четверг, 6 июля 2017 г.

DelegatedNtdll

It seems that since est. w10 build 15007 you can have more than one loaded 32bit ntdll.dll
Function LdrpLoadDelegatedNtdll query key DelegatedNtdll via LdrQueryImageFileKeyOption then appends this value to \\SystemRoot\\system32\\ and loads it. Sure this required changes in callbacks propagation logic

There is table LdrpDelegatedNtdllExports which just hold pairs of exported symbol and offset to it "delegated" ptr:
  • LdrInitializeThunk -> LdrDelegatedLdrInitializeThunk
  • RtlUserThreadStart -> LdrDelegatedRtlUserThreadStart
  • RtlDispatchAPC -> LdrDelegatedRtlDispatchAPC
  • KiUserExceptionDispatcher -> LdrDelegatedKiUserExceptionDispatcher
  • KiUserApcDispatcher -> LdrDelegatedKiUserApcDispatcher
  • KiUserCallbackDispatcher -> LdrDelegatedKiUserCallbackDispatcher
  • KiRaiseUserExceptionDispatcher -> LdrDelegatedKiRaiseUserExceptionDispatcher
  • LdrSystemDllInitBlock -> LdrDelegatedSystemDllInitBlock
  • LdrpChildNtdll -> LdrpChildNtdllPointer
  • LdrParentInterlockedPopEntrySList -> LdrpParentInterlockedPopEntrySListPointer
  • LdrParentRtlInitializeNtUserPfn -> LdrpParentRtlInitializeNtUserPfnPointer
  • LdrParentRtlResetNtUserPfn -> LdrpParentRtlResetNtUserPfnPointer
  • LdrParentRtlRetrieveNtUserPfn -> LdrpParentRtlRetrieveNtUserPfnPointer

Lets see how this "delegated" pfns works

for example function KiUserExceptionDispatcher

_KiUserExceptionDispatcher@8 proc near
var_C= dword ptr -0Ch
var_8= dword ptr -8
var_4= dword ptr -4
arg_0= dword ptr  4

  cmp     _LdrDelegatedKiUserExceptionDispatcher, 0
  jz      short loc_6A28DB27
  mov     ecx, _LdrDelegatedKiUserExceptionDispatcher
  call    ds:___guard_check_icall_fptr

  jmp     ecx
loc_6A28DB27:

  ...

Image base of "delegated" ntdll stored in LdrpDelegatedNtdllBase

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