Recently I have been reading up articles about DLL injection and I understand them fairly well.
However, what I don't understand is why APIs such as CreateRemoteThread, WriteProcessMemory(in being able to write to the memory of another process) and VirtualAllocEx(in being able to allocat memory in the context of another process) were implemented in the first place.
What was the original need for such APIs? Just curious.
WriteProcessMemory was made for ring3 debuggers that need to securely write process memory, most commonly for INT 3 breakpoints or user provided memory edits.
along the same line, CreateRemoteThread can also be used for debugging purposes, however, MSDN can enlighten us on CreateRemoteThread a bit more:
A common use of this function is to inject a thread into a process
that is being debugged to issue a break. However, this use is not
recommended, because the extra thread is confusing to the person
debugging the application and there are several side effects to using
this technique:
It converts single-threaded applications into
multithreaded applications.
It changes the timing and memory layout of
the process.
It results in a call to the entry point of each DLL in
the process.
IIRC, CreateRemoteThread is also used by debuggers to hook application native expection handlers, commonly set by SetExceptionHandler, which requires call from the target process as the handler is stored in the PEB.
VirtualAllocEx is just how windows virtual memory system operates, it needs a context to allocate in, be it in the current process, a child process or a remote process. VirtualAlloc in fact is nothing more than a pass through wrapper of the Ex variant, it just passes a special constant that indicates the handle of the caller process is to be used.
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I have a large, complex application written in C++ (no MFC or .NET). The client that uses the software most aggressively will, within an hour or so of starting it, get to a state where all the windows stop painting. We get reports that the application has "hung" because as far as they can tell nothing is happening. In reality, the application is functioning, just not displaying anything.
I've tried a lot of different things to no avail. I'm out of ideas...
You probably already have a hunch of what it is - you give it away in the first sentence
... large, complex application ...
It sounds like you have a GDI resource leak somewhere. To confirm this try looking in task manager at GDI objects for your process. At some point most GDI operations will fail for your application.
Make sure you are freeing all handles correctly. Note that different GDI objects require different methods of freeing the object. For example GetDC is freed by ReleaseDC, but CreateDC is freed by DeleteDC.
This is why RAII smart objects (like smart pointers) are recommended for resource management in C++ (where freeing is managed by the smart object to reduce the likelihood of leaks and errors).
I'd bet that the application is leaking GDI objects, and when the GDI dedicated space for this process is exhausted, it can no longer paint itself.
You can check if this is the case by adding to the Windows Task Manager (or any other process manager such as Process Monitor) the column GDI Objects and see if this number grows unbounded with time.
Your application may actually be suffering from an exception that is getting ignored. See Microsoft KB article 976038.
I'm writting a Windows Java application that needs to call unsafe native code, and I need to prevent this code from having access to Java objects and JVM data structures, otherwise it might crash the JVM or hack into sensitive data. Before you ask, this native code is previously verified - it can only call a few APIs and cannot have certain instructions, so we know it won't VirtualProtect itself or other memory regions to gain more access and mess around.
Anyway, my first attempt was to wrap this code into a separate process (sandbox) and use IPC to talk with Java. There's a JNI DLL that does IPC stuff on Java side. Basically, every time we need to run unsafe native code, our Java app calls a JNI function that wakes up the sandbox using an auto-reset Windows Event, and then awaits completion. The sandbox runs unsafe native code and wakes up the JVM using another auto-reset Windows Event, and life continues. It would be perfect if it weren't so slow.
The problem is that unsafe native code can contain some functions that perform very quick calculations and can be called millions of times from Java, hence the call overhead should be minimum. But this overhead is huge because JVM wakes-up sandbox with a Windows Event, and vice-versa when the sandbox returns. This process is 8x the time of an in-process (non-IPC) solution, where unsafe native code is wrapped in JNI DLL (and hence the call happens in the same thread, in the same time slice).
My first guess is that when JVM wakes-up the sandbox, Windows only puts the sandbox thread on the ready set, so it runs only after some milliseconds. And the same happens when the sandbox returns. Not to count for two (possibly expensive) context switches.
Microsoft documentation here says the following:
If a higher-priority thread becomes available to run, the system ceases to execute the lower-priority thread (without allowing it to finish using its time slice), and assigns a full time slice to the higher-priority thread.
To test this theory, I assigned THREAD_PRIORITY_TIME_CRITICAL to the sandbox thread. There was some gains. Performance went from 8x to 5x the time of the in-process (non-IPC) solution. But I need more, otherwise this Java app might not get a change to go into production!
You can help me in two ways:
Tell me if there's a faster method to wake-up another process, such as forcing a context switch or performing an inter-process procedure call.
Tell me how can I protect JVM while running unsafe native code in-process. I heard that Google Native Client does this, but I only found this documentation. If you know more, please provide links to more detailed information about how this is implemented.
I solved the problem by performing JVM-sandbox interactions using spinlock over a shared memory variable accessed from JVM through file mapping. This question explains how to implement in a C++ environment. Porting to Java is easy with MappedByteBuffer.
I want to unload a DLL from another process. Is it possible?
If yes, how to do that? (I'm using Win32 API)
Thank you very much.
Yes, this is possible. It is called DLL ejection and is featured by some DLL injectors. The way a DLL is usually loaded is via LoadLibrary and it is subsequently unloaded via FreeLibrary. FreeLibrary takes only one parameter which is a handle to the module to be unloaded. If you injected the DLL in the first place, you should be able to find this very easily. Otherwise there are ways of obtaining the handle such as CreateToolHelp32Snapshot with further enumeration with Module32First/Module32Next. Suppose you have obtained the handle through some means, then the steps to eject the DLL are simple:
Get the address of FreeLibrary with GetProcAddress. This address will match the one for the same function in the target because of how Windows works.
Call CreateRemoteThread on the target process, specifying lpStartAddress as the address of FreeLibrary, and with lpParameter as the module's handle
There are several caveats to DLL ejection.
You should only ever eject a DLL which you are certain no code is going to make use of again in the future. If any dynamically linked code attempts to make a call to your code after it has been freed, it will most likely trigger some form of page access violation.
You should ensure that no threads are executing within the code of the DLL whilst ejection is being performed for similar reasons.
DLL ejection should be avoided with general. If the library wants to have the option of being freed, it should supply some interface which users can access it through which eventually calls FreeLibraryAndExitThread.
If you require a code example for this, I have written an ejector as part of an injector I wrote in the past in C. I can search it up and find it but it's from many years ago and the code quality is not likely to be good.
You don't want to do this.
"Loading" a DLL is much more than simply opening (and locking) a file. When the NT loader starts an executable, it processes all the DLLs referenced by the image (recursively) and wires up the function calls (recursively): loading the DLLs, calling the DLL initialization code, etc.
Unloading a DLL would mean that you'd need to stop all processes that loaded your DLL, load the new DLL, and perform all the operations the NT loader would. Of course, unloading and re-loading a DLL would need to restore that old DLL's state (initialized variables etc), an operation which is not specified in Win32.
For a bit of background information, see this article on MSDN and this Under the Hood article in MSJ.
Short answer: No, it is impossible.
Win32 doesn't provide an API to unload a DLL of another process. If a library is freed unexpectedly, the process will crash. This leads to a serious security hole as it breaks process protection mechanism.
If you can modify both of the processes, you can modify the application and add routines to free a library, and let the other application to send the message.
I would instead look to change the function called when the process tries to invoke the functions in that dll. I know this is possible in theory.
It would mean a bit of memory hacking and knowing where the pointers to the functions are stored, but all of that can be found easily enough (ollydbg manages to do it), it would be harder if they use ordinals, even harder if they hard code the pointers, but no one does that nowadays. You could then inject your own code that (ideally) mimics the functions they mask, but does not actually do anything. They will probably have to be injected into the process, and that way you could get it to work without the process ever knowing, and without any crashes.
Here is a little background information. I'm working on replacing a dll that has been used in a dll injection technique via the AppInit_DLLs registry entry. Its purpose was to be present in every process and set hooks into the GDI32.dll to gather information about printing. This is kind of a funky way to get what we want. The .dll itself is over 10 years old (written in Visual Studio 97) and we'd like to replace it with something a little less invasive than an injected dll.
It appears SetWindowsHookEx() maybe what we are looking for. I've been having some trouble with it, but I've also had some discussions with co-workers about whether or not this tree is worth barking up. Here are some questions that we could not determine:
When we hook a routine out of a dll, for example StartDoc() from GDI32.dll, do we really get a notification every time any other process uses that rotuine out of that dll? This is kind of the functionality we were getting with our injected .dll and we need the same functionality going forward.
When the hook is triggered, does the hook handling procedure run in the process space of the process that initiated the actual call, or in the process space of the process that set up the hook? My opinion is that it has to run in the process space of the process that called the routine. For example, if a program calls StartDoc() from GDI32.dll, it will have the hook handling procedure code "injected" into its space and executed. Otherwise, there would have to be some inter-process communication that automatically gets set up between the calling process and the process that set up the hook, and I just don't see that as being the case. Also, its kind of necessary that this hook handling routine run in the process space of the calling process since one of the things it needs to know is the name of that calling process, and I'm not sure on how to get that information if it wasn't actually running in that process.
If the hook handling routine is written using the .NET managed environment, will it break when getting hooked into a process not utilizing the .NET managed environment? We'd really like to move away from C++ here and use C#, but what would happen if we our hook gets called from a process that isn't managed? As stated before, I think that our hook handling procedure will run in the process that originally called the routine that was hooked. But if this is true, than I would think that we'd run into trouble if this process was not using the .NET run time environment, but the incoming hooked handling code is.
Yes.
Generally, it's the former: it executes in the context of the process whose event it is hooking.
After a successful call to SetWindowsHookEx, the operating system automatically injects the hook DLL (the one that contains the callback function) into the address space of all target processes that meet the requirements for the specified hook type. (Of course, the hooking code is not necessarily injected immediately.)
The exception to this general rule are the low-level keyboard and mouse hooks (WH_LL_KEYBOARD and WH_LL_MOUSE). Since those hook types are not injected into the client processes, the callback is called in the same thread that originally called SetWindowsHookEx.
That last point is important to keep in mind to answer your third question. Because the low-level keyboard and mouse hooks are the only two global hooks that do not require DLL injection, they are also the only two types of hooks that can be written in managed .NET code.
For the other hook types, your concerns expressed in the question are precisely correct. You would need to write these hook DLLs in C or C++. Of course, the rest of your application's pieces could still be written in a managed language. The only thing that matters is the hook DLL.
You might consider looking into either Microsoft Detours or EasyHook.
My Win32 console applicaton uses a third-party library. After it exits WinMain global objects destruction begins and an AV happens somewhere deep inside. I'm really tempted to just write
TerminateProcess( GetCurrentProcess(), 0 );
somewhere near the end of WinMain. If I do this the application ends gracefully.
But MSDN says that doing so can compromise the state of global data maintained by dynamic-link libraries (DLLs) which is not clear. I understand that if I have some global object its destructor is not run and I risk not finalizing a database connection or something similar. I don't have anything like that in my program.
What exactly is the risk when using TerminateProcess? How do I determine if I can use it for my purpose?
Based on the documentation for that and ExtiProcess it seems the primary concern is that DLL's are unloaded without a call to DllMain with the flag DLL_PROCESS_DETACH.
My 2cents: The documentation is being paranoid that you will upset some critical operation which runs in DllMain + DLL_PROCESS_DETACH. Anyone who depends on that to maintain critical state is already at the mercy of task manager so I don't see a huge risk in using this API.
Generally the bad things will happen when interacting with objects outside of your process. For an example say you have some shared memory used by multiple processes that your process will write to and other processes read and or write to. Typically to synchronize the reading and writing a mutex is used. If a thread in your process has acquired the mutex and is in the middle of making changes when TerminatePorcess is called, the mutex will be abandoned and the shared memory potentially left in an inconsistent state.
I suspect you are miss using one of the third party libraries. DllMain is somewhat limiting so the library may have initialize and uninitialize functions that you are supposed to call.
AFAIK, if you're not doing anything "fancy" (which includes but is not limited to: creating threads, locks, DB connections, using COM objects), nothing terrible will happen. But as Earwicker says, you don't know what OS-wide stuff a DLL is doing, and you certainly don't know if that will change in the future, so relying on this is very fragile.
Aren't you curious to know why this access violation is occurring? It may well be the sign of something that became corrupted much earlier on. Please at least confirm that the bug is caused by this 3rd-party library, e.g. by writing a program that links with the library but whose main() does nothing, and confirming that this causes the same crash.
It depends how you interpret "global data". If you take it to mean (as I normally would) data stored in the process's address space, then the advice makes no sense - we know that memory is going to disappear, so who cares what happens to that?
So it may be referring to OS-wide stuff that a DLL may have done, that persists outside the lifetime of any process. A simple example would be a temporary file that might need to be cleaned up; crash the process too many times and you'll run out of disk space, so probably best not to make a habit of it.