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# readelf

| Outside of a dog, a book is a man's best friend. Inside a dog it's too dark to read.
| Groucho Marx

Let's get a bit more serious and examine the assembly program from In the language of evil with readelf, part of the binutils package.

Command.

    #!/bin/sh
    strip tmp/evil_magic/nasm
    ls -l tmp/evil_magic/nasm
    readelf -l tmp/evil_magic/nasm

Output.

    -rwxrwxr-x    1 alba     alba          476 Mar 15 22:02 tmp/evil_magic/nasm

    Elf file type is EXEC (Executable file)
    Entry point 0x8048080
    There are 1 program headers, starting at offset 52

    Program Header:
      Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
      LOAD           0x000000 0x08048000 0x08048000 0x00097 0x00097 R E 0x1000

     Section to Segment mapping:
      Segment Sections...
       00     .text

Nice to see the entry point we retrieved through od again. Program layout is a simplified variation of Sort of an answer. The value of `FileSiz` includes ELF header and program header. The size of this overhead is:

> overhead = `Entry point` - `VirtAddr` = 0x08048080 - 0x8048000 = 0x80 bytes

So effective code size is:

> code size = `FileSiz` - overhead = 0x97 - 0x80 = 0x17 = 23 bytes

This matches with the disassembly listing. However, the ratio of file size to effective code deserves the title "Bloat", with capital B.

> code size / file size = 23 / 476 = 0.048

Only 5 percent of the file actually do something useful!

Anyway, we see that even for trivial examples the code is surrounded by lots of other stuff. Let's zoom in on our target.

## Bashful glance

Command.

    #!/bin/sh
    ls -l /bin/bash
    readelf -l /bin/bash

Output.

    -rwxr-xr-x    1 root     root       519964 Jul  9  2001 /bin/bash

    Elf file type is EXEC (Executable file)
    Entry point 0x8059380
    There are 6 program headers, starting at offset 52

    Program Headers:
      Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
      PHDR           0x000034 0x08048034 0x08048034 0x000c0 0x000c0 R E 0x4
      INTERP         0x0000f4 0x080480f4 0x080480f4 0x00013 0x00013 R   0x1
          [Requesting program interpreter: /lib/ld-linux.so.2]
      LOAD           0x000000 0x08048000 0x08048000 0x79273 0x79273 R E 0x1000
      LOAD           0x079280 0x080c2280 0x080c2280 0x057e0 0x09bd0 RW  0x1000
      DYNAMIC        0x07e980 0x080c7980 0x080c7980 0x000e0 0x000e0 RW  0x4
      NOTE           0x000108 0x08048108 0x08048108 0x00020 0x00020 R   0x4

     Section to Segment mapping:
      Segment Sections...
       00
       01     .interp
       02     .interp .note.ABI-tag .hash .dynsym .dynstr .gnu.version .gnu.version_r .rel.got .rel.bss .rel.plt .init .plt .text .fini .rodata
       03     .data .eh_frame .ctors .dtors .got .dynamic .bss
       04     .dynamic
       05     .note.ABI-tag

Looks intimidating. But then the ELF specification says that only segments of type "LOAD" are considered for execution. Since the flags of the first one are `R E`, meaning "read & execute", we know that it must be the code segment. The other one has `RW`, meaning "read & write", so it must be the data segment.

`MemSiz` is larger than `FileSiz` in the data segment. Just like with mmap(2) excessive bytes are defined to be initialized with 0. The linker takes advantages of that by grouping all variables that should be initialized to zero at the end. Note that the last section of segment 3 (counting starts with 0) is called `.bss`, the traditional name for this kind of area.

The mapping for segment 2 looks even more complex. But I would guess that `.rodata` means "read-only data" and `.text` contains productive code, as opposed to the administrative stuff in the other sections.

## Turn the pages

The distance between the two LOAD segments is interesting:

> `VirtAddr`[2] - `VirtAddr`[1] - `FileSiz`[1] = 0x80c2280 - 0x8048000 - 0x79273 = 0x100d = 4109 bytes

Only 13 bytes (0xd) would be needed to align the first LOAD segment up to the alignment of 0x1000. For some reason at least one complete page lies between code segment and data segment. This would be easy target for a tiny virus. So lets check out whether this is a unique phenomenon.

Source.

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

    my $min = 0xFFFFFFFF;
    my $max = 0;
    while(my $filename = <>)
    {
      chomp $filename;
      open(ELF, '-|', "readelf -l $filename 2>&1") || die "$1 ($filename)";

      my $nrLoad = 0;
      my $end = 0;
      while(my $line = <ELF>)
      {
        chomp $line;
        if ($line =~ m/^\s*LOAD\s*/)
        {
          $nrLoad++;

          my @number = split / +/, $line;
          my $virtaddr = hex($number[3]);
          my $filesiz = hex($number[5]);

          if ($end != 0)
          {
            my $dist = $virtaddr - $end;
    	if ($dist < 0x1000)
    	{
    	  printf "%-32s virtaddr=%#08x dist=%#08x\n",
    	    $filename, $virtaddr, $dist;
            }
    	$max = $dist if ($dist > $max);
    	$min = $dist if ($dist < $min);
          }
          $end = $virtaddr + $filesiz;
        }
      }
      if ($nrLoad != 2)
      {
        printf "%-32s has %d LOAD segments.\n", $filename, $nrLoad;
      }
      close ELF;
    }
    printf "\n%d files; min_distance=%#08x max_distance=%#08x\n",
      $., $min, $max;

Command.

    #!/bin/sh
    find /bin -type f -maxdepth 1 | src/check_dist/check_dist.pl
    echo ""
    echo tmp/evil_magic/nasm | src/check_dist/check_dist.pl

Output.

    /bin/igawk                       has 0 LOAD segments.
    /bin/vimtutor                    has 0 LOAD segments.

    73 files; min_distance=0x001000 max_distance=0x00101f

    tmp/evil_magic/nasm              has 1 LOAD segments.

    1 files; min_distance=0xffffffff max_distance=00000000

Yes, this empty page is common usage, at least in `/bin`.

## The plan

You may have heard that Linux is a difficult target for malware because there are so many different distributions. Well, basically they all use the same compiler, producing the same idiosyncrasies. This allows us to cheat in big style.

1. Insert our code between code segment and data segment.

2. Modify inserted code to jump to original entry point afterwards.

3. Change entry point to start of our code.

4. Modify program header

   1. To include increased amount of code in entry of code segment.

   2. To move all following entries down the file.

5. Move all following sections down the file.

This setup has two big problems, however.

- Code size is limited to 0x1000 bytes. Manageable with assembly. Tough luck for C.

- Infected executables will be detected by above perl script. Yes, I actually wrote the scanner before the virus. The truly paranoid doesn't trust himself.

Of course the naive implementation through parsing readelf's output significantly limits performance. But use of file as a fast file-type filter will lower noise ("has 0 LOAD segments") and duration to acceptable regions.

Command.

    #!/bin/sh
    find /usr/bin -type f -maxdepth 1 -print0 \
    | xargs -r0 file -i \
    | sed -ne 's/: *application\/x-executable-file,.*//p' \
    | src/check_dist/check_dist.pl

Output.

    file: Using regular magic file `/usr/share/magic.mime'
    file: Using regular magic file `/usr/share/magic.mime'

    1031 files; min_distance=0x001000 max_distance=0x00101f

## Paranoid android

Since all executables in `/bin` and `/usr/bin` follow the same layout, a heuristic scanner can easily spot deviations. A "perfect infection", resulting in a executable indistinguishable from the real thing, is well beyond the scope of this document. But then there are bigger issues an innocent virus seeking a warm nest in the wild would face.

For example RPM-based distributions maintain a checksum database. Verifying a single file, a complete package, or even all installed packages takes just one command.

If you know what you are looking for:

    rpm --verify -f /bin/sh

For dedicated people with enough time to read the output:

    /bin/nice -n 19 rpm --verify --all

A possible counter attack is to patch the database after infection. This is distribution dependent and requires root permissions. And it won't help against people who have the checksums offline, e.g. with tripwire.

Another possible attack is to hide the original (uninfected) executable on the file system, and patch the kernel via an inserted module to fake calculation of the checksum. And if the kernel is compiled without module-support, there is still direct access to `/dev/kmem` to install a kernel-patch…

On this road lies madness.

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