I'm currently making one of those game trainers as a small project. I've already ran into a problem; when you "go into a different level", the addresses for things such as fuel, cash, bullets, their addresses change. This would also happen say, if you were to restart the application.
How can I re-locate these addresses?
I feel like it's a fairly basic question, but it's one of those "it is or is not possible" questions to me. Should I just stop looking and forget the concept entirely? "Too hard?"
It's a bit hard to describe exactly how to do this since it heavily dependents on the program you're studying and whether the author went out if his way to make your life difficult. Note that I've only done this once but it worked reasonably well even if I only knew a little assembly.
What is probably happening is that the values are allocated on the heap using a call to malloc/new and everytime you change level they are cleaned up and re-allocated somewhere else. So the idea is to look at the assembly code of the program to find where the pointer returned by malloc is stored and figure out a way to reliably read the content of the pointer and find the value you're looking for.
First thing you'll want is a debugger like OllyDbg and a basic knowledge of assembly. After that, start by setting a read and write breakpoint on the variable you want to examine. Since you said that you can't tell exactly where the variable is, you'll have to pause the process while it's running and search the program's memory for the value. Hopefully you'll end up with only a few results to sift through but be suspicious of anything that is on the stack since it might just be a copy for a function call or for local use.
Once the breakpoint is set just run the program until a break occurs. Now all you have to do is look at the code and examine how the variable is being accessed. If it's being passed as a parameter, go examine the call site of the function. If it's being accessed through a pointer, make a note of it and start examining the pointer. If it's being accessed as an offset of a pointer, that means it's part of a data structure so make a note of it and start examining the other variable. And so on.
Stay focused on your variable and just keep examining the code until you eventually find the root which can be one of two things:
A global variable that has a static address. This is the easiest scenario since you have a static address hardcoded straight into the code that you can use to reliably walk through the data structures.
A stack allocated variable. This is trickier and I'm not entirely sure how to deal with this scenario reliably. It's possible that its address will have the same offset from the beginning of the stack most of the time but it might not. You could also walk the stack to find the corresponding function and its parameters but this a bit tricky to get right.
Once you have an address all that's left to do is use ReadProcessMemory to locate your variable using the information you found. For example, if the address you have represents a pointer to a data structure where at offset 0x40 your fuel value is stored, then you'll have to read the value at the address, add 0x40 to it and do another read on the result.
Note that the address is only valid as long as the executable doesn't change in any way. If it's recompiled or patched then you have to start over. I believe you'll also have to be careful about Windows' ASLR which might change the address around every time you start the program.
Comment box was too small to fit this so I'll put it here.
If it's esp plus a constant then I believe that this is a parameter and not a local variable (do confirm by checking the layout of the calling convention). If that's the case, then you should step the program until it returns to its caller, figure out how the parameter is being set (look for push instructions before the call instruction) and continue exploring from there. When I did this I had to unwind the stack once or twice before I found the global pointer to the data structure.
Also the esi register is not related to the stack (I had to look it up) so I'd check how it's being set. It could be that it contains the address of the data structure and the constant is the offset to the variable. If you figure out how the register is set you'll be that much closer to the pointer.
Related
I have been reading up on HeapValidate in an existing code and trying to figure out what does it do. The documentation says that it checks whether the heap control structures are in a consistent state. What does that mean?
The heap is a data structure like any other, and in its metadata-state-variables there are certain conditions that should always be true. As a made-up example, the number of children in a heap-tree-node should always be a non-negative number; so if HeapValidate() reads a child-count variable and sees that it is negative, it knows something has gone badly wrong and can flag that block as broken.
You might wonder, assuming Microsoft’s heap code does not have any bugs, how the heap’s metadata might get in to an invalid/“impossible” state like that in the first place. Since the heap’s metadata structures live in the same address space that the user-code has access to, it’s usually the result of buggy user code writing some other data via an invalid pointer that happens to point at a memory location where the heap’s metadata field happens to reside, silently overwriting/corrupting the metadata.
I'm trying to make a program that can read some information from another process. I use Cheat Engine to find the memory address of whatever I'm looking for and ReadProcessMemory in c++ to get the value.
So far so good. Here is my problem: The process I'm trying to get the information from can have multiples windows open at the same time. I will take notepad++ as an example. With notepad++, I can open as many .txt files as I want. Each of these files' content will have their own memory address. So what I think I need is a memory address with every pointer to every files content as value
Example:
Address A = Value 1
Address B = Value 2
Address C = Value 3
etc... for all files opened
I would need to find a static address with the value: (address A, address B, address C, etc...).
I don't even know how to look for that... Can a memory address hold an array of values...?
EDIT: Really guys, you think the ONLY purpose of cheat engine is cheating? I'm not trying to cheat or hack anything. I didn't know trying to learn about memory address was wrong... For your info, I'm trying to make my own interface for a program I like. AN INTERFACE, that's not cheating.
In a program where windows are dynamically allocated, variables related to those windows will generally be dynamically allocated as well. That means the addresses may be different each time the program runs (depending on what else is in the program's heap at the time). In order to reliably get those locations, you'd need to start at the top-level static pointer (e.g. the address of the list of windows) and then dynamically follow the chain of dynamically-allocated pointers down to the addresses you are looking for. Whether CheatEngine can do this, or if it can even be done safely at all, I don't know.
Memory only holds numbers. It may be helpful to think of process memory as a huge list of numbers, each taking a value in the range (0-255). How those numbers are interpreted as is entirely up to the process (i.e., notepad in your example). This includes whether or not they are some "value" or perhaps a pointer to some value, etc.
I have read from Google that it is used for controlling buffer overruns at application level and it is called by CRT.
It also says that
" Essentially, on entry to an overrun-protected function, the cookie is put on the stack, and on exit, the value on the stack is compared against the global cookie. Any difference between them indicates that a buffer overrun has occurred and results in immediate termination of the program."
But I could not much understand how it works? Please help.
The "cookie" is basically nothing more than an arbitrary value.
So, the basic idea is that you write the chosen value on the stack before calling a function. Although it's probably not a very good value, let's arbitrarily chose 0x12345678 as the value.
Then it calls the function.
When the function returns, it goes back to the correct spot on the stack, and compares that value to 0x12345678. If the value has changed, this indicates that the function that was called wrote outside the area of the stack where it was allowed to write, so it (and that process in general) are deemed untrustworthy, and shut down.
In this case, instead of choosing 0x12345678, the system chooses a different value on a regular basis, such as every time the system is started. This means it's less likely to hit the correct value by accident -- it might happen to do so once, but if it's writing a specific value there, when the correct/chosen value changes, it'll end up writing the wrong value, and the problem will be detected.
It's probably also worth noting that this basic idea isn't particularly new. Just for example, back in the MS-DOS days, both Borland's and Microsoft's compilers would write some known value at the very bottom of the stack before calling main in your program. After main returned, they'd re-check that value. It would then print out an error message (right as the program exited) if the value didn't match what was expected.
It's exactly what the explanation says, but you can replace "cookie" with "some value". When the function is called, it puts some value on the stack. When the function returns, it checks it again to see if it changed.
The normal behavior of the function is to not touch the memory location. If the value there changed, it means that function code somehow overwrote it, and this means there was a buffer overflow.
I recently used the /FAsu Visual C++ compiler option to output the source + assembly of a particularly long member function definition. In the assembly output, after the stack frame is set up, there is a single call to a mysterious _chkstk() function.
The MSDN page on _chkstk() does not explain the reason why this function is called. I have also seen the Stack Overflow question Allocating a buffer of more a page size on stack will corrupt memory?, but I do not understand what the OP and the accepted answer are talking about.
What is the purpose of the _chkstk() CRT function? What does it do?
Windows pages in extra stack for your thread as it is used. At the end of the stack, there is one guard page mapped as inaccessible memory -- if the program accesses it (because it is trying to use more stack than is currently mapped), there's an access violation. The OS catches the fault, maps in another page of stack at the same address as the old guard page, creates a new guard page just beyond the old one, and resumes from the instruction that caused the violation.
If a function has more than one page of local variables, then the first address it accesses might be more than one page beyond the current end of the stack. Hence it would miss the guard page and trigger an access violation that the OS doesn't realise is because more stack is needed. If the total stack required is particularly huge, it could perhaps even reach beyond the guard page, beyond the end of the virtual address space assigned to stack, and into memory that's actually in use for something else.
So, _chkstk ensures that there is enough space for the local variables. You can imagine that it does this by touching the memory for the local variables at page-sized intervals, in increasing order, to ensure that it doesn't miss the guard page (so-called "stack probes"). I don't know whether it actually does that, though, possibly it takes a more direct route and instructs the OS to map in a certain amount of stack. Either way, if the total required is greater than the virtual address space available for stack, then the OS can complain about it instead of doing something undefined.
I looked at the code for __chkstk and it does do the repeated stack probes at one-page intervals. So this way, it doesn't need to make any calls to the OS. The parameter in rax is size of data you want to add. It ensures that the target address (current rsp - rax) is accessible. If rax > rsp, it does this for address 0. As an interesting shortcut, it first compares the address with gs:[10h], which is the current lowest page that is mapped; if the target address >= this, then it does nothing.
By the way, for 64-bit code at least, it is spelled with two underscores: __chkstk__.
I just came to know that there are data breakpoints. I have worked for the last 5 years in C++ using Visual Studio, and I have never used data breakpoints.
Can someone throw some light on what data breakpoints are, when to use them and how to use them with VS?
As per my understanding we can set a data breakpoint when we want to check for changes to a variable's value. In this case, we can set a data breakpoint with a condition on the variable value.
Any other examples?
Good ol' Daniel LeCheminant has a solid answer on what a data breakpoint does, so i'll toss in some anecdotes that highlight useful uses:
Any scenario where you know what will change, but have little or no idea where the code changing it lives (since otherwise you could simply use a conditional breakpoint). Specifically,
"Impossible" scenarios - program is crashing, because variable X is NULL, when variable X should never be NULL because no code anywhere ever sets variable X to NULL. Put a normal breakpoint in the code that initializes X, and when it is hit, set up a data breakpoint to watch for the change to NULL. Somewhat more common is the case where memory is released too early, and there are still pointers to it hanging around: use data breakpoints to find out who's releasing the memory.
Tedious scenarios - a 3rd-party library is doing bad, nasty, horrible things to your data structures. You know it's happening, because someone is trashing your data and obviously your code is perfect. But you don't know where, or when. Sure, you could single-step through a megabyte of disassembled DLL... but why bother, when you can set a data breakpoint on your data, sit back, and wait for it to get trashed!
Heisenbugs - similar to the impossible scenario, but they go away when you watch too closely, such that normal breakpoints - even conditional breakpoints - are useless. Timing and user-input sensitive logic is particularly vulnerable to this sort of thing. Since data breakpoints don't require the debugger to actually break at all until the time is right, assuming you can come up with a memory location that will only change when that elusive bug actually occurs you can use data breakpoints to set a trap for the Heisenbug and catch it in flagrante delicto.
Spaghetti scenarios - common in old, rotten code bases where global data is accessed everywhere. Yeah, you could use plain ol' conditional breakpoints... but you'd need hundreds of them. Data breakpoints make it easy.
Definition:
Data breakpoints allow you to break
execution when the value stored at a
specified memory location changes.
From MSDN: How to: Set a Data Breakpoint:
How to Set a Memory Change Breakpoint
From the Debug Menu, choose New Breakpoint and click New Data Breakpoint
—or—
in the Breakpoints window Menu, click the New dropdown and choose New Data Breakpoint.
The New Breakpoint dialog box appears.
In the Address box, enter a memory address or expression that evaluates to a memory address. For example, &foo to break when the contents of variable foo change.
In the Byte Count box, enter the number of bytes you want the debugger to watch. For example, if you enter 4, the debugger will watch the four bytes starting at &foo and break if any of those bytes change value.
Click OK.
So far we've got a great definition and a bunch of great theoretical explanations.
Let's have a concrete example!
I'm currently working on a rather large and convoluted codebase. I made a small safe change to one bit of code and started getting - in a completely unrelated chunk of the codebase - crashes in the memory allocator. This is generally a sign that you're doing something Very Wrong with memory management - either double-deletion or writing out-of-bounds.
Thankfully, we have an option to turn on a debug memory manager that checks for things like this. I turned it on and it immediately started reporting a memory block guard violation, which means that something wrote out of bounds. The problem is that this report shows up only once the memory is deallocated - essentially saying "hey, something was broken. Hope you can figure out what!"
Unfortunately this particular chunk of memory, at the point of deallocation, is completely indistinguishable from literally thousands of other chunks of memory. Fortunately, our debug framework tags each allocation with a consecutive ID, and the memory that got corrupted had a consistent ID (#9667, if you're curious.) One quick breakpoint in the memory manager later and I was able to find where that memory was allocated. Which, as it turned out, wasn't immediately helpful either.
But at that point, I had several important components:
I knew the address of a block of memory
I knew the intended length of that memory
I knew that, at some point in the future, a specific byte past the intended length of that memory would be overwritten
Given this, I could set up a data breakpoint on that specific byte, then hit "go" and find out where the corruption occured.
Which I did - it led to an off-by-one error which I am now in the process of fixing.
And that's a concrete example of how data breakpoints can be useful. :)
I believe data breakpoints are breakpoints which will occur when some memory is set to a certain value. For example, you can set a breakpoint when i == 10 in a typical for loop to stop after the 10th iteration. You can also watch for changes to variables on the heap, like wait for a member of a class to be modified.