display linux module vermagic and CRC value

modcrc.c

a/*
 * Linux kernel module fucker
 *
 * by wzt       <[email protected]>
 *
 */

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <elf.h>
#include <sys/stat.h>
#include <sys/mman.h>

#define MODULE_NAME_LEN	 (64 - sizeof(unsigned long))

struct modversion_info
{
	unsigned long crc;
	char name[MODULE_NAME_LEN];
};

Elf32_Ehdr *ehdr = NULL;
Elf32_Phdr *phdr = NULL;
Elf32_Shdr *shdr = NULL;
Elf32_Shdr *shstrtab = NULL;
Elf32_Sym *dynsym_ptr = NULL;
Elf32_Sym *symtab_ptr = NULL;
Elf32_Sym *dynstr_ptr = NULL;

char *Real_strtab = NULL;
char *dynstr = NULL;
char *strtab_ptr = NULL;
char dynstr_buffer[2048];    
char strtab_buffer[4096]; 
char *real_strtab = NULL;

unsigned int shstrtab_off, shstrtab_len, shstrtab_num;
unsigned int strtab_off, strtab_size;

int elf_fd;
struct stat f_stat;

void usage(char *pro)
{
	fprintf(stderr, "usage: %s <options> <module>\n\n", pro);
	fprintf(stderr, "-w -v\tCheck vermgaic in module.\n");
	fprintf(stderr, "-w -c\tCheck crc value in module.\n");
	fprintf(stderr, "-s -v <new_vermagic>\tSet vermagic in module.\n");
	fprintf(stderr, "-s -c\tSet crc value in module.\n");

	exit(0);
}

int init_load_elf(char *elf_file)
{
	char buff[1024];

	elf_fd = open(elf_file, O_RDWR);
	if (elf_fd == -1) {
		perror("open");
		return 0;
	}
	fprintf(stderr, "[+] Open %s ok.\n", elf_file);

	if (fstat(elf_fd, &f_stat) == -1) {
		perror("fstat");
		return 0;
	}

	ehdr = (Elf32_Ehdr *)mmap(NULL, f_stat.st_size, PROT_WRITE|PROT_READ, MAP_SHARED, elf_fd, 0);
	if(ehdr == MAP_FAILED) {
		perror("mmap");
		return 0;
	}

	phdr = (Elf32_Phdr *)((unsigned long)ehdr + ehdr->e_phoff);
	shdr = (Elf32_Shdr *)((unsigned long)ehdr + ehdr->e_shoff);

	shstrtab = &shdr[ehdr->e_shstrndx];
	shstrtab_off = (unsigned int)shstrtab->sh_offset;
	shstrtab_len = shstrtab->sh_size;

	real_strtab = (char *)( (unsigned long)ehdr + shstrtab_off );

	printf("[+] .Shstrtab Size :0x%x,%d\n", shstrtab->sh_size, shstrtab->sh_name);
	printf("[+] .Shstrtab Off: 0x%x\n", shstrtab_off);

	return 1;
}

int display_module_crc_info(void)
{
	struct modversion_info *versions;
	char *buff = NULL;
	unsigned int version_off, version_size, num_versions;
	int i, j;

	buff = (char *)malloc(shstrtab_len + 2);
	if (!buff) {
		fprintf(stderr, "[-] Malloc failed.\n");
		return 0;
	}

	memcpy(buff, real_strtab, shstrtab_len + 1);
	for (i = 0 ; i < (int)ehdr->e_shnum ; i++){
		if (!strcmp(buff + shdr[i].sh_name,"__versions")){
			printf("[+] found section %s.\n", buff + shdr[i].sh_name);
			version_off = (unsigned int)shdr[i].sh_offset;
			version_size = (unsigned int)shdr[i].sh_size;
			printf("[+] version off: 0x%x\n", version_off);
			printf("[+] version size: 0x%x\n", version_size);
			break;
		}
	}

	printf("[+] %x,%x\n", (unsigned long)ehdr + version_off, shdr[i].sh_addr);
	versions = (void *)((unsigned long)ehdr + version_off);
	num_versions = version_size / sizeof(struct modversion_info);
	printf("[+] num_versions: %d\n", num_versions);

	for (j = 0; j < num_versions; j++) {
		printf("[+] %s:0x%08x.\n", versions[j].name, versions[j].crc);
	}

	free(buff);
	return 1;
}

int set_module_crc_info(void)
{
	struct modversion_info *versions;
	char *buff = NULL;
	unsigned int version_off, version_size, num_versions;
	int i, j;

	buff = (char *)malloc(shstrtab_len + 2);
	if (!buff) {
		fprintf(stderr, "[-] Malloc failed.\n");
		return 0;
	}

	memcpy(buff, real_strtab, shstrtab_len + 1);
	for (i = 0 ; i < (int)ehdr->e_shnum ; i++){
		if (!strcmp(buff + shdr[i].sh_name,"__versions")){
			printf("[+] found section %s.\n", buff + shdr[i].sh_name);
			version_off = (unsigned int)shdr[i].sh_offset;
			version_size = (unsigned int)shdr[i].sh_size;
			printf("[+] version off: 0x%x\n", version_off);
			printf("[+] version size: 0x%x\n", version_size);
			break;
		}
	}

	printf("[+] %x,%x\n", (unsigned long)ehdr + version_off, shdr[i].sh_addr);
	versions = (void *)((unsigned long)ehdr + version_off);
	num_versions = version_size / sizeof(struct modversion_info);
	printf("[+] num_versions: %d\n", num_versions);

	for (j = 0; j < num_versions; j++) {
		printf("[+] %s:0x%08x.\n", versions[j].name, versions[j].crc);
		if (!strcmp(versions[j].name, "struct_module")) {
			fprintf(stderr, "[+] Found symbol struct_module.\n");
			versions[j].name[0] = 'T';
			break;
		}
	}

	for (j = 0; j < num_versions; j++) {
		printf("[+] %s:0x%08x.\n", versions[j].name, versions[j].crc);
	}

	free(buff);
	return 1;
}

static char *next_string(char *string, unsigned long *secsize)
{
	/* Skip non-zero chars */
	while (string[0]) {
		string++;
		if ((*secsize)-- <= 1)
			return NULL;
	}

	/* Skip any zero padding. */
	while (!string[0]) {
		string++;
		if ((*secsize)-- <= 1)
			return NULL;
	}
	return string;
}

static char *get_modinfo(Elf32_Shdr *sechdrs, unsigned int info, const char *tag)
{
	char *p;
	unsigned int taglen = strlen(tag);
	unsigned long size = sechdrs[info].sh_size;

	for (p = (char *)(ehdr +sechdrs[info].sh_offset); p; p = next_string(p, &size)) {
		if (strncmp(p, tag, taglen) == 0 && p[taglen] == '=')
			return p + taglen + 1;
	}
	return NULL;
}

int display_module_vermagic_info(void)
{
	char *buff, *p;
	char *ver = "vermagic";
	unsigned int taglen = strlen(ver);
	int size, i;

	buff = (char *)malloc(shstrtab_len + 2);
	if (!buff) {
		fprintf(stderr, "[-] Malloc failed.\n");
		return 0;
	}

	memcpy(buff, real_strtab, shstrtab_len + 1);
	for (i = 0 ; i < (int)ehdr->e_shnum ; i++){
		if (!strcmp(buff + shdr[i].sh_name,".modinfo")){
			printf("[+] found section %s.\n", buff + shdr[i].sh_name);
			break;
		}
	}

	size = shdr[i].sh_size;
	printf("[+] size: 0x%x.\n", size);

	p = (char *)((unsigned long)ehdr + shdr[i].sh_offset);
	printf("[+] 0x%08x\n", p);

	for (; p; p = next_string(p, &size)) {
		printf("[+] %s\n", p);
		if (strncmp(p, "vermagic", taglen) == 0 && p[taglen] == '=') {
			printf("[+] %s\n", p + taglen + 1);
			//memset(p + taglen + 1, 'A', 30);
		}
	}

	return 1;
}

int set_module_vermagic_info(char *new_vermagic)
{
	char *buff, *p;
	char *ver = "vermagic";
	unsigned int taglen = strlen(ver);
	int size, i;

	buff = (char *)malloc(shstrtab_len + 2);
	if (!buff) {
		fprintf(stderr, "[-] Malloc failed.\n");
		return 0;
	}

	memcpy(buff, real_strtab, shstrtab_len + 1);
	for (i = 0 ; i < (int)ehdr->e_shnum ; i++){
		if (!strcmp(buff + shdr[i].sh_name,".modinfo")){
			printf("[+] found section %s.\n", buff + shdr[i].sh_name);
			break;
		}
	}

	size = shdr[i].sh_size;
	printf("[+] size: 0x%x.\n", size);

	p = (char *)((unsigned long)ehdr + shdr[i].sh_offset);
	printf("[+] 0x%08x\n", p);

	for (; p; p = next_string(p, &size)) {
		printf("[+] %s\n", p);
		if (strncmp(p, "vermagic", taglen) == 0 && p[taglen] == '=') {
			printf("[+] %s\n", p + taglen + 1);
			if (strlen(p + taglen + 1) < strlen(new_vermagic)) {
				printf("[-] New vermagic len must < current magic len.\n");
				return 0;
			}
			memset(p + taglen + 1, '\0', strlen(new_vermagic));
			memcpy(p + taglen + 1, new_vermagic, strlen(new_vermagic));
		}
	}

	return 1;
}

int exit_elf_load(void)
{
	close(elf_fd);
	if (munmap(ehdr, f_stat.st_size) == -1) {
		return 0;
	}

	return 1;
}

int main(int argc, char **argv)
{
	if (argc == 1) {
		usage(argv[0]);
	}

	if (!strcmp(argv[1], "-w") && !strcmp(argv[2], "-c")) {
		fprintf(stderr, "[+] Display %s module crc value.\n", argv[3]);
		if (!init_load_elf(argv[3])) {
			fprintf(stderr, "[-] Init elf load failed.\n");
			return 0;
		}
		display_module_crc_info();
		exit_elf_load();
	}
	else if (!strcmp(argv[1], "-s") && !strcmp(argv[2], "-c")) {
		fprintf(stderr, "[+] Set %s module crc value.\n", argv[3]);
		if (!init_load_elf(argv[3])) {
			fprintf(stderr, "[-] Init elf load failed.\n");
			return 0;
		}
		set_module_crc_info();
		exit_elf_load();
	}
	if (!strcmp(argv[1], "-w") && !strcmp(argv[2], "-v")) {
		fprintf(stderr, "[+] Display %s module crc value.\n", argv[3]);
		if (!init_load_elf(argv[3])) {
			fprintf(stderr, "[-] Init elf load failed.\n");
			return 0;
		}
		display_module_vermagic_info();
		exit_elf_load();
	}
	if (!strcmp(argv[1], "-s") && !strcmp(argv[2], "-v")) {
		fprintf(stderr, "[+] Display %s module crc value.\n", argv[4]);
		if (!init_load_elf(argv[4])) {
			fprintf(stderr, "[-] Init elf load failed.\n");
			return 0;
		}
		set_module_vermagic_info(argv[3]);
		exit_elf_load();
	}
	else {
		return 0;
	}
}