How unhook LdrLoadDll function?

[flashcoder]

I had created this code below where i try prevent a dll injection that will use LdrLoadDll function through of a kernel driver.

Now i want know how i can unhook this api (based in my Hook() function) when the dll of protection is unloaded or when protected process is finalized?

Code:

library mydll;

uses
  Windows,
  SysUtils,
  Classes;

{$R *.res}

type
  NTSTATUS = UINT;

CONST
  STATUS_ACCESS_DENIED = NTSTATUS($C0000022);

type
  PUNICODE_STRING = ^UNICODE_STRING;

  UNICODE_STRING = packed record
    Length: Word;
    MaximumLength: Word;
    Buffer: PWideChar;
  end;

var
  Old_LdrLoadDll: function(szcwPath: PWideChar; dwFlags: DWORD;
    pUniModuleName: PUNICODE_STRING; pResultInstance: PPointer)
    : NTSTATUS; stdcall;

function LdrLoadDll(szcwPath: PWideChar; dwFlags: DWORD;
  pUniModuleName: PUNICODE_STRING; pResultInstance: PPointer)
  : NTSTATUS; stdcall;
begin
  Result := Old_LdrLoadDll(szcwPath, dwFlags, pUniModuleName, pResultInstance);
end;

function NewLdrLoadDll(szcwPath: PWideChar; dwFlags: DWORD;
  pUniModuleName: PUNICODE_STRING; pResultInstance: PPointer)
  : NTSTATUS; stdcall;
begin
  if (CompareStr(pUniModuleName^.Buffer, 'hackdll.dll') = 0) or
    (CompareStr(szcwPath, 'Hack') = 0) then
    Result := STATUS_ACCESS_DENIED
  else
    Result := LdrLoadDll(szcwPath, dwFlags, pUniModuleName, pResultInstance);
end;

procedure Hook(target, newfunc: pointer);
var
  jmpto: DWORD;
  OldProtect: Cardinal;
begin
  jmpto := DWORD(newfunc) - DWORD(target) - 5;
  VirtualProtect(target, 5, PAGE_EXECUTE_READWRITE, @OldProtect);
  pbyte(target)^ := $E9;
  pdword(DWORD(target) + 1)^ := jmpto;
end;

procedure DllEntryPoint(Reason: Integer); stdcall;
begin
  case Reason of
    DLL_PROCESS_ATTACH:
      begin
        DisableThreadLibraryCalls(HInstance);
        Hook(GetProcAddress(GetModuleHandle('ntdll.dll'), 'LdrLoadDll'),
          @NewLdrLoadDll);
      end;

    DLL_THREAD_ATTACH:
      ; //
    DLL_THREAD_DETACH:
      ; //

    DLL_PROCESS_DETACH:
      begin
        // Unhook();
      end;
  end;
end;

begin
  DllProc := @DllEntryPoint;
  DllEntryPoint(DLL_PROCESS_ATTACH);
end.

[himitsu]

HOOK overwrites a few bytes, so what do you have to do to undo?

Save old content and write back.

[flashcoder]

Zitat von himitsu:

HOOK overwrites a few bytes, so what do you have to do to undo?

Save old content and write back.

How?

could be:

Code:

Unhook(@NewLdrLoadDll, GetProcAddress(GetModuleHandle('ntdll.dll'), 'LdrLoadDll'));

?

[himitsu]

Zitat:

pbyte(target)^ := $E9;

Assignments can also be read out.
variable := pbyte(target)^;

[flashcoder]

Zitat von himitsu:

Zitat:

pbyte(target)^ := $E9;

Assignments can also be read out.
variable := pbyte(target)^;

Like this:

Code:

procedure Unhook(hookedfunc, oldfunc: pointer);
var
  jmpto: DWORD;
  OldProtect: Cardinal;
begin
  jmpto := DWORD(oldfunc) - DWORD(hookedfunc) - 5;
  VirtualProtect(hookedfunc, 5, PAGE_EXECUTE_READWRITE, @OldProtect);
  hookedfunc := pbyte(oldfunc)^;
  pdword(DWORD(hookedfunc) + 1)^ := jmpto;
end;

Usage:

Code:

Unhook(@NewLdrLoadDll, GetProcAddress(GetModuleHandle('ntdll.dll'), 'LdrLoadDll'));

right?