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Memory editor is used as game trainer on iOS/OSX by most power users, but very few App Store developers know how to write one. Today I'll be here guiding you through the very basic knowledge of writing a memory editor, we'll review some basic concepts which may be ignored by Objective-C coders, describe how a memory editor works, explain the usage of some memory manipulation functions, and write the code at last. Our target is a working memory editor (game trainer) who successfully cheats "Blood & Glory 2" from OSX App Store and "大富翁4FunLite" from iOS App Store. Feel free to correct me if anything is wrong, and have fun
Part I Basic concepts
We write HelloWorld.c, compile it and get HelloWorld.exe, an executable. On iOS/OSX, executables are all MachO format, containing data and instructions needed for execution. When the executable is run by OS, it becomes a process. So actually executable and process are different status of a same object, i.e. some data and a set of instructions. We can view executable as static, process as dynamic. Executable is stored on hard disk, while process is "stored" in memory. So bare in mind, process contains data and instructions, and they're in memory.
2. Virtual memory, memory page and memory region
When OS runs an executable, it loads this executable from hard disk to memory. Where in memory is this process located? This is a very complicated procedure that ATM, the only thing you need to know is that every process has its own separate memory space (i.e. process A and B run in different memory areas), and this is achieved by memory virtualization. Most of the memory operations we do are based on virtual memory, and for a 32-bit processor like ARMv7, the virtual memory size for each process is 2^32 = 4G bytes large. But most processes don't take 4GB memory to run, they consume only a small part of 4GB. The actual size of memory a process uses is called the process' virtual memory address space, and the 4GB virtual memory space is divided into many many memory pages for distribution, while memory page is "a fixed-length (4096 bytes on iOS/OSX) contiguous block of virtual memory, and it is the smallest unit of data for memory operation". The virtual memory address space of a process consists of numerous regions of memory. Each memory region contains a number of virtual memory pages, and pay attention, memory regions may not be contiguous. So the above concepts can be depicted by the following scrawls:
Part II Modeling
Address : Byte
0x02cb2000 : 0xfe
0x02cb2001 : 0xed
0x02cb2002 : 0xfa
0x02cb2003 : 0xce
0x02cb2004 : 0x00
0x02cb200c : 0x00
0x02cb200d : 0x00
0x02cb200e : 0x00
0x02cb200f : 0x08
If our target value is int targetValue = 21592 == 0x5458, we can find it in the memory segment from address 0x02cb2009 to 0x02cb200c, right? However, because instructions and other data are also stored in memory, 0x5458 may be only part of an instruction, or part of a long number (say 0x0000545845545f5f. ARM is little-endian), it may not present our int target value. This situation happens in most of our memory searchs, i.e. more than one match can be found. If that's the case, we have to search again for a more specific value (we'll look into this later in the example). Now we know what "memory searching" means
With this information, we can try to model our memory editor. It can be divided into 3 steps:
1. Get the virtual memory address space of the target process, by enumerating all its virtual memory regions;
2. Search our target value in the address space, and get the possible address(es) of our target value (by some tricks, one of which we'll discuss);
3. Modify the value at the target address(es).
That's it. Simpler and clearer than you originally thought, huh?
Part III Functions
Before coding, let's take a quick look of the main functions we're gonna use frequently. They're all mach_vm functions, which can be found at http://www.opensource.apple.com/source/xnu/xnu-1456.1.26/osfmk/vm/vm_user.c. They're: (words between dollar symbols are $arguments$)
Description: Read/copy a range from one address space and return it to the caller.
$map$ is the port for the task whose memory is to be read;
$addr$ is the address at which to start the read;
$size$ is the number of bytes to read;
$data$ is a buffer to store the read bytes;
$data_size$ on input, is a pointer to the maximum size of the buffer; on output, points to the size read.
__unused mach_msg_type_number_t size);
Description: Write data to the specified address in the target task's address space.
$map$ is the port for the task whose memory is to be written;
$address$ is the address at which to start the write;
$data$ is a buffer to be written;
$size$ is the size of
Description: Return description of a virtual memory region.
$map$ is the port for the task whose address space contains the region;
$addr$ on input, is the address at which to start looking for a region; on output, returns the starting address actually used;
$size$ on output is the number of bytes in the located region;
$flavor$ is the type of information to be returned, should be
$info$ returns region information, should be of type
vm_region_basic_info_data_64_t on both 32-bit and 64-bit OSes;
$count$ on input, should be
VM_REGION_BASIC_INFO_COUNT_64; on output, the size of the region.
memmem(const void *big, size_t big_len, const void *little, size_t little_len);
Description: Locates the first occurrence of the byte string
$little$ in the byte string
$big$. If it matches, a pointer to the first character of the first occurrence of
$little$ is returned; else it returns
Part IV Coding
There's only one thing worth mentioning before we get coding: the example we give only works on Apps with no custom memory protection. The coder himself (a.k.a. myself) is a n00b on memory operation, so the code may not be very efficient and elegant. Feel free to make optimization BTW, I've tried to comment most of the obscure code, so if you still can't catch it, just ask!
Click me for the code
Part V Testing
(I got inspiration of the name from a mini movie, which is "海马洗头" in Chinese and tells a story about erasing people's memory. It's one of 20 mini movie series, all by famous Taiwanese directors. I strongly suggest you watch them because their visions are really original.)
Open up Blood & Glory 2 and head to the store at the right down corner directly.
See the silver coins and rubies on top of the screen?
Write down the rubies' number, mine is 69166 ATM. Then run HippocampHairSalon_OSX as root and find the process "Blood And Glory". Enter its process ID, mine is 1283
Enter the rubies' number, i.e. 69166, and view the results
5 results are found and we've talked about the reason. Which one is for rubies? We have 2 simple ways to figure it out
1) Change the rubies to another number, and search again to see if we get less results. To that end, let's buy some Health Potions and re-search
Unfortunately, we've gotten 8 more results for the new value 69136 You can repeat this process until you find the only result, or turn to the following trick
2) Change the rubies and review the address. Again let's buy some Health Potions to make our rubies 69106 (I forgot to screenshot), but review rather than re-search
See something interesting? The value at 0x1a8fae0c has changed to 69106, and that matches our rubies! So this is very likely the target value, let's modify it to verify
Go back to the game, buy one more Health Potion to refresh the UI. Our rubies grew to 99993 now
Being a rich guy, we can now buy the best weapons and armours to finish our enemies in 1 single round. Isn't that cool?!
Quit and re-enter, you'll find the rubies are still 99993. We've successfully cheated Blood & Glory 2, yay!
(Copy the compiled and entitled HippocampHairSalon_iOS to your iOS now)
Open up 大富翁4FunLite and start a new game
When it's our turn, write down the money we have, mine is 25000
Launch HippocampHairSalon_iOS as root and enter PID of "RichManLite"
Enter the amount of money, 25000, and we get 14 results. Good
Whatever, change the money
Review previous search results
Modify our target value
We're RichManReal now
Quit and enter to verify our hack.
Part VI Conclusion
We've walked through the writing of a universal memory editor on iOS/OSX, and it works like a charm as game trainer. This post only demonstrates the possibilty of such a tool on Apple systems, things may get way more complicated in practice, especially on Apps who possess strong memory protection. In that case, you'll have to combine memory editor with necessary reverse engineering skills to make it. Stick to @iOSAppRE and iosre.com, more interesting stuff are right on their way Thank you for reading!