metasm_shell.rb

#!/usr/bin/env ruby

##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

#
# This tool provides an easy way to see what opcodes are associated with
# certain x86 instructions by making use of Metasm! Also allows to get
# friendly output from a GAS assembler source code file.
#

#
#    This file is part of Metasm, the Ruby assembly manipulation suite
#    Copyright (C) 2007 Yoann GUILLOT
#
#    Licence is LGPL, see LICENCE in the top-level directory
#

msfbase = __FILE__
while File.symlink?(msfbase)
  msfbase = File.expand_path(File.readlink(msfbase), File.dirname(msfbase))
end

gem 'rex-text'

$:.unshift(File.expand_path(File.join(File.dirname(msfbase), '..', '..', 'lib')))
require 'msfenv'

$:.unshift(ENV['MSF_LOCAL_LIB']) if ENV['MSF_LOCAL_LIB']

require 'rex'
require 'rex/ui'
require 'metasm'

#PowerPC, seems broken for now in metasm
#@Arch = ['Ia32','MIPS','PowerPC','ARM','X86_64']
@Arch = ['Ia32','MIPS','ARM','X86_64']
@Endian = ['little','big']
@architecture = ""
@endianess = ""

def usage
  $stderr.puts("\nUsage: #{$0} <options>\n" + $args.usage)
  exit
end

$args = Rex::Parser::Arguments.new(
  "-a" => [ true, "The architecture to encode as (#{@Arch.sort.collect{|a| a + ', ' }.join.gsub(/\, $/,'')})"],
  "-e" => [ true, "The endianess to encode as (#{@Endian.sort.collect{|a| a + ', ' }.join.gsub(/\, $/,'')})" ],
  "-h" => [ false, "Display this help information" ])

$args.parse(ARGV) { |opt, idx, val|
  case opt
  when "-a"
    found = nil
    @Arch.each { |a|
      if val.downcase == a.downcase
        @architecture = a
        found = true
      end
    }
    usage if not found
  when "-e"
    found = nil
    @Endian.each { |e|
      if val.downcase == e.downcase
        @endianess = e
        found = true
      end
    }
    usage if not found
  when "-h"
    usage
  else
    usage
  end
}

unless @architecture.empty?
  if @endianess.empty?
    String.class_eval("@@cpu = Metasm::#{@architecture}.new")
  else
    String.class_eval("@@cpu = Metasm::#{@architecture}.new(:#{@endianess})")
  end
end

class String
  @@cpu ||= Metasm::Ia32.new
  class << self
    def cpu()   @@cpu   end
    def cpu=(c) @@cpu=c end
  end

  # encodes the current string as a Shellcode, returns the resulting EncodedData
  def metasm_encode_edata
    s = Metasm::Shellcode.assemble @@cpu, self
    s.encoded
  end

  # encodes the current string as a Shellcode, returns the resulting binary String
  # outputs warnings on unresolved relocations
  def metasm_encode
    ed = metasm_encode_edata
    if not ed.reloc.empty?
      puts 'W: encoded string has unresolved relocations: ' + ed.reloc.map { |o, r| r.target.inspect }.join(', ')
    end
    ed.fill
    ed.data
  end

  # decodes the current string as a Shellcode, with specified base address
  # returns the resulting Disassembler
  def metasm_decode_blocks(base_addr=0, eip=base_addr)
    sc = Metasm::Shellcode.metasm_decode(self, @@cpu)
    sc.base_addr = base_addr
    sc.metasm_disassemble(eip)
  end

  # decodes the current string as a Shellcode, with specified base address
  # returns the asm source equivallent
  def metasm_decode(base_addr=0, eip=base_addr)
    metasm_decode_blocks(base_addr, eip).to_s
  end

  def metasm_disassemble(str, eip=0)
    Metasm::Shellcode.metasm_disassemble(@@cpu, str, eip)
  end

end

def parse_gas_file(filename)
  filename = File.expand_path(filename)
  unless ::File.exist?(filename)
    puts "File #{filename} not found"
    return
  end
  shellcode = ""
  puts "Reading file #{filename}"
  ::File.open(filename, "rb") do |f|
    f.each_line do |l|
      l.gsub!(/#.*$/, "")    # Delete comments
      l.gsub!(/@.*$/, "")    # Delete comments
      l.gsub!(/\..*$/, "")   # Delete directives
      l.gsub!(/(\r|\n)/, '') # Delete newlines... just in case...
      next if l.strip.empty?
      shellcode << "#{l}\n"
    end
  end

  begin
    encoded = shellcode.metasm_encode
    puts Rex::Text.to_ruby(encoded)
    puts encoded.metasm_disassemble(shellcode.metasm_encode)
  rescue Metasm::Exception => e
    puts "Error: #{e.class} #{e.message}"
  end
end

# Start a pseudo shell and dispatch lines to be assembled and then
# disassembled.
shell = Rex::Ui::Text::PseudoShell.new("%bldmetasm%clr")
shell.init_ui(Rex::Ui::Text::Input::Stdio.new, Rex::Ui::Text::Output::Stdio.new)

puts [
  'type "exit" or "quit" to quit',
  'use ";" or "\\n" for newline',
  'type "file <file>" to parse a GAS assembler source file',
  '']

shell.run { |l|
  l.gsub!(/(\r|\n)/, '')
  l.gsub!(/\\n/, "\n")
  l.gsub!(';', "\n")

  break if %w[quit exit].include? l.chomp
  if l.chomp.index(/^file (.*)/)
    parse_gas_file($1)
    next
  end
  next if l.strip.empty?

  begin
    l = l.metasm_encode
    puts '"' + l.unpack('C*').map { |c| '\\x%02x' % c }.join + '"'
  rescue Metasm::Exception => e
    puts "Error: #{e.class} #{e.message}"
  end
}