dynamic,
struct library *mapping, struct dynsym_section *dynsym)
{
dynsym->vaddr = 0;
dynsym->syment = 0;
for (unsigned long i = 0; i < dynamic->count; i++) {
if (dynamic->entries[i].d_tag == DT_SYMTAB)
dynsym->vaddr = dynamic->entries[i].d_un.d_ptr;
if (dynamic->entries[i].d_tag == DT_SYMENT)
dynsym->syment = dynamic->entries[i].d_un.d_val;
if (dynamic->entries[i].d_tag == DT_NULL)
break;
}
if (!dynsym->vaddr) {
fprintf(stderr, "[!] missing DT_SYMTAB entry\n");
return -1;
}
if (!dynsym->syment) {
fprintf(stderr, "[!] missing DT_SYMENT entry\n");
return -1;
}
dynsym->symbols = resolve_vaddr(dynsym->vaddr, mapping);
dynsym->count = 0;
if (!dynsym->symbols) {
fprintf(stderr, "[!] invalid .dynsym section vaddr: %lx\n",
dynsym->vaddr);
return -1;
}
return 0;
}
static int locate_hash_section(struct dynamic_section *dynamic,
struct library *mapping, struct hash_section *hash)
{
hash->vaddr = 0;
for (unsigned long i = 0; i < dynamic->count; i++) {
if (dynamic->entries[i].d_tag == DT_HASH)
hash->vaddr = dynamic->entries[i].d_un.d_ptr;
if (dynamic->entries[i].d_tag == DT_NULL)
break;
}
if (!hash->vaddr) {
fprintf(stderr, "[!] missing DT_HASH entry\n");
return -1;
}
hash->table = resolve_vaddr(hash->vaddr, mapping);
if (!hash->table) {
fprintf(stderr, "[!] invalid .hash section vaddr: %lx\n",
hash->vaddr);
return -1;
}
return 0;
}
struct symbol_table* find_dynamic_symbol_table(struct library *mapping)
{
if (mapping->region_count < 1) {
fprintf(stderr, "[*] no memory regions to look at: %zu\n",
mapping->region_count);
return NULL;
}
/*
* This ELF image has been successfully loaded so we can be sure that
* it is valid (and don't perform all the safety checks). However, we
* should still check whether we have a valid base address, and if it
* contains an ELF image that we can parse. The first mapped region
* should start with an ELF header from which we can go further.
*/
if (!valid_elf_header(mapping->regions[0].content)) {
fprintf(stderr, "[*] sorry, this does not look like an ELF\n");
return NULL;
}
printf("[.] validated ELF header\n");
/*
* After we're sure the ELF is fine, we need to know where the
* ".dynamic" section has been loaded into memory. This section
* describes all symbols exported for dynamic linking.
*/
struct dynamic_section dynamic;
if (locate_dynamic_section(mapping, &dynamic) < 0) {
fprintf(stderr, "[*] .dynamic section not found\n");
return NULL;
}
printf("[.] found .dynamic section at %lx (%ld bytes)\n",
dynamic.vaddr, dynamic.size);
/*
* Now we need to scan through the ".dynamic" section which
* contains an array of tagged structures. We are interested
* in the information about the dynamic string and symbol tables
* (stored in the ".dynstr" and ".dynsym" sections accordingly)
* as well as the hash table of symbol names (".hash" section).
*/
struct dynstr_section dynstr;
struct dynsym_section dynsym;
struct hash_section hash;
if (locate_dynstr_section(&dynamic, mapping, &dynstr) < 0) {
fprintf(stderr, "[*] .dynstr section not found\n");
return NULL;
}
if (locate_dynsym_section(&dynamic, mapping, &dynsym) < 0) {
fprintf(stderr, "[*] .dynsym section not found\n");
return NULL;
}
if (locate_hash_section(&dynamic, mapping, &hash) < 0) {
fprintf(stderr, "[*] .hash section not found\n");
return NULL;
}
printf("[.] found .dynstr section at %lx (%ld bytes)\n",
dynstr.vaddr, dynstr.size);
printf("[.] found .dynsym section at %lx (%ld bytes per entry)\n",
dynsym.vaddr, dynsym.syment);
printf("[.] found .hash section at %lx\n", hash.vaddr);
/*
* The number of entries in the symbol table is inferred from
* the size of the symbol hash table. Because of ELF reasons.
*/
dynsym.count = hash.table->nchain;
printf("[.] dynamic symbol count: %ld\n", dynsym.count);
/*
* And now we can finally repackage and return the result.
*/
struct symbol_table *table = calloc(1, sizeof(*table));
if (!table) {
fprintf(stderr, "[*] failed to allocate symbol table\n");
return NULL;
}
table->base_vaddr = mapping->regions[0].vaddr_low;
table->dynstr = dynstr;
table->dynsym = dynsym;
table->hash = hash;
return table;
}
void free_symbol_table(struct symbol_table *table)
{
free(table);
}
static Elf64_Word elf_hash(const char *name)
{
Elf64_Word h = 0;
while (*name) {
h = (h << 4) + *name++;
Elf64_Word g = h & 0xF0000000;
if (g)
h ^= g >> 24;
h &= ~g;
}
return h;
}
static const char* symbol_name(const Elf64_Sym *symbol,
struct symbol_table *symbols)
{
if (!symbol->st_name)
return "";
return &symbols->dynstr.strings[symbol->st_name];
}
static unsigned long symbol_address(const char *name,
const Elf64_Sym *symbol, struct symbol_table *symbols)
{
uint8_t bind = ELF64_ST_BIND(symbol->st_info);
uint8_t type = ELF64_ST_TYPE(symbol->st_info);
if (symbol->st_shndx == STN_UNDEF) {
fprintf(stderr, "[!] undefined symbol: %s\n", name);
return 0;
}
if ((bind != STB_GLOBAL) && (bind != STB_WEAK)) {
fprintf(stderr, "[!] local symbol: %s\n", name);
return 0;
}
if ((type != STT_FUNC) && (type != STT_OBJECT)) {
fprintf(stderr, "[!] not a runtime object: %s\n", name);
return 0;
}
return symbols->base_vaddr + symbol->st_value;
}
unsigned long resolve_symbol(const char *name, struct symbol_table *symbols)
{
Elf64_Word nbucket = symbols->hash.table->nbucket;
const Elf64_Word *buckets = &symbols->hash.table->entries[0];
const Elf64_Word *chains = &symbols->hash.table->entries[nbucket];
Elf64_Word hash = elf_hash(name);
Elf64_Word bucket = hash % nbucket;
Elf64_Word index = buckets[bucket];
for (;;) {
if (index > symbols->dynsym.count) {
fprintf(stderr, "[*] invalid hash index: %d (> %ld)\n",
index, symbols->dynsym.count);
return 0;
}
const Elf64_Sym *symbol = &symbols->dynsym.symbols[index];
if (strcmp(symbol_name(symbol, symbols), name) == 0)
return symbol_address(name, symbol, symbols);
index = chains[index];
if (index == STN_UNDEF) {
fprintf(stderr, "[!] symbol not found: %s\n", name);
return 0;
}
}
}