Create and test x86-64 ELF executable shellcode on

2019-05-10 11:21发布

站内文章 / Linux
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问题:

I am creating a training on buffer overflows and stack/heap attacks. I am working on an Ubuntu 12.04 x86_64 machine and want to show some sample buggy programs and the ways you could exploit those vulnerabilities.

I am trying to start with the most basic shellcode I have found so far, the simple exit call, which should exit the program being overflowed.

Hereby the exitcall.asm:

;exitcall.asm

[SECTION .text]

global _start

_start:
    xor ebx,ebx     ; zero out ebx, same function as mov ebx,0
    mov al, 1       ; exit command to kernel
    int 0x80

I've got this asm file from other tutorials, written for i386 architectures however. Next thing to do is generate an object file and make it a binary executable:

# nasm -f elf64 exitcall.asm

# ld -o exitcall exitcall.o 

# file exitcall

        exitcall: ELF 64-bit LSB  executable, x86-64, version 1 (SYSV), statically linked, not stripped

# strace ./exitcall

        execve("./exitcall", ["./exitcall"], [/* 73 vars */]) = 0
        write(0, NULL, 0 <unfinished ...>
        +++ exited with 0 +++

# objdump -d exitcall

        exitcall:     file format elf64-x86-64


        Disassembly of section .text:

        0000000000400080 <_start>:
          400080:   31 db                   xor    %ebx,%ebx
          400082:   b0 01                   mov    $0x1,%al
          400084:   cd 80                   int    $0x80

As you can see the binary result executes well (exit 0 verified with strace), somehow giving me the confidence that the asm file is correct as well. So what I am supposed to do now is to create a shellcode character array from it, so I can test is in the following sample shellprogram.c executor. I just took the HEX values from objdump and started reading left to right, top to bottom, resulting in the following test:

char code[] = "\x31\xdb\xb0\x01\xcd\x80";

int main(int argc, char **argv) {

    int (*exeshell)();
    exeshell = (int (*)()) code;
    (int)(*exeshell)();

}

When I compile this file and execute it, I get a segmentation fault, however:

# gcc shellprogram.c -o shellprogram

# file shellprogram
        shellprogram: ELF 64-bit LSB  executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=765bdf6201099b9784b63a0111dc16c1115118bb, not stripped

# strace ./shellprogram 
        execve("./shellprogram", ["./shellprogram"], [/* 73 vars */]) = 0
        brk(0)                                  = 0x602000
        access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
        mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ffff7ff8000
        access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
        open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
        fstat(3, {st_mode=S_IFREG|0644, st_size=134914, ...}) = 0
        mmap(NULL, 134914, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7ffff7fd7000
        close(3)                                = 0
        access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
        open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
        read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\320\37\2\0\0\0\0\0"..., 832) = 832
        fstat(3, {st_mode=S_IFREG|0755, st_size=1845024, ...}) = 0
        mmap(NULL, 3953344, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7ffff7a14000
        mprotect(0x7ffff7bcf000, 2097152, PROT_NONE) = 0
        mmap(0x7ffff7dcf000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1bb000) = 0x7ffff7dcf000
        mmap(0x7ffff7dd5000, 17088, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7ffff7dd5000
        close(3)                                = 0
        mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ffff7fd6000
        mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ffff7fd4000
        arch_prctl(ARCH_SET_FS, 0x7ffff7fd4740) = 0
        mprotect(0x7ffff7dcf000, 16384, PROT_READ) = 0
        mprotect(0x600000, 4096, PROT_READ)     = 0
        mprotect(0x7ffff7ffc000, 4096, PROT_READ) = 0
        munmap(0x7ffff7fd7000, 134914)          = 0
        --- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_ACCERR, si_addr=0x601038} ---
        +++ killed by SIGSEGV (core dumped) +++
        Segmentation fault (core dumped)

Does anyone have an idea where I am doing things wrong. Is my strategy to convert the binary elf64 file to shellcode string wrong, or is the original asm not 64 bit compatible as well?

I read somewhere that in order to generate the shellcode I could use the following xxd linux command:

# xxd -i exitcall

    unsigned char exitcall[] = {
      0x7f, 0x45, 0x4c, 0x46, 0x02, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x02, 0x00, 0x3e, 0x00, 0x01, 0x00, 0x00, 0x00,
      0x80, 0x00, 0x40, 0x00, 0x00, 0x00, 0x00, 0x00, 0x40, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0xa8, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x40, 0x00, 0x38, 0x00, 0x01, 0x00, 0x40, 0x00,
      0x05, 0x00, 0x02, 0x00, 0x01, 0x00, 0x00, 0x00, 0x05, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x40, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x40, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x86, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x31, 0xdb, 0xb0, 0x01,
      0xcd, 0x80, 0x00, 0x2e, 0x73, 0x79, 0x6d, 0x74, 0x61, 0x62, 0x00, 0x2e,
      0x73, 0x74, 0x72, 0x74, 0x61, 0x62, 0x00, 0x2e, 0x73, 0x68, 0x73, 0x74,
      0x72, 0x74, 0x61, 0x62, 0x00, 0x2e, 0x74, 0x65, 0x78, 0x74, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x1b, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00,
      0x06, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x40, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x06, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x11, 0x00, 0x00, 0x00,
      0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x86, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x21, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x01, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0xe8, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xc0, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00,
      0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x18, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x09, 0x00, 0x00, 0x00, 0x03, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0xa8, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x2d, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x03, 0x00, 0x01, 0x00, 0x80, 0x00, 0x40, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00,
      0x04, 0x00, 0xf1, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x04, 0x00, 0xf1, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x0e, 0x00, 0x00, 0x00,
      0x10, 0x00, 0x01, 0x00, 0x80, 0x00, 0x40, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x15, 0x00, 0x00, 0x00,
      0x10, 0x00, 0x01, 0x00, 0x86, 0x00, 0x60, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x21, 0x00, 0x00, 0x00,
      0x10, 0x00, 0x01, 0x00, 0x86, 0x00, 0x60, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x28, 0x00, 0x00, 0x00,
      0x10, 0x00, 0x01, 0x00, 0x88, 0x00, 0x60, 0x00, 0x00, 0x00, 0x00, 0x00,
      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x65, 0x78, 0x69,
      0x74, 0x63, 0x61, 0x6c, 0x6c, 0x2e, 0x61, 0x73, 0x6d, 0x00, 0x5f, 0x73,
      0x74, 0x61, 0x72, 0x74, 0x00, 0x5f, 0x5f, 0x62, 0x73, 0x73, 0x5f, 0x73,
      0x74, 0x61, 0x72, 0x74, 0x00, 0x5f, 0x65, 0x64, 0x61, 0x74, 0x61, 0x00,
      0x5f, 0x65, 0x6e, 0x64, 0x00
    };
    unsigned int exitcall_len = 725;

# objdump -d exitcall

        exitcall:     file format elf64-x86-64

        Disassembly of section .text:

        0000000000400080 <_start>:
          400080:   31 db                   xor    %ebx,%ebx
          400082:   b0 01                   mov    $0x1,%al
          400084:   cd 80                   int    $0x80

This array is considerably longer then the one from objdump (display assembler contents of executable sections) and also contains a lot of null bytes, which is not desirable in shell code, since the shellcode after a null (00) won't get executed.

I tried the same strategy with a sample Hello World.asm, trying to make it x86-64 compatible and testing it with shellprogram... failed as well. Does anyone have some good material on how to write shellcode compatible asm (i.e. not using the data sections for strings and making your addresses independant)?

Any help really appreciated, Boeboe.

回答1:

The problem was that the test program shellprogram.c was not correct for the purposes I wanted to use it for, as mentioned by @Iwillnotexist Idonotexist. You can't get data executed due to memory protection enforced by the OS.

The final result that worked (making the data section containing the char[] shellcode readable & executable) was calling:

mprotect((void*)((intptr_t)code & ~0xFFF), 8192, PROT_READ|PROT_EXEC);

Final result for simple exitcall example:

#include <unistd.h>
#include <sys/mman.h>

unsigned char code[] = {
  0x31, 0xdb, 0xb0, 0x01, 0xcd, 0x80
};

int main(int argc, char **argv) {

    mprotect((void*)((intptr_t)code & ~0xFFF), 8192, PROT_READ|PROT_EXEC);  

    int (*exeshell)();
    exeshell = (int (*)()) code;
    (int)(*exeshell)();

    printf("Failed to execute shellcode");

}

Final result for printing "you win!\r\n" to the console:

#include <unistd.h>
#include <sys/mman.h>

unsigned char code[] = {
  0xeb, 0x19, 0x31, 0xc0, 0x31, 0xdb, 0x31, 0xd2, 0x31, 0xc9, 0xb0, 0x04,
  0xb3, 0x01, 0x59, 0xb2, 0x0a, 0xcd, 0x80, 0x31, 0xc0, 0xb0, 0x01, 0x31,
  0xdb, 0xcd, 0x80, 0xe8, 0xe2, 0xff, 0xff, 0xff, 0x79, 0x6f, 0x75, 0x20,
  0x77, 0x69, 0x6e, 0x21, 0x0d, 0x0a
};


int main(int argc, char **argv) {

    mprotect((void*)((intptr_t)code & ~0xFFF), 8192, PROT_READ|PROT_EXEC);  

    int (*exeshell)();
    exeshell = (int (*)()) code;
    (int)(*exeshell)();

    printf("Failed to execute shellcode");

}

Thanks again for showing the solution!



回答2:

If it helps you are using the memory register pointers from 32-bit applications - you need to replace EAX and EBX with RAX and RBX for 64-bit.

A year late, I know, and doubtless you've moved on but I thought I'd mention it!



标签： linux nasm x86-64 elf shellcode
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