250 lines
9.3 KiB
Ruby
250 lines
9.3 KiB
Ruby
# This file is part of Metasm, the Ruby assembly manipulation suite
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# Copyright (C) 2008 Yoann GUILLOT
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#
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# Licence is LGPL, see LICENCE in the top-level directory
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#
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# To use your own patterns, create a script that defines Deobfuscate::Patterns, then eval() this file.
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# Use your script as argument to --custom.
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#
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# This script is to be used with the --custom option of samples/disassemble(-gtk).rb
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# It holds methods to ease the definition of instruction patterns that are to be replaced
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# by another arbitrary instruction sequence, using mostly a regexp syntax
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#
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# The pattern search&replace is done every time the disassembler, found in variable 'dasm',
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# finds a new instruction, through the callback_newinstr callback.
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#
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# The patterns can use shortcuts for frequently-used regexps (like 'any machine registers'),
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# defined in the PatternMacros hash.
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#
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# The patterns are matched first against the sequence of instruction opcode names, then
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# each instruction is rendered as text (using Instruction#to_s), and the global regexp
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# is checked.
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# Backreferences can be used in the substitution instruction sequence, through the %1 ... %9
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# special values.
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#
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# A pattern consists of a sequence of regexp for instructions, separated by ' ; '
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# Each subregexps should not match multiple instructions (ie a patterns matches a fixed-length
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# instruction sequence, whose length equals the number of ' ; '-separated regexps)
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# The first word of each regexp should match only the instruction opcode name.
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#
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# The substitution may be a Proc, which will receive |dasm object, matched decodedinstr list| as
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# arguments, and should return:
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# a String, holding a sequence of instructions separated by ' ; ', which will be parsed by the CPU (no labels allowed)
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# nil if the pattern did not match, continue searching
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# an Array of Instruction/DecodedInstruction. If the array is the original di list, same as returning nil
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#
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# If the substitution array is different from the matched sequence, the new instructions are passed
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# to dasm.replace_instrs, which will patch the disassembler decoded instruction graph ; and each
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# new instruction is passed through the callback once more, allowing for recursive patterns.
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#
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module Deobfuscate
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# special constructs : %i => an integer (immediate/standard label)
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# %r => standard x86 register (except esp), all sizes
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# %m => modr/m 32 (memory indirection or reg)
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PatternMacros = {
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'%i' => '(?:-|loc_|sub_|xref_)?[0-9][0-9a-fA-F]*h?',
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'%r' => '(?:[re]?[abcd]x|[re]?[sd]i|[re]?bp|[abcd][lh])',
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'%m' => '(?:dword ptr \[.*?\]|eax|ebx|ecx|edx|edi|esi|ebp)',
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} if not defined? PatternMacros
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# instructions are separated by ' ; '
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# instruction must be '<simple regexp matching opcode> <arbitrary regexp>'
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# in the pattern target, %1-%9 are used for backreferences from the regexp match
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Patterns = {
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'nop ; (.*)' => '%1', # concat 'nop' into following instruction
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'mov (%r|esp), \1' => 'nop',
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'lea (%r|esp), dword ptr \[\1(?:\+0)?\]' => 'nop',
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'(.*)' => lambda { |dasm, list| # remove 'jmp imm' preceding us without interfering with running dasm
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if pdi = prev_di(dasm, list.last) and pdi.opcode.name == 'jmp' and
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pdi.instruction.args[0].kind_of? Metasm::Expression
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dasm.replace_instrs(pdi.address, pdi.address, [])
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end
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nil
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},
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#'call %i ; pop (%r)' => lambda { |dasm, list| "mov %1, #{list.first.next_addr}" },
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} if not defined? Patterns
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# returns an array of strings matching the regexp (only |,?,[], non-nested allowed, no special chars)
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# expand_regexp['a[bcd]?(ef|gh)'] => [abef acef adef aef abgh acgh adgh agh]
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def self.expand_regexp(str)
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case str
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when nil, '', '.*'; return [str.to_s]
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when /^\\\./
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l1, p2 = ['.'], $'
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when /^(\w+)(\?)?/
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s1, q, p2 = $1, $2, $'
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l1 = (q ? [s1, s1.chop] : [s1])
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when /^\[(.*?)\](\?)?/
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p1, q, p2 = $1, $2, $'
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l1 = p1.split(//)
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l1 << '' if q
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when /^\((?:\?:)?(.*?)\)(\?)?/
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p1, q, p2 = $1, $2, $'
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l1 = p1.split('|').map { |p| expand_regexp(p) }.flatten
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l1 << '' if q
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else raise "bad pattern #{str.inspect}"
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end
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expand_regexp(p2).map { |s2| l1.map { |s1_| s1_ + s2 } }.flatten.uniq
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end
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# find the instr preceding adi ; follows from_normal if it is a single element array
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def self.prev_di(dasm, di)
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if di.block.list.first != di
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di.block.list[di.block.list.index(di)-1]
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elsif di.block.from_normal.to_a.length == 1
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dasm.decoded[di.block.from_normal.first]
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end
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end
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# preprocess the pattern list to optimize matching on each new instruction
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# last pattern instr opname => prev instr opname => prev instr opname => :pattern => [patterns]
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def self.generate_precalc(next_hash, next_ops, pattern)
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if next_ops.empty?
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next_hash[:pattern] ||= []
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next_hash[:pattern] << pattern
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else
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(expand_regexp(next_ops[-1]) rescue ['.*']).each { |op|
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nh = next_hash[op] ||= {}
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generate_precalc(nh, next_ops[0...-1], pattern)
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}
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end
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end
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PrecalcPatterns = {} if not defined? PrecalcPatterns
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# replace Macros in patterns, do some precalc to speedup pattern matching
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def self.init
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PrecalcPatterns.clear
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Patterns.keys.each { |pat|
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# replace PatternMacros in patterns
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newp = pat.dup
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PatternMacros.each { |mk, mv| newp.gsub!(mk, mv) }
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Patterns[newp] = Patterns.delete(pat) if pat != newp
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pat = newp
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# TODO handle instructions with prefix (lock/rep), conditional regexp over multiple instructions..
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ops = pat.split(' ; ').map { |instr| instr[/^\S+/] }
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generate_precalc(PrecalcPatterns, ops, pat)
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}
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end
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# the actual disassembler callback
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# checks the current instruction opname against the end of patterns using precomputed tree, then check previous instr etc
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# once full pattern may match, convert each instr to string, and run the regexp match
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# on match, reuse the captures in the pattern target, parse the target, generate decoded instrs, and replace in the dasm graph.
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# on match, rerun the callback on each replaced instruction (for recursive patterns)
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def self.newinstr_callback(dasm, di)
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# compute the merged subtree of t1 and t2
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# merges patterns if found
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mergetree = lambda { |t1, t2|
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if t1 and t2
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case t1
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when Array; t1 + t2
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when Hash; (t1.keys | t2.keys).inject({}) { |t, k| t.update k => mergetree[t1[k], t2[k]] }
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end
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else t1 || t2
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end
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}
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di_seq = [di]
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lastdi = di
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tree = PrecalcPatterns
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tree = mergetree[tree['.*'], tree[lastdi.instruction.opname]]
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newinstrs = match = nil
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# walk the Precalc tree
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while tree
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if tree[:pattern]
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strs = di_seq.map { |pdi| pdi.instruction.to_s }
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break if tree[:pattern].find { |pat|
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if match = /^#{pat}$/.match(strs.join(' ; '))
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newinstrs = Patterns[pat]
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newinstrs = newinstrs[dasm, di_seq] if newinstrs.kind_of? Proc
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newinstrs = nil if newinstrs == di_seq
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else newinstrs = nil
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end
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newinstrs
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} or tree.length == 1
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end
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if lastdi = prev_di(dasm, lastdi)
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di_seq.unshift lastdi
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tree = mergetree[tree['.*'], tree[lastdi.instruction.opname]]
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else break
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end
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end
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# match found : create instruction stream, replace in dasm, recurse
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if newinstrs
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# replace %1-%9 by the matched substrings
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newinstrs = newinstrs.gsub(/%(\d)/) { match.captures[$1.to_i-1] }.split(' ; ').map { |str| dasm.cpu.parse_instruction(str) } if newinstrs.kind_of? String
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if newinstrs.last.kind_of? Metasm::Instruction and newinstrs.last.opname != 'jmp' and
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lastdi.address + di_seq.inject(-di.bin_length) { |len, i| len + i.bin_length } != di.address
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# ensure that the last instr ends the same place as the original last instr (to allow disassemble_block to continue)
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newinstrs << dasm.cpu.parse_instruction("jmp #{Metasm::Expression[di.next_addr]}")
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# nop ; jmp => jmp
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newinstrs.shift if newinstrs.length >= 2 and newinstrs.first.kind_of? Metasm::Instruction and newinstrs.first.opname == 'nop'
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end
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# remove instructions from the match to have only 2 linked blocks passed to replace_instrs
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unused = di_seq[1..-2] || []
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unused.delete_if { |udi| udi.block.address == di_seq[0].block.address or udi.block.address == di_seq[-1].block.address }
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dasm.replace_instrs(unused.shift.address, unused.shift.address, []) while unused.length > 1
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dasm.replace_instrs(unused.first.address, unused.first.address, []) if not unused.empty?
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# patch the dasm graph
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if dasm.replace_instrs(lastdi.address, di.address, newinstrs)
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puts ' deobfuscate', di_seq, ' into', newinstrs, ' ---' if $DEBUG
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# recurse, keep the last generated di to return to caller as replacement
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newinstrs.each { |bdi| di = newinstr_callback(dasm, bdi) || di }
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else
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di = nil
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end
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end
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di
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end
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# call newinstr_callback on all existing instructions of dasm
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def self.deobfuscate_existing(dasm)
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dasm.each_instructionblock { |b|
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b.list.dup.each { |di| newinstr_callback(dasm, di) }
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}
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end
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# calls dasm.merge_blocks(true) on all instruction blocks to merge sequences of blocks
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def self.merge_blocks(dasm)
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dasm.each_instructionblock { |b|
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pv = dasm.decoded[b.from_normal.to_a.first]
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if pv.kind_of? Metasm::DecodedInstruction and not pv.block.list.last.opcode.props[:setip] and
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b.from_normal.length == 1 and pv.block.to_normal.to_a.length == 1
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dasm.merge_blocks(pv.block, b, true)
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end
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}
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end
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end
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if $DEBUG
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# update DecodedInstr.to_s to include instr length
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class Metasm::DecodedInstruction
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def to_s ; "#{Metasm::Expression[address] if address} +#{bin_length} #{instruction}" end
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end
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end
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# do the pattern precalc
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Deobfuscate.init
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# setup the newinstr callback
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dasm.callback_newinstr = lambda { |di| Deobfuscate.newinstr_callback(dasm, di) } if defined? dasm
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