Magic revealed

The ELF Virus Writing HOWTO: sparc-debian2.2-linux
Prev |  | Next

# 3. Magic revealed

| It looks worse than you can imagine! I can imagine some pretty bad things! That's why I said *worse*!
| Terry Pratchett, Moving Pictures

The fancy output format of The address of main was chosen for a reason. It is valid input for `/bin/sh`. Let's check out whether the program from The magic of the Elf has `main` at the same offset.

## 3.1. objdump -d

The output of `objdump` includes function labels. Filtering the complete disassembly can yield the desired code without prior knowledge of the function address. But since we already have the value we use `--start-address` for symmetry with `ndisasm`. That option accepts only numeric values, not symbol names.

Command: pre/sparc-debian2.2-linux/magic_elf/objdump.sh

    #!/bin/bash
    . out/sparc-debian2.2-linux/magic_elf/addr_of_main
    /usr/bin/objdump -d  \
    	--start-address=0x${addr_main_x} \
    	tmp/sparc-debian2.2-linux/magic_elf/magic_elf \
    2>&1 | pre/sparc-debian2.2-linux/magic_elf/objdump_format.pl \
    	-start_address=${addr_main_x}

The filter to pretty up this disassembly is at objdump_format.pl (i)

Output: out/sparc-debian2.2-linux/magic_elf/objdump.asm

       1073c:   9d e3 bf 98         save    %sp, -104, %sp
       10740:   90 10 20 01         mov     1, %o0
       10744:   13 00 00 40         sethi   %hi(0x10000), %o1
       10748:   92 12 60 01         or      %o1, 1, %o159 ! 10001 <*ABS*+0x10001>
       1074c:   40 00 44 5a         call    218b4 <_PROCEDURE_LINKAGE_TABLE_+0x30>
       10750:   94 10 20 03         mov     3, %o2
       10754:   81 c7 e0 08         ret
       10758:   91 e8 20 00         restore %g0, 0, %o0

This looks like a real `main`. So both programs indeed have `main` at the same offset. Unfortunately a brief look through `/bin` proves this to be pure chance. And instead of a real system call for write(2) we see something strange. It resolves to a location in a shared library. But what function in what library?

## 3.2. GDB to the rescue

Command: pre/sparc-debian2.2-linux/magic_elf/gdb_core.sh

    #!/bin/bash
    /usr/bin/gdb ${1} -q <<EOF 2>&1

    	disassemble ${2}
    EOF

The filter to pretty up this disassembly is at gdb_format.pl (i)

Command: pre/sparc-debian2.2-linux/magic_elf/gdb.sh

    #!/bin/bash
    file=${1:-tmp/sparc-debian2.2-linux/magic_elf/magic_elf}
    func=${2:-main}

    /bin/echo "[func=${func}]"
    pre/sparc-debian2.2-linux/magic_elf/gdb_core.sh ${file} ${func} \
    | pre/sparc-debian2.2-linux/magic_elf/gdb_format.pl

Output: out/sparc-debian2.2-linux/magic_elf/gdb

    [func=main]
    0x1073c <main>:               save          %sp, -104, %sp
    0x10740 <main+4>:             mov           1, %o0
    0x10744 <main+8>:             sethi         %hi(0x10000), %o1
    0x10748 <main+12>:            or            %o1, 1, %o1         ! 0x10001
    0x1074c <main+16>:            call          0x218b4 <write>
    0x10750 <main+20>:            mov           3, %o2
    0x10754 <main+24>:            ret
    0x10758 <main+28>:            restore       %g0, 0, %o0

Looks better. We need a way to retrieve the function name, `write`, from this output. Then we can feed `gdb` this argument for disassembly.

Command: pre/sparc-debian2.2-linux/evil_magic/first_gdb_func.sed

    #!/bin/sed -nf
    /.*<\(.*\)>$/ {
    	s//\1/
    	p
    	q
    }

Command: pre/sparc-debian2.2-linux/evil_magic/gdb_write.sh

    #!/bin/bash
    file=${1:-tmp/sparc-debian2.2-linux/magic_elf/magic_elf}
    func=$( pre/sparc-debian2.2-linux/evil_magic/first_gdb_func.sed \
    	< out/sparc-debian2.2-linux/magic_elf/gdb )

    /bin/echo "[func=${func}]"
    pre/sparc-debian2.2-linux/magic_elf/gdb_core.sh ${file} ${func} \
    | pre/sparc-debian2.2-linux/magic_elf/gdb_format.pl

Output: out/sparc-debian2.2-linux/evil_magic/write.gdb

    [func=write]
    0x218b4 <write>:              sethi         %hi(0xc000), %g1
    0x218b8 <write+4>:            b,a           0x21884 <_PROCEDURE_LINKAGE_TABLE_>
    0x218bc <write+8>:            nop

Oops. Shared libraries don't share their secrets with everyone.

## 3.3. 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: pre/sparc-debian2.2-linux/magic_elf/cc_static.sh

    #!/bin/bash
    /usr/bin/gcc -static \
    	-Wall -O1 -I . -I out/sparc-debian2.2-linux -D NDEBUG \
    	-o tmp/sparc-debian2.2-linux/magic_elf/magic_elf_static \
    	pre/sparc-debian2.2-linux/magic_elf/magic_elf.c \
    && /bin/ls -l tmp/sparc-debian2.2-linux/magic_elf/ \
    && tmp/sparc-debian2.2-linux/magic_elf/magic_elf_static

Output: out/sparc-debian2.2-linux/magic_elf/magic_elf_static

    total 304
    -rwxr-xr-x    1 alba     alba        11252 Feb 15 23:55 magic_elf
    -rwxr-xr-x    1 alba     alba       291434 Feb 15 23:55 magic_elf_static
    ELF

Seems we found an easy way to fill up the hard disk. Anyway, what has gdb(1) to say about it?

Output: out/sparc-debian2.2-linux/evil_magic/static_main.gdb

    [func=main]
    0x101ec <main>:               save          %sp, -104, %sp
    0x101f0 <main+4>:             mov           1, %o0
    0x101f4 <main+8>:             sethi         %hi(0x10000), %o1
    0x101f8 <main+12>:            or            %o1, 1, %o1         ! 0x10001
    0x101fc <main+16>:            call          0x184e8 <write>
    0x10200 <main+20>:            mov           3, %o2
    0x10204 <main+24>:            ret
    0x10208 <main+28>:            restore       %g0, 0, %o0

The function was called `write` before, it is called `write` now. Let's look what is behind the name.

Source: pre/sparc-debian2.2-linux/evil_magic/static_write.sh

    #!/bin/bash
    file=${1:-tmp/sparc-debian2.2-linux/magic_elf/magic_elf_static}
    func=$( pre/sparc-debian2.2-linux/evil_magic/first_gdb_func.sed \
    	< out/sparc-debian2.2-linux/evil_magic/static_main.gdb )

    /bin/echo "[func=${func}]"
    pre/sparc-debian2.2-linux/magic_elf/gdb_core.sh ${file} ${func} \
    | pre/sparc-debian2.2-linux/magic_elf/gdb_format.pl

Output: out/sparc-debian2.2-linux/evil_magic/static_write.gdb

    [func=write]
    0x184e8 <write>:              mov           4, %g1              ! 0x4
    0x184ec <write+4>:            ta            0x10
    0x184f0 <write+8>:            bcc,a         0x18514 <write+44>
    0x184f4 <write+12>:           nop
    0x184f8 <write+16>:           save          %sp, -96, %sp
    0x184fc <write+20>:           call          0x113d8 <__errno_location>
    0x18500 <write+24>:           nop
    0x18504 <write+28>:           st            %i0, [ %o0 ]
    0x18508 <write+32>:           restore

## 3.4. Write your name

Above disassembly is not guaranteed to work. The names of symbols imported by libraries differ from one platform to the other, and from one compiler to the other. A more rational approach is to search the listing of all symbols for similar names and identical addresses.

Command: pre/sparc-debian2.2-linux/evil_magic/nm.sh

    #!/bin/bash
    # -p produces same output format on SunOS and GNU
    /usr/bin/nm -p tmp/sparc-debian2.2-linux/magic_elf/magic_elf_static \
    | /bin/grep '[^[:alnum:]]write\>' \
    | /usr/bin/sort

Output: out/sparc-debian2.2-linux/evil_magic/nm

    0000000000012624 t _IO_unbuffer_write
    00000000000129a8 T _IO_default_write
    00000000000184e8 T __libc_write
    00000000000184e8 W __write
    00000000000184e8 W write
    0000000000019e44 T _IO_do_write
    0000000000019e44 T _IO_new_do_write
    0000000000019e78 t new_do_write
    000000000001a81c T _IO_file_write
    000000000001a81c T _IO_new_file_write

I suspect there is actually order behind the chaos. The symbol `__write`, with a varying number of leading underscores, seems to be "the real thing" on all platforms. The aliases for the value, `0x184e8`, differ a lot.

Command: pre/sparc-debian2.2-linux/evil_magic/gdb_nm.sh

    #!/bin/bash
    file=${1:-tmp/sparc-debian2.2-linux/magic_elf/magic_elf_static}

    # \< and \> don't work on i386-freebsd4.7
    func=$( /usr/bin/nm -p ${file} \
    	| /bin/sed -ne '/.*[tTwW] \(__*write\)/ {
    		s//\1/
    		p
    		q
    	}' )

    /bin/echo "[func=${func}]"
    pre/sparc-debian2.2-linux/magic_elf/gdb_core.sh ${file} ${func} \
    | pre/sparc-debian2.2-linux/magic_elf/gdb_format.pl

Output: out/sparc-debian2.2-linux/evil_magic/gdb_nm

    [func=__write]
    0x184e8 <write>:              mov           4, %g1              ! 0x4
    0x184ec <write+4>:            ta            0x10
    0x184f0 <write+8>:            bcc,a         0x18514 <write+44>
    0x184f4 <write+12>:           nop
    0x184f8 <write+16>:           save          %sp, -96, %sp
    0x184fc <write+20>:           call          0x113d8 <__errno_location>
    0x18500 <write+24>:           nop
    0x18504 <write+28>:           st            %i0, [ %o0 ]
    0x18508 <write+32>:           restore

There are two man pages giving some overview of system calls, intro(2) and syscalls(2). `/usr/include/unistd.h` declares a traditional general purpose interface called `syscall`. Not all Linux system have man page syscall(2), though. Anyway, the statement `mov 4,%g1` corresponds to the value of `__NR_write` in `/usr/include/asm/unistd.h`. Note that Linux uses `ta 0x10` while Solaris uses `ta 8`. There are other differences, but they are beyond the scope of this document.

Prev | Home | Next
The magic of the Elf |  | The language of evil