1+++
  2title = "Writing our own Cheat Engine: Introduction"
  3date = 2021-02-07
  4updated = 2021-02-12
  5[taxonomies]
  6category = ["sw"]
  7tags = ["windows", "rust", "hacking"]
  8+++
  9
 10This is part 1 on the *Writing our own Cheat Engine* series.
 11
 12[Cheat Engine][ce] is a tool designed to modify single player games and contains other useful tools within itself that enable its users to debug games or other applications. It comes with a memory scanner, (dis)assembler, inspection tools and a handful other things. In this series, we will be writing our own tiny Cheat Engine capable of solving all steps of the tutorial, and diving into how it all works underneath.
 13
 14Needless to say, we're doing this for private and educational purposes only. One has to make sure to not violate the EULA or ToS of the specific application we're attaching to. This series, much like cheatengine.org, does not condone the illegal use of the code shared.
 15
 16Cheat Engine is a tool for Windows, so we will be developing for Windows as well. However, you can also [read memory from Linux-like systems][linux-readmem]. [GameConqueror][game-conqueror] is a popular alternative to Cheat Engine on Linux systems, so if you feel adventurous, you could definitely follow along too! The techniques shown in this series apply regardless of how we read memory from a process. You will learn a fair bit about doing FFI in Rust too.
 17
 18We will be developing the application in Rust, because it enables us to interface with the Windows API easily, is memory safe (as long as we're careful with `unsafe`!), and is speedy (we will need this for later steps in the Cheat Engine tutorial). You could use any language of your choice though. For example, [Python also makes it relatively easy to use the Windows API][python-ctypes]. You don't need to be a Rust expert to follow along, but this series assumes some familiarity with C-family languages. Slightly advanced concepts like the use of `unsafe` or the `MaybeUninit` type will be briefly explained. What a `fn` is or what `let` does will not be explained.
 19
 20[Cheat Engine's source code][ce-code] is mostly written in Pascal and C. And it's *a lot* of code, with a very flat project structure, and files ranging in the thousand lines of code each. It's daunting[^1]. It's a mature project, with a lot of knowledge encoded in the code base, and a lot of features like distributed scanning or an entire disassembler. Unfortunately, there's not a lot of comments. For these reasons, I'll do some guesswork when possible as to how it's working underneath, rather than actually digging into what Cheat Engine is actually doing.
 21
 22With that out of the way, let's get started!
 23
 24## Welcome to the Cheat Engine Tutorial
 25
 26<details open><summary>Cheat Engine Tutorial: Step 1</summary>
 27
 28> This tutorial will teach you the basics of cheating in video games. It will also show you foundational aspects of using Cheat Engine (or CE for short). Follow the steps below to get started.
 29>
 30> 1. Open Cheat Engine if it currently isn't running.
 31> 2. Click on the "Open Process" icon (it's the top-left icon with the computer on it, below "File".).
 32> 3. With the Process List window now open, look for this tutorial's process in the list. It will look something like > "00001F98-Tutorial-x86_64.exe" or "0000047C-Tutorial-i386.exe". (The first 8 numbers/letters will probably be different.)
 33> 4. Once you've found the process, click on it to select it, then click the "Open" button. (Don't worry about all the > other buttons right now. You can learn about them later if you're interested.)
 34>
 35> Congratulations! If you did everything correctly, the process window should be gone with Cheat Engine now attached to the > tutorial (you will see the process name towards the top-center of CE).
 36>
 37> Click the "Next" button below to continue, or fill in the password and click the "OK" button to proceed to that step.)
 38>
 39> If you're having problems, simply head over to forum.cheatengine.org, then click on "Tutorials" to view beginner-friendly > guides!
 40
 41</details>
 42
 43## Enumerating processes
 44
 45Our first step is attaching to the process we want to work with. But we need a way to find that process in the first place! Having to open the task manager, look for the process we care about, noting down the process ID (PID), and slapping it in the source code is not satisfying at all. Instead, let's enumerate all the processes from within the program, and let the user select one by typing its name.
 46
 47From a quick [DuckDuckGo search][ddg-enumproc], we find an official tutorial for [Enumerating All Processes][tut-enumproc], which leads to the [`EnumProcesses`][api-enumproc] call. Cool! Let's slap in the [`winapi`][winapi-crate] crate on `Cargo.toml`, because I don't want to write all the definitions by myself:
 48
 49```toml
 50[dependencies]
 51winapi = { version = "0.3.9", features = ["psapi"] }
 52```
 53
 54Because [`EnumProcesses`][api-enumproc] is in `Psapi.h` (you can see this in the online page of its documentation), we know we'll need the `psapi` crate feature. Another option is to search for it in the [`winapi` documentation][winapi-doc] and noting down the parent module where its stored.
 55
 56The documentation for the method has the following remark:
 57
 58> It is a good idea to use a large array, because it is hard to predict how many processes there will be at the time you call **EnumProcesses**.
 59
 60*Sidenote: reading the documentation for the methods we'll use from the Windows API is extremely important. There's a lot of gotchas involved, so we need to make sure we're extra careful.*
 61
 621024 is a pretty big number, so let's go with that:
 63
 64```rust
 65use std::io;
 66use std::mem;
 67use winapi::shared::minwindef::{DWORD, FALSE};
 68
 69pub fn enum_proc() -> io::Result<Vec<u32>> {
 70    let mut pids = Vec::<DWORD>::with_capacity(1024);
 71    let mut size = 0;
 72    // SAFETY: the pointer is valid and the size matches the capacity.
 73    if unsafe {
 74        winapi::um::psapi::EnumProcesses(
 75            pids.as_mut_ptr(),
 76            (pids.capacity() * mem::size_of::<DWORD>()) as u32,
 77            &mut size,
 78        )
 79    } == FALSE
 80    {
 81        return Err(io::Error::last_os_error());
 82    }
 83
 84    todo!()
 85}
 86```
 87
 88We allocate enough space[^2] for 1024 `pids` in a vector[^3], and pass a mutable pointer to the contents to `EnumProcesses`. Note that the size of the array is in *bytes*, not items, so we need to multiply the capacity by the size of `DWORD`. The API likes to use `u32` for sizes, unlike Rust which uses `usize`, so we need a cast.
 89
 90Last, we need another mutable variable where the amount of bytes written is stored, `size`.
 91
 92> If the function fails, the return value is zero. To get extended error information, call [`GetLastError`][getlasterr].
 93
 94That's precisely what we do. If it returns false (zero), we return the last OS error. Rust provides us with [`std::io::Error::last_os_error`][lasterr], which essentially makes that same call but returns a proper `io::Error` instance. Cool!
 95
 96> To determine how many processes were enumerated, divide the *lpcbNeeded* value by `sizeof(DWORD)`.
 97
 98Easy enough:
 99
100```rust
101let count = size as usize / mem::size_of::<DWORD>();
102// SAFETY: the call succeeded and count equals the right amount of items.
103unsafe { pids.set_len(count) };
104Ok(pids)
105```
106
107Rust doesn't know that the memory for `count` items were initialized by the call, but we do, so we make use of the [`Vec::set_len`][vecsetlen] call to indicate this. The Rust documentation even includes a FFI similar to our code!
108
109Let's give it a ride:
110
111```rust
112fn main() {
113    dbg!(enum_proc().unwrap().len());
114}
115```
116
117```
118>cargo run
119   Compiling memo v0.1.0
120    Finished dev [unoptimized + debuginfo] target(s) in 0.20s
121     Running `target\debug\memo.exe`
122[src\main.rs:27] enum_proc().unwrap().len() = 178
123```
124
125It works! But currently we only have a bunch of process identifiers, with no way of knowing which process they refer to.
126
127> To obtain process handles for the processes whose identifiers you have just obtained, call the [`OpenProcess`][openproc] function.
128
129Oh!
130
131## Opening a process
132
133The documentation for `OpenProcess` also contains the following:
134
135> When you are finished with the handle, be sure to close it using the [`CloseHandle`](closehandle) function.
136
137This sounds to me like the perfect time to introduce a custom `struct Process` with an `impl Drop`! We're using `Drop` to cleanup resources, not behaviour, so it's fine. [Using `Drop` to cleanup behaviour is a bad idea][drop-behaviour]. But anyway, let's get back to the code:
138
139```rust
140use std::ptr::NonNull;
141use winapi::ctypes::c_void;
142
143pub struct Process {
144    pid: u32,
145    handle: NonNull<c_void>,
146}
147
148impl Process {
149    pub fn open(pid: u32) -> io::Result<Self> {
150        todo!()
151    }
152}
153
154impl Drop for Process {
155    fn drop(&mut self) {
156        todo!()
157    }
158}
159```
160
161For `open`, we'll want to use `OpenProcess` (and we also need to add the `processthreadsapi` feature to the `winapi` dependency in `Cargo.toml`). It returns a `HANDLE`, which is a nullable mutable pointer to `c_void`. If it's null, the call failed, and if it's non-null, it succeeded and we have a valid handle. This is why we use Rust's [`NonNull`][nonnull]:
162
163```rust
164// SAFETY: the call doesn't have dangerous side-effects.
165NonNull::new(unsafe { winapi::um::processthreadsapi::OpenProcess(0, FALSE, pid) })
166    .map(|handle| Self { pid, handle })
167    .ok_or_else(io::Error::last_os_error)
168```
169
170`NonNull` will return `Some` if the pointer is non-null. We map the non-null pointer to a `Process` instance with `Self { .. }`. `ok_or_else` converts the `Option` to a `Result` with the error builder function we provide if it was `None`.
171
172The first parameter is a bitflag of permissions we want to have. For now, we can leave it as zero (all bits unset, no specific permissions granted). The second one is whether we want to inherit the handle, which we don't, and the third one is the process identifier. Let's close the resource handle on `Drop` (after adding `handleapi` to the crate features):
173
174```rust
175// SAFETY: the handle is valid and non-null.
176unsafe { winapi::um::handleapi::CloseHandle(self.handle.as_mut()) };
177```
178
179`CloseHandle` can actually fail (for example, on double-close), but given our invariants, it won't. You could add an `assert!` to panic if this is not the case.
180
181We can now open processes, and they will be automatically closed on `Drop`. Does any of this work though?
182
183```rust
184fn main() {
185    let mut success = 0;
186    let mut failed = 0;
187    enum_proc().unwrap().into_iter().for_each(|pid| match Process::open(pid) {
188        Ok(_) => success += 1,
189        Err(_) => failed += 1,
190    });
191
192    eprintln!("Successfully opened {}/{} processes", success, success + failed);
193}
194```
195
196```
197>cargo run
198   Compiling memo v0.1.0
199    Finished dev [unoptimized + debuginfo] target(s) in 0.36s
200     Running `target\debug\memo.exe`
201Successfully opened 0/191 processes
202```
203
204…nope. Maybe the documentation for `OpenProcess` says something?
205
206> `dwDesiredAccess`
207>
208> The access to the process object. This access right is checked against the security descriptor for the process. This parameter can be **one or more** of the process access rights.
209
210One or more, but we're setting zero permissions. I told you, reading the documentation is important[^4]! The [Process Security and Access Rights][proc-rights] page lists all possible values we could use. `PROCESS_QUERY_INFORMATION` seems to be appropriated:
211
212> Required to retrieve certain information about a process, such as its token, exit code, and priority class
213
214```rust
215OpenProcess(winapi::um::winnt::PROCESS_QUERY_INFORMATION, ...)
216```
217
218Does this fix it?
219
220```rust
221>cargo run
222   Compiling memo v0.1.0
223    Finished dev [unoptimized + debuginfo] target(s) in 0.36s
224     Running `target\debug\memo.exe`
225Successfully opened 69/188 processes
226```
227
228*Nice*. It does solve it. But why did we only open 69 processes out of 188? Does it help if we run our code as administrator? Let's search for `cmd` in the Windows menu and right click to Run as administrator, then `cd` into our project and try again:
229
230```
231>cargo run
232    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
233     Running `target\debug\memo.exe`
234Successfully opened 77/190 processes
235```
236
237We're able to open a few more, so it does help. In general, we'll want to run as administrator, so normal programs can't sniff on what we're doing, and so that we have permission to do more things.
238
239## Getting the name of a process
240
241We're not done enumerating things just yet. To get the "name" of a process, we need to enumerate the modules that it has loaded, and only then can we get the module base name. The first module is the program itself, so we don't need to enumerate *all* modules, just the one is enough.
242
243For this we want [`EnumProcessModules`][mod-enumproc] and [`GetModuleBaseNameA`][mod-name]. I'm using the ASCII variant of `GetModuleBaseName` because I'm too lazy to deal with UTF-16 of the `W` (wide, unicode) variants.
244
245```rust
246use std::mem::MaybeUninit;
247use winapi::shared::minwindef::HMODULE;
248
249pub fn name(&self) -> io::Result<String> {
250    let mut module = MaybeUninit::<HMODULE>::uninit();
251    let mut size = 0;
252    // SAFETY: the pointer is valid and the size is correct.
253    if unsafe {
254        winapi::um::psapi::EnumProcessModules(
255            self.handle.as_ptr(),
256            module.as_mut_ptr(),
257            mem::size_of::<HMODULE>() as u32,
258            &mut size,
259        )
260    } == FALSE
261    {
262        return Err(io::Error::last_os_error());
263    }
264
265    // SAFETY: the call succeeded, so module is initialized.
266    let module = unsafe { module.assume_init() };
267    todo!()
268}
269```
270
271`EnumProcessModules` takes a pointer to an array of `HMODULE`. We could use a `Vec` of capacity one to hold the single module, but in memory, a pointer a single item can be seen as a pointer to an array of items. `MaybeUninit` helps us reserve enough memory for the one item we need.
272
273With the module handle, we can retrieve its base name:
274
275```rust
276let mut buffer = Vec::<u8>::with_capacity(64);
277// SAFETY: the handle, module and buffer are all valid.
278let length = unsafe {
279    winapi::um::psapi::GetModuleBaseNameA(
280        self.handle.as_ptr(),
281        module,
282        buffer.as_mut_ptr().cast(),
283        buffer.capacity() as u32,
284    )
285};
286if length == 0 {
287    return Err(io::Error::last_os_error());
288}
289
290// SAFETY: the call succeeded and length represents bytes.
291unsafe { buffer.set_len(length as usize) };
292Ok(String::from_utf8(buffer).unwrap())
293```
294
295Similar to how we did with `EnumProcesses`, we create a buffer that will hold the ASCII string of the module's base name[^5]. The call wants us to pass a pointer to a mutable buffer of `i8`, but Rust's `String::from_utf8` wants a `Vec<u8>`, so instead we declare a buffer of `u8` and `.cast()` the pointer in the call. You could also do this with `as _`, and Rust would infer the right type, but `cast` is neat.
296
297We `unwrap` the creation of the UTF-8 string because the buffer should contain only ASCII characters (which are also valid UTF-8). We could use the `unsafe` variant to create the string, but what if somehow it contains non-ASCII characters? The less `unsafe`, the better.
298
299Let's see it in action:
300
301```rust
302fn main() {
303    enum_proc()
304        .unwrap()
305        .into_iter()
306        .for_each(|pid| match Process::open(pid) {
307            Ok(proc) => match proc.name() {
308                Ok(name) => println!("{}: {}", pid, name),
309                Err(e) => println!("{}: (failed to get name: {})", pid, e),
310            },
311            Err(e) => eprintln!("failed to open {}: {}", pid, e),
312        });
313}
314```
315
316```
317>cargo run
318   Compiling memo v0.1.0
319    Finished dev [unoptimized + debuginfo] target(s) in 0.32s
320     Running `target\debug\memo.exe`
321failed to open 0: The parameter is incorrect. (os error 87)
322failed to open 4: Access is denied. (os error 5)
323...
324failed to open 5940: Access is denied. (os error 5)
3255608: (failed to get name: Access is denied. (os error 5))
326...
3271704: (failed to get name: Access is denied. (os error 5))
328failed to open 868: Access is denied. (os error 5)
329...
330```
331
332That's not good. What's up with that? Maybe…
333
334> The handle must have the `PROCESS_QUERY_INFORMATION` and `PROCESS_VM_READ` access rights.
335
336…I should've read the documentation. Okay, fine:
337
338```rust
339use winapi::um::winnt;
340OpenProcess(winnt::PROCESS_QUERY_INFORMATION | winnt::PROCESS_VM_READ, ...)
341```
342
343```
344>cargo run
345   Compiling memo v0.1.0 (C:\Users\L\Desktop\memo)
346    Finished dev [unoptimized + debuginfo] target(s) in 0.35s
347     Running `target\debug\memo.exe`
348failed to open 0: The parameter is incorrect. (os error 87)
349failed to open 4: Access is denied. (os error 5)
350...
3519348: cheatengine-x86_64.exe
3523288: Tutorial-x86_64.exe
3538396: cmd.exe
3544620: firefox.exe
3557964: cargo.exe
35610052: cargo.exe
3575756: memo.exe
358```
359
360Hooray 🎉! There's some processes we can't open, but that's because they're system processes. Security works!
361
362## Finale
363
364That was a fairly long post when all we did was print a bunch of pids and their corresponding name. But in all fairness, we also laid out a good foundation for what's coming next.
365
366You can [obtain the code for this post][code] over at my GitHub. At the end of every post, the last commit will be tagged, so you can `git checkout step1` to see the final code for any blog post.
367
368In the [next post](/blog/woce-2), we'll tackle the second step of the tutorial: Exact Value scanning.
369
370### Footnotes
371
372[^1]: You could say I simply love reinventing the wheel, which I do, but in this case, the codebase contains *far* more features than we're interested in. The (apparent) lack of structure and documentation regarding the code, along with the unfortunate [lack of license][lack-license] for the source code, make it a no-go. There's a license, but I think that's for the distributed program itself.
373
374[^2]: If it turns out that there are more than 1024 processes, our code will be unaware of those extra processes. The documentation suggests to perform the call again with a larger buffer if `count == provided capacity`, but given I have under 200 processes on my system, it seems unlikely we'll reach this limit. If you're worried about hitting this limit, simply use a larger limit or retry with a larger vector.
375
376[^3]: C code would likely use [`GlobalAlloc`][global-alloc] here, but Rust's `Vec` handles the allocation for us, making the code both simpler and more idiomatic. In general, if you see calls to `GlobalAlloc` when porting some code to Rust, you can probably replace it with a `Vec`.
377
378[^4]: This will be a recurring theme.
379
380[^5]: …and similar to `EnumProcesses`, if the name doesn't fit in our buffer, the result will be truncated.
381
382[ce]: https://cheatengine.org/
383[python-ctypes]: https://lonami.dev/blog/ctypes-and-windows/
384[ce-code]: https://github.com/cheat-engine/cheat-engine/
385[linux-readmem]: https://stackoverflow.com/q/12977179/4759433
386[game-conqueror]: https://github.com/scanmem/scanmem
387[ddg-enumproc]: https://ddg.gg/winapi%20enumerate%20all%20processes
388[tut-enumproc]: https://docs.microsoft.com/en-us/windows/win32/psapi/enumerating-all-processes
389[api-enumproc]: https://docs.microsoft.com/en-us/windows/win32/api/psapi/nf-psapi-enumprocesses
390[winapi-crate]: https://crates.io/crates/winapi
391[winapi-doc]: https://docs.rs/winapi/
392[getlasterr]: https://docs.microsoft.com/en-us/windows/win32/api/errhandlingapi/nf-errhandlingapi-getlasterror
393[lasterr]: https://doc.rust-lang.org/stable/std/io/struct.Error.html#method.last_os_error
394[vecsetlen]: https://doc.rust-lang.org/stable/std/vec/struct.Vec.html#method.set_len
395[openproc]: https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-openprocess
396[closehandle]: https://docs.microsoft.com/en-us/windows/win32/api/handleapi/nf-handleapi-closehandle
397[drop-behaviour]: https://internals.rust-lang.org/t/pre-rfc-leave-auto-trait-for-reliable-destruction/13825
398[nonnull]: https://doc.rust-lang.org/stable/std/ptr/struct.NonNull.html
399[proc-rights]: https://docs.microsoft.com/en-us/windows/win32/procthread/process-security-and-access-rights
400[mod-enumproc]: https://docs.microsoft.com/en-us/windows/win32/api/psapi/nf-psapi-enumprocessmodules
401[mod-name]: https://docs.microsoft.com/en-us/windows/win32/api/psapi/nf-psapi-getmodulebasenamea
402[code]: https://github.com/lonami/memo
403[lack-license]: https://github.com/cheat-engine/cheat-engine/issues/60
404[global-alloc]: https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-globalalloc