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##
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# This module requires Metasploit: https://metasploit.com/download
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# Current source: https://github.com/rapid7/metasploit-framework
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##
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# This class implements a simple NOP generator for RISC-V 32-bit (Little Endian)
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class MetasploitModule < Msf::Nop
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  def initialize
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    super(
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      'Name' => 'Simple',
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      'Alias' => 'riscv32le_simple',
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      'Description' => 'Simple NOP generator',
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      'License' => MSF_LICENSE,
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      'Author' => ['bcoles'],
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      'Arch' => ARCH_RISCV32LE)
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    register_advanced_options([
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      OptBool.new('RandomNops', [false, 'Generate a random NOP sled', true]),
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    ])
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  end
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  def generate_sled(length, opts)
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    badchars = opts['BadChars'] || ''
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    random = opts['Random'] || datastore['RandomNops']
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    nops = [
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      # Safe NULL-free nops using temporary registers (t0 - t6)
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      [0x400282b3].pack('V'),    # sub t0, t0, 0
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      [0x40030333].pack('V'),    # sub t1, t1, 0
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      [0x400383b3].pack('V'),    # sub t2, t2, 0
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      [0x400e0e33].pack('V'),    # sub t3, t3, 0
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      [0x400e8eb3].pack('V'),    # sub t4, t4, 0
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      [0x400f0f33].pack('V'),    # sub t5, t5, 0
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      [0x400f8fb3].pack('V'),    # sub t6, t6, 0
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      # Safe NULL-free nops using zero register (x0)
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      [0x01102013].pack('V'),    # slti x0, x0, 0x11
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      [0x7ff02013].pack('V'),    # slti x0, x0, 0x7ff
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      [0x01103013].pack('V'),    # sltiu x0, x0, 0x11
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      [0x7ff03013].pack('V'),    # sltiu x0, x0, 0x7ff
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      [0x01105013].pack('V'),    # srli x0, x0, 0x11
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      [0x01f05013].pack('V'),    # srli x0, x0, 0x1f
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      [0x01101013].pack('V'),    # slli x0, x0, 0x11
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      [0x01f01013].pack('V'),    # slli x0, x0, 0x1f
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      [0x41105013].pack('V'),    # srai x0, x0, 0x11
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      [0x41f05013].pack('V'),    # srai x0, x0, 0x1f
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      [0x01106013].pack('V'),    # ori x0, x0, 0x11
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      [0x7ff06013].pack('V'),    # ori x0, x0, 0x7ff
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      [0x01104013].pack('V'),    # xori x0, x0, 0x11
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      [0x7ff04013].pack('V'),    # xori x0, x0, 0x7ff
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      [0x01107013].pack('V'),    # andi x0, x0, 0x11
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      [0x7ff07013].pack('V'),    # andi x0, x0, 0x7ff
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      [0x10101037].pack('V'),    # lui x0, 0x10101
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      [0xfffff037].pack('V'),    # lui x0, 0xfffff
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      # Safe NULL-free numeric nops using zero register (x0)
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      # lui x0, 0x????3037
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      "\x37\x30" + Rex::Text.rand_text_numeric(2, badchars),
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      # Safe NULL-free alphanumeric nops using zero register (x0)
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      # lui x0, 0x????[357]037
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      "\x37\x30" + Rex::Text.rand_text_alphanumeric(2, badchars),
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      "\x37\x50" + Rex::Text.rand_text_alphanumeric(2, badchars),
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      "\x37\x70" + Rex::Text.rand_text_alphanumeric(2, badchars),
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      # Safe NULL-free english nops using zero register (x0)
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      # lui x0, 0x????[34567]037
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      "\x37\x30" + Rex::Text.rand_text_english(2, badchars),
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      "\x37\x40" + Rex::Text.rand_text_english(2, badchars),
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      "\x37\x50" + Rex::Text.rand_text_english(2, badchars),
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      "\x37\x60" + Rex::Text.rand_text_english(2, badchars),
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      "\x37\x70" + Rex::Text.rand_text_english(2, badchars),
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    ]
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    # Remove nops containing BadChars
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    nops.delete_if do |nop|
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      nop.bytes.any? { |byte| badchars.force_encoding('BINARY').include?(byte.chr) }
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    end
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    # Give up if no safe nops are available
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    return if nops.empty?
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    # Use random instructions for all NOPs
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    if random
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      sled = ''
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      (length / 4).times do
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        sled << nops.sample
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      end
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      return sled
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    end
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    # Use a single instruction for all NOPs
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    return (nops.sample * (length / 4))
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  end
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end
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