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# The magic of the Elf

| Any sufficiently advanced technology is indistinguishable from magic.
| Arthur C. Clarke

Building executables from C source code is a complex task. An innocent looking call of gcc will invoke a pre-processor, a multi-pass compiler, an assembler and finally a linker. Using all these tools to plant virus code into another executable makes the result either prohibitively large, or very dependent on the completeness of the target installation.

Real viruses approach the problem from the other end. They are aggressively optimized for code size and do only what's absolutely necessary. Basically they just copy one chunk of code and patch a few addresses at hard coded offsets.

However, this has drastic effects:

- Since we directly copy binary code, the virus is restricted to a particular hardware architecture.

- Code must be position independent.

- Code cannot use shared libraries; not even the C runtime library.

- We cannot allocate global variables in the data segment.

There are ways to circumvent these limitations. But they are complicated and make the virus more likely to fail.

For the first example I'll present the simplest piece of code that still gives sufficient feedback. Our aim is to implant it into /bin/sh. On practically every recent installation of Linux/i386 the following code will emit three magic letters instead of just dumping core.

## In the language of mortals

Source.

    #include <unistd.h>
    int main() { write(1, (void*)0x08048001, 3); return 0; }

Command.

    #!/bin/sh
    gcc -Wall -O2 src/magic_elf/magic_elf.c -o tmp/magic_elf/magic_elf \
    && tmp/magic_elf/magic_elf

Output.

    ELF

## How it works

### Digested answer

The three letters are part of the signature of ELF files. Executables created by ld are always mapped into the same memory region. That's why the program can find its own header at a predictable virtual address.

### Short answer

RTFM.

The raw details are in `/usr/include/elf.h`. The canonical document describing the ELF file format for Intel-386 architectures can be found at ftp://tsx.mit.edu/pub/linux/packages/GCC/ELF.doc.tar.gz. A flat-text version is http://www.muppetlabs.com/~breadbox/software/ELF.txt. And finally http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html humorously describes how far you can bend the rules to reach minimal size.

### Sort of an answer

`0x8048000` is not a natural constant, but happens to be the default base address of ELF executables produced by ld. As of version 2.11 of binutils it should be possible to change that with options `-Ttext ORG` and `--section-start SECTIONNAME=ORG`, but I didn't get it working. Anyway, the layout of executables produced by ld is straight forward.

1. One ELF header - Elf32_Ehdr

2. Program headers - Elf32_Phdr

3. Program interpreter (not if statically linked)

4. Code

5. Data

6. Section headers - Elf32_Shdr

Everything from the start of the file to the last byte of code is loaded into one segment (named "code" or "text") that begins at the base address. There is a whole section called readelf describing a command to view all these details. In the meantime I will show fancy ways to get by without.

## Showing off some tools

What would you do if you knew nothing about ELF and just asked yourself how that example works? How can you go sure that the executable file really contains those three letters?

A good start for finding text in binary files is strings.

Command.

    #!/bin/sh
    # without "-a -n 3" we don't get any output
    strings -a -n 3 tmp/magic_elf/magic_elf | grep -n ELF

Output.

    1:ELF

The leading `1:` is written by grep and tells that our three-letter word is the first found string. This gives some help where we can find it in a hex dump. It is difficult to search strings in such a dump because of line breaks. Interactive tools like hexedit might be useful.

Command.

    #!/bin/sh

    # select ASCII characters or backslash escapes (octal)
    od -N 16 -c tmp/magic_elf/magic_elf | head -1

    # named characters (ASCII)
    od -N 16 -a tmp/magic_elf/magic_elf | head -1

    # plain bytewise hex
    od -N 16 -t x1 tmp/magic_elf/magic_elf | head -1

Output.

    0000000 177   E   L   F 001 001 001  \0  \0  \0  \0  \0  \0  \0  \0  \0
    0000000 del   E   L   F soh soh soh nul nul nul nul nul nul nul nul nul
    0000000 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00

At this point we can guess that file offset 1 and `0x8048000 + 1` are not coincidental. A test program might help.

Source - addr_of_main.c.

    #include <stdio.h>

    int main()
    {
      printf("0x08048000=%#02x\n", *(unsigned char*)0x08048000);
      printf("0x08048001=%.3s\n", (char*)0x08048001);
      printf("main=%p\n", main);
      return 0;
    }

Output.

    0x08048000=0x7f
    0x08048001=ELF
    main=0x8048460

Looks good. The byte at address `0x8048000 + 0` is equal to that at file offset 0. And the address of function `main` is plausible.

Command.

    #!/bin/sh
    ndisasm -e 0x460 -U tmp/magic_elf/magic_elf | sed -e '/ret/q'

Output.

    00000000  55                push ebp
    00000001  89E5              mov ebp,esp
    00000003  83EC0C            sub esp,byte +0xc
    00000006  6A03              push byte +0x3
    00000008  6801800408        push dword 0x8048001
    0000000D  6A01              push byte +0x1
    0000000F  E8A4FEFFFF        call 0xfffffeb8
    00000014  31C0              xor eax,eax
    00000016  C9                leave
    00000017  C3                ret

Both programs have `main` at the same file offset. Unfortunately a brief look through `/bin` proves this to be pure chance. The really bad news is the generated code, however. Instead of a real system call for `write` we see a strange negative address. Let's have another try.

Command.

    #!/bin/sh
    file=${1:-tmp/magic_elf/magic_elf}
    func=${2:-main}
    gdb ${file} -q <<EOT | sed -ne '/:$/,/ret *$/p'
    	set disassembly-flavor intel
    	disassemble ${func}
    EOT

Output.

    (gdb) (gdb) Dump of assembler code for function main:
    0x8048460 <main>:	push   ebp
    0x8048461 <main+1>:	mov    ebp,esp
    0x8048463 <main+3>:	sub    esp,0xc
    0x8048466 <main+6>:	push   0x3
    0x8048468 <main+8>:	push   0x8048001
    0x804846d <main+13>:	push   0x1
    0x804846f <main+15>:	call   0x8048318 <write>
    0x8048474 <main+20>:	xor    eax,eax
    0x8048476 <main+22>:	leave
    0x8048477 <main+23>:	ret

That strange negative address resolves to a function in a shared library. Not shown is a pathetic attempt to single-step to the actual code of `write`.

## In doubt use force

We can now search for a fine manual explaining how to debug shared libraries. Or just compile the bugger static.

Command.

    #!/bin/sh
    gcc -Wall -O2 -static src/magic_elf/magic_elf.c \
    	-o tmp/magic_elf/magic_elf_static \
    && ls -l tmp/magic_elf \
    && tmp/magic_elf/magic_elf_static

Output.

    total 1660
    -rwxrwxr-x    1 alba     alba        13699 Apr  1 23:09 magic_elf
    -rwxrwxr-x    1 alba     alba      1683208 Apr  1 23:09 magic_elf_static
    ELF

Seems we found an easy way to fill up the hard disk. Anyway, what has gdb to say about it?

Output.

    (gdb) (gdb) Dump of assembler code for function main:
    0x80481e0 <main>:	push   ebp
    0x80481e1 <main+1>:	mov    ebp,esp
    0x80481e3 <main+3>:	sub    esp,0xc
    0x80481e6 <main+6>:	push   0x3
    0x80481e8 <main+8>:	push   0x8048001
    0x80481ed <main+13>:	push   0x1
    0x80481ef <main+15>:	call   0x804cc60 <__libc_write>
    0x80481f4 <main+20>:	xor    eax,eax
    0x80481f6 <main+22>:	leave
    0x80481f7 <main+23>:	ret

The name of the function changed for no apparent reason. But it is reachable for disassembly now.

Output.

    (gdb) (gdb) Dump of assembler code for function __libc_write:
    0x804cc60 <__libc_write>:	push   ebx
    0x804cc61 <__libc_write+1>:	mov    edx,DWORD PTR [esp+16]
    0x804cc65 <__libc_write+5>:	mov    ecx,DWORD PTR [esp+12]
    0x804cc69 <__libc_write+9>:	mov    ebx,DWORD PTR [esp+8]
    0x804cc6d <__libc_write+13>:	mov    eax,0x4
    0x804cc72 <__libc_write+18>:	int    0x80
    0x804cc74 <__libc_write+20>:	pop    ebx
    0x804cc75 <__libc_write+21>:	cmp    eax,0xfffff001
    0x804cc7a <__libc_write+26>:	jae    0x8052bb0 <__syscall_error>
    0x804cc80 <__libc_write+32>:	ret

## In the language of evil

The code generated by gcc is not suitable for a virus. So here comes hand crafted code optimized for size (twenty three is the perfect number of bytes). I prefer nasm to GNU as.

Source.

    		global	_start
    _start:		push	byte 4
    		pop	eax		; eax = 4 = write(2)
    		xor	ebx,ebx
    		inc	ebx		; ebx = 1 = stdout
    		mov	ecx,0x08048001	; ecx = magic address
    		push	byte 3
    		pop	edx		; edx = 3 = three characters
    		int	0x80

    		xor	eax,eax
    		inc	eax		; eax = 1 = exit(2)
    		xor	ebx,ebx		; ebx = 0 = return code
    		int	0x80

Command.

    #!/bin/sh
    nasm -f elf -o tmp/evil_magic/nasm.o src/evil_magic/evil_magic.asm \
    && ld -o tmp/evil_magic/nasm tmp/evil_magic/nasm.o \
    && tmp/evil_magic/nasm

Output.

    ELF

Output is good. But how do we get the resulting machine code? We can't just add a call to printf(3) to the assembly code. Above example is not linked with glibc; it does not even have a function called `main`.

### Entry point

On the other hand things became a lot easier. There is no initialization code that gets executed before `_start`, so the address of `_start` is really the ELF entry point of the executable. A look into `/usr/include/elf.h` shows that `Elf32_Ehdr::e_entry` is at file offset 24.

Command.

    #!/bin/sh
    od -Ad -j24 -w4 -tx4 tmp/evil_magic/nasm | head -1

Output.

    0000024 08048080

The entry point is specified as a virtual address in memory. By subtracting the base address we get the file offset:

> 0x08048080 - 0x8048000 = 0x80

### Resulting code

Command.

    #!/bin/sh
    ndisasm -e 0x80 -U tmp/evil_magic/nasm | head -12

Output.

    00000000  6A04              push byte +0x4
    00000002  58                pop eax
    00000003  31DB              xor ebx,ebx
    00000005  43                inc ebx
    00000006  B901800408        mov ecx,0x8048001
    0000000B  6A03              push byte +0x3
    0000000D  5A                pop edx
    0000000E  CD80              int 0x80
    00000010  31C0              xor eax,eax
    00000012  40                inc eax
    00000013  31DB              xor ebx,ebx
    00000015  CD80              int 0x80

That's the code we need. There is just one thing left: Dressing up the hex dump as C source. A filter written in Perl will do.

Filter.

    #!/usr/bin/perl -sw
    use strict;

    $::identfier = 'main' if (!defined($::identfier));
    $::size = '' if (!defined($::size));

    printf "const unsigned char %s[%s] =\n", $::identfier, $::size;
    while(<>)
    {
      chomp;
      my @word = split;
      my $code = $word[1];

      my $escape = '"';
      for(my $i = 0; $i < length($code); $i += 2)
      {
        $escape .= '\\x' . substr($code, $i, 2);
      }
      $escape .= '"';
      s/\s+[^\s]*\s+/: /;
      printf "  %-24s /* %-30s */\n", $escape, $_;
    }
    print "  ;\n";

Output.

    const unsigned char main[] =
      "\x6A\x04"               /* 00000000: push byte +0x4       */
      "\x58"                   /* 00000002: pop eax              */
      "\x31\xDB"               /* 00000003: xor ebx,ebx          */
      "\x43"                   /* 00000005: inc ebx              */
      "\xB9\x01\x80\x04\x08"   /* 00000006: mov ecx,0x8048001    */
      "\x6A\x03"               /* 0000000B: push byte +0x3       */
      "\x5A"                   /* 0000000D: pop edx              */
      "\xCD\x80"               /* 0000000E: int 0x80             */
      "\x31\xC0"               /* 00000010: xor eax,eax          */
      "\x40"                   /* 00000012: inc eax              */
      "\x31\xDB"               /* 00000013: xor ebx,ebx          */
      "\xCD\x80"               /* 00000015: int 0x80             */
      ;

Calling the string constant `main` is not a mistake. Above output is a complete and valid C program.

Command.

    #!/bin/sh
    gcc -Wall -O2 out/evil_magic/evil_magic.c -o tmp/evil_magic/cc \
    && tmp/evil_magic/cc

Output.

    out/evil_magic/evil_magic.c:1: warning: `main' is usually a function
    ELF

## Other roads to ELF

Source.

    #!/usr/bin/perl -w
    syscall 4, 1, 0x8048001, 3

Output.

    ELF

Command.

    #!/bin/sh
    dd if=/proc/self/mem bs=1 skip=134512641 count=3 2>/dev/null

Output.

    ELF

Command.

    #!/bin/sh
    dd if=/proc/self/exe bs=1 skip=1 count=3 2>/dev/null

Output.

    ELF

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