Is it possible to read process memory of a 64 bit process from a 32bit app?

Question

On windows 64 bit, I've got a 32 bit process that reads the memory of other 32 bit processes, and I'd like it to be able to read 64 bit processes too.

ReadProcessMemory is being used to read the memory, but it has a 32 bit limitation. Is there any way of doing the equivalent of a ReadProcessMemory on a 64 bit process?

I know I could write a 64 bit process and launch that from my 32 bit process to do the work, but I'm wondering if there's some other option so that I don't need to write a 64 bit process.

Thanks.

Answer 1

It's possible.

For an example you may refer to the excellent sample in the answer of tofucoder. For one more sample you may refer to this link.

For explanation why it actually works please check this thread.

Another sample may be found here.

The whole trick is to call 64-bit version of ReadProcessMemory function. Intuitively it's not an option from 32-bit process however the link above explains: x64 version of ntdll.dll is also loaded as a part of 32-bit process in Windows WOW64 emulator. It has a function called NtReadVirtualMemory with the same prototype as ReadProcessMemory64:

__declspec(SPEC)BOOL __cdecl ReadProcessMemory64(HANDLE hProcess, DWORD64 lpBaseAddress, LPVOID lpBuffer, SIZE_T nSize, SIZE_T *lpNumberOfBytesRead);

The address is 64-bit long and thus the whole virtual address space of 64-bit process may be referred.

You may wonder how to get the address of this function. It's when another function in ntdll.dll comes in handy: LdrGetProcedureAddress. Its prototype is the same as of GetProcAddress:

__declspec(SPEC)DWORD64 __cdecl GetProcAddress64(DWORD64 hModule, char* funcName);

We are to examine export directory of x64 ntdll.dll and manually found this function's entry. Then we can obtain address of any other function.

Another question is left uncovered so far: how to obtain start address of x64 ntdll.dll? We need to manually walk through x64 PEB structure of our process and traverse loaded modules' list - as one of the variants. And how to get PEB address? Please refer to the links above, not to overflow this post with too many details.

All this is covered in sample from the first link. Alternative variants with usage of NtReadVirtualMemory & NtWow64ReadVirtualMemory64 functions are provided in second & third links (as well as alternative ways to get PEB address).

Summary: it is possible to interact with x64 process from x86 one. It can be done either with direct call to x64 version of function (from x64 ntdll.dll which is loaded as a part of WOW64 process) or with the call of specific x86 function which is intended to work with x64 process (namely NtWow64ReadVirtualMemory64).

P.S. One may say it's undocumented and is more like hack - but it's just not officially documented. Soft like Unlocker, ProcessHacker or ProcessExplorer, for example, makes use of these undocumented features (and many more), and it's up to you to decide, of course.