[Project] Add heuristics for ole files

lualib/lua_magic/heuristics.lua

@@ -0,0 +1,130 @@
+--[[
+Copyright (c) 2019, Vsevolod Stakhov <vsevolod@highsecure.ru>
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+]]--
+
+--[[[
+-- @module lua_magic/heuristics
+-- This module contains heuristics for some specific cases
+--]]
+
+local rspamd_trie = require "rspamd_trie"
+local rspamd_util = require "rspamd_util"
+local lua_util = require "lua_util"
+local bit = require "bit"
+local fun = require "fun"
+
+local N = "lua_magic"
+local msoffice_trie
+local msoffice_patterns = {
+  doc = [[WordDocument]],
+  xls = [[Workbook]],
+  ppt = [[PowerPoint Document]]
+}
+
+local exports = {}
+
+local function compile_msoffice_trie(log_obj)
+  if not msoffice_trie then
+    local strs = {}
+    for ext,pat in pairs(msoffice_patterns) do
+      strs[#strs + 1] = '^' ..
+         table.concat(
+             fun.totable(
+                 fun.map(function(c) return c .. [[\x{00}]] end,
+                     fun.iter(pat))))
+      msoffice_patterns[ext] = #strs
+    end
+    msoffice_trie = rspamd_trie.create(strs, rspamd_trie.flags.re)
+  end
+end
+
+local function detect_ole_format(input, log_obj)
+  local inplen = #input
+  if inplen < 0x31 + 4 then
+    lua_util.debugm(N, log_obj, "short length: %s", inplen)
+    return nil
+  end
+
+  compile_msoffice_trie(log_obj)
+  local bom,sec_size = rspamd_util.unpack('<I2<I2', input:span(29, 4))
+  if bom == 0xFFFE then
+    bom = '<'
+  else
+    lua_util.debugm(N, log_obj, "bom file!: %s", bom)
+    bom = '>'; sec_size = bit.bswap(sec_size)
+  end
+
+  if sec_size < 7 or sec_size > 9 then
+    lua_util.debugm(N, log_obj, "bad sec_size: %s", sec_size)
+    return nil
+  end
+
+  sec_size = 2 ^ sec_size
+
+  -- SecID of first sector of the directory stream
+  local directory_offset = (rspamd_util.unpack(bom .. 'I4', input:span(0x31, 4)))
+      * sec_size + 512 + 1
+  lua_util.debugm(N, log_obj, "directory: %s", directory_offset)
+
+  if inplen < directory_offset then
+    lua_util.debugm(N, log_obj, "short length: %s", inplen)
+    return nil
+  end
+
+  local function process_dir_entry(offset)
+    local dtype = input:at(offset + 66)
+    lua_util.debugm(N, log_obj, "dtype: %s", dtype)
+
+    if dtype == 5 then
+      -- Skip root dentry
+      return true,nil
+    elseif dtype == 2 then
+      local matches = msoffice_trie:match(input:span(offset, 64))
+      if matches then
+        for n,_ in pairs(matches) do
+          for ext,num in pairs(msoffice_patterns) do
+            if num == n then
+              return true,ext
+            end
+          end
+        end
+      end
+      return true,nil
+    elseif dtype < 5 then
+      -- Bad type
+      return true,nil
+    end
+
+    return false,nil
+  end
+
+  repeat
+    local res,ext = process_dir_entry(directory_offset)
+
+    if res and ext then
+      return ext,60
+    end
+
+    if not res then
+      break
+    end
+
+    directory_offset = directory_offset + 128
+  until directory_offset >= inplen
+end
+
+exports.ole_format_heuristic = detect_ole_format
+
+return exports