Interactive Disassembler Pro v3.7 Demo (II)

(How to load the previous databases)



by Quine

(30 October 1997)



Advanced cracking series Courtesy of Fravia's page of reverse engineering



Well, this is SERIOUS ADVANCED CRACKING once more. Once more a fundamental tool of the trade (IDA). Once more a function reenabling work (the loading of the previous databases, i.e. one of the most important crippled functions of the crippled version: you do not want to start everything anew every time you use IDA, do you?). Once more something we all need: new knowledge that you can at once apply to other targets and reverse engineering endeavours.

Quine is getting us used to this kind of well-crafted essays. I'm afraid newbyes will not understand much here, please read the 'basic' essays first, and peruse the other +HCU page (where you'll find a lot of help for newbyes) before delving in this.

For all other Fravia: some considerations.

1) Read it twice (at least) what you'll find daunting at the first quick glance will be understandable at the second and easy to follow at the third pass. In my opinion if you read this at least a couple of times you will not even need to have a crippled IDA target on your HD to follow the path;

2) Note how all this is built and buids on the contributes of others. Quine is an extraordinary Fravia and a very good didact, witty and deep, whose mighty essays never forget (as unfortunately some other do :-( to quote all contributions to the crumb trail he has followed and left behind ;

3) I believe it is time to tackle a VITAL sector: compiler specifities.

Actually that what we are doing is not very professional. This has some advantages (only non programmers can see the non-obvious) and some disadvantages (we find relocation tables and we don't even know that they are called vtables :-)

Time to correct the disadvantages keeping the advantages. Let's begin to work on 'compiler specific differences' or whatever you want to call that what we should understand perfectly if we really want to migrate from 'good dilettante' to 'elite' reverse engineers.



This said, here you have a real reverse engineering essay in all its glory... enjoy!

Target: Interactive Disassembler Pro v3.7 Demo We're going to enable the loading of saved databases . Source: http://www.datarescue.com/ida.htm (homepage) http://www.datarescue.com/ida/demo37.zip (9,884,100 bytes) or, rather, ftp://195.0.122.253/pub/ida/375evl.zip (thank Ed!) read on for more web resources :-) Tools used: IDA Pro 3.7 itself (nothing comes close to comparing to it) SoftICE for NT v3.2 (that new video driver is amazing) (if you can't get Pinnacle's version to install, change the byte at 2B6C in setup.ins from 2B to B8) HexWorkshop32 (for patching - any hex editor will do) In this essay, I will assume that you are familiar with my previous essay here on Fravia+'s site about cracking the file size limit on the IDA Pro demo. I am also assuming that you have ida.wll loaded into IDA Pro. Ok, the first thing to do is to find the place where it puts up the message that it can't load old databases. Our previous work on the file size limit suggests that this message will be in ida.hlp, which it is. Using the method I outlined in that article, compute the index number for the help message and use IDA's search for immediate function to find the place where 36Eh is moved into a register or pushed on the stack. Sure enough, we find it early on at 403520. This routine is directly called anywhere in the program, but IDA very helpfully tells us that the value 403520 is referenced at 403D3F (actually it will give you the starting address of the function it occurs in and an offset, but I will usually translate that into an address in this article). Follow the hyperlink and we find ourselves in a familiar place: just past the explicit file size check and before the rather vulgar expiration date check (now would be a good time to patch it before you forget-- Jan 1st isn't that far away). 403520 is moved into ebx, then we get the expiration check, then call ebx. But look, depending on the value of esi at 403CDE, either 403520 or 40352C is moved into ebx. Let's jump back to 40352C and see what's going on there. 40352C calls two functions, one indirectly, and returns. A little thought will tell you that this routine must be called when a new file (i.e., a file to be disassembled rather than a saved database) is loaded into IDA. Why? Well, we ought to assume that the expiration date is always checked and therefore, since execution follows immediately to the call ebx and we don't get the message about loading old databases when we load new files (obviously), it must call 40352C (this can be verified in SoftICE if you feel like it). Therefore, the value of esi at 403CDE must indicate whether or not we've got a new file or a saved database. This, unfortunately, is the point at which pedagogy must depart from actual practice, because explaining everything I tried at this point would take far too long and furthermore I can't even remember everything I did that might be significant. Instead, I will attempt a rational reconstruction of the process and try to cover the major points of interest. Remember, though, that this crack required a lot of tedious pouring through code and slowing but surely putting together a rather detailed picture of everything that IDA does between starting up and displaying the message saying that it can't load old databases. I could not have hoped to put such a picture of IDA together without IDA itself. The commenting and renaming features and all the other features that make it a truly interactive disassembler (unlike w32dasm which is basically a text viewer) are what saved me hours of scratching done notes and trying to remember where I had been and what functions did what. Enough of that and on with the crack. There are two reasonable things to do here. One is to trace back the value of esi and see how it gets set. The other is to simply force the code to jump to 40352C no matter what. Let's try the second, but it sounds like more fun. Fire up the text editor and change the byte at file offset 3340h from 20h to 2Ch. Start up IDA, load a saved database and see what happens. Well, it happens is that it crashes at location 44D2BB trying to access memory at 00000064. That's no good. No memory access there for humble console applications. Load up SoftICE and try it again. Now when it crashes, we'll be able to look at what's going on. Turns out that the function in which it crashes got a null pointer from sonewhere, because the offending instruction is mov edi, [eax+64h] and eax is 0. This is bad news for us because there are any number of ways that that pointer could be set. Also, patching the code to jump to 40352C could have introduced further problems. This is a tough position to be in when cracking a target. So, let's sit back and evaluate the situation and try to gather everything we know about IDA's start up code so far. First, how perceptive where you when you loaded your old database? I'll tell you that I (stupidly) wasn't very perceptive at all for quite some time. Part of the problem is that I was using as a test database one that I had created from the tiny hello.exe sample program included in IDA. The fact that this program is small means that it disassembles quickly and produces a small database (which is why I chose it). With a bigger database what I'm about to point out is much more obvious. Look at the screen when you load the old database. IDA allocates memory for the database, it unpacks the file, it compiles the default macro (ida.idc) and at least begins to execute it. Furthermore, it does this whether you've patched the program or not. Only after having done all this does it display the mesg box/crash. What does this tell us? Well, it tells that Mr. Guilfanov did not remove large sections of the code that have to do with loading saved files and furthermore that that code actually executes. However, there's still something that needs to be done to get the database loaded. Here's the crucial point: INSTEAD OF DOING THAT EXTRA THING, IDA CALLS THE ROUTINE THAT DISPLAYS THE MSG BOX. In other words, the call to 403520 needs to be replaced with a call to a function that works the missing magic. I wouldn't expect anyone to figure out this last point exactly without more work, because it certainly took me forever to figure it out, but it does seem rather obvious in hindsight. Of course, we still don;t know what the missing magic is or what function does it. We do know that 40352C doesn't do it. Also, having 403520 simply return instantly doesn't work either. Now, let's get back into SoftICE and learn a little more about our crashing patched program. When the crash puts you into SoftICE, we're going to walk the stack back as far we can go and find out where that null pointer comes from (see my previous article on IDA for a description of my particular stack walking technique). 44D2BB is sub_44D2AC which was called at 417CC4 in sub_417CA4. eax came into sub_44D2AC live and it was assigned a value just before the call in sub_417CA4 with the following command: mov eax, [ebx+0Eh]. Great. Another pointer. What are these pointers that have immediate values added to them? Brief digression about the importance of understanding the compiler When I ask this question, I am asking "What is the program's author doing here that causes the compiler to generate such commands?". This is THE SINGLE MOST IMPORTANT QUESTION a reverse engineer can ask him/herself when dealing with compiled code (which is almost always unless you're in a library routine where you shouldn't be in the first place). No person wrote the code you are looking at. Who would write the following? 0041809A mov edi, edx 0041809C mov ebx, eax 0041809E mov edx, edi 004180A0 mov eax, ebx Only an idiot or a compiler. This is taken straight out of ida.wll which was compiled by Borland C++ 5.01 with the optimization level set to maximize speed of execution (I know because I have the makefile---read on :-). Compilers just have their ways of doing things and it is very helpful to figure out just what those ways are. End digression. They are pointers into structures when the immediate value is not an address offset. Here's the situation. You pass a pointer or a reference as an argument to a function and that function is then able to get at the members of the structure by adding values to the pointer that was passed. Keep in mind, also, that the same goes for C++ classes (but they have added complexities that I'll get into in a moment). So, in the struct/class based at [ebx] in sub_417CA4 there is a member at offset 0Eh which is itself a pointer to a struct/class which in turn has another pointer at offset 64h and that pointer is 00000000h and we don't want it to be. On with the tracing! sub_417CA4 is called at 417B89, which is in sub_417B50. We see that again in this function ebx is used to hold the pointer to the struct. (Since writing my last article, i have read that most Win32 compilers use ebx, edi, and esi as holders for enregistered local variables). Anyway, it came in through eax from a call at 41E941 in sub_41E934. Here's where we get our first break. eax is assigned the address of export _637. Now, let's dump the memory at _637 and see what we get. What we get are mostly zeros and FFFFh at one point. And of course at _637+0Eh we get zeros. However, significant progress has been made. We can safely assume that the struct/class at _637, which was declared at global scope in the program or else it wouldn't be able to be executed, isn't filled porperly. Furthermore, this must be a fairly significant struct or else it wouldn't be exported. Before we get too excited, though, let's continue tracing backwards as far as we can. There are quite a few functions that you will go through before you get back to the function that has the file size check, the expiration check, etc. I won't go through all the details here, but it is worth looking at one function call in particular. sub_422AF8 calls sub_403300 at 422B23, but it does it with the following instruction: call dword ptr [esi+2Ch]. This is interesting. I trust that everyone has read Fravia+'s fine article on call relocation tables. But now we must ask what these call relocation tables are. Are they a structure or array of pointers to functions declared explicitly by the programmer? Almost always not. In fact, the average programmer probably only has the vaguest idea that they exist at all. They are an invention of the compiler used to deal with virtual function calls in C++ and are commonly referred to as vtables. I am still working on the details of how they are implemented (it differs from compiler to compiler and the optimization settings also affect it), but notice that the pointer to the beginning of the vtable is gotten in a fairly roundabout way. You will see these offsets (7D9, 7DD) in other places in the program and if you get an indirect call right after that, you'll be able to know which vtable you are dealing with. This particular vtable starts at 489191. Another thing you know is that all the functions in a particular vtable are member functions of the same class and are therefore related. Ok, so you end up in sub_403934, which is the big routine that has the date check, etc. at loc_403E2E, which calls sub_408100. This tells us quite a lot. First, the call to 40352C does not crash when we're loading a saved file. Second, an obvious test with SoftICE will tell us that sub_408100 is only called when a saved file is loaded. That means that at 403E23 esi==0 if and only if we're loading a database. Furthermore, sub_408044 isn't called at 403E27 when we are loading a database. Finally, at 403E33, the load new file and the load database paths meet up. Something has happened in the routines that handle loading a new file, that hasn't happened in the routines that load a saved database and that something has to do with the struct/class at export _637. So, let's start up IDA, load a new file, let it run for a minute and then break in with SoftICE and see what's going on at _637. With ida.wll loaded into IDA, this is what I get (remember, ida.wll is relocated to BB0000 on my machine): 00C47A74 09 00 00 FF 29 00 90 AE-C6 00 00 00 00 00 28 E6 00C47A84 C7 00 90 AE C6 00 E0 B8-C6 00 B0 B8 C6 00 00 00 Good, we've got some pointers in here. In particular we've got one at _637+0Eh. Dumping the memory at [_637+0Eh] doesn't tell us much (try it), so let's look at some of these other pointers: 00C6AE90: 00401000 00489000 FF001CF5 FF001CF5 00C6AEA0: 00000000 01000203 00010000 FFFFFFFF 00C6B8E0: 00489000 004A1000 FF001CF6 FF001CF6 00C6B8F0: 00000000 01000203 00020000 FFFFFFFF 00C6B8B0: 004A118C 004A12DC FF001CF7 FF001CF6 00C6B8C0: 00000000 01000203 00030000 FFFFFFFF Now we're getting somewhere. These ought to look familiar because the first two dwords at each pointer are the begin and end addresses of the various segments in ida.wll! So, it looks like the struct at _637 holds information about all the segments in the open file. No wonder the program couldn't get anywhere without this information. What we need to do now is figure out how to get this information out of the saved database and into the struct at _637 before getting to the call to sub_408100. Is this our missing magic that 403520 was supposed to do? Well, this is where I got stuck for a long time. I pretty sure I knew what had to be done, but had no idea how to do it. Furthermore, I wasn't sure that this was the only thing that had to be done. Where there other structures that needed to be filled in? I wouldn't know until I figured out how to get the segment structure filled in. What saved me is what some might consider cheating, because it involves having access to way more information than you usually do when reversing. Here's the story. On IDA's US web site (www.datarescue.com) there is a mention of an SDK (Software Developer's Kit) for IDA that enables you to write processor modules for IDA (see my first essay on IDA). This sounded very helpful, but it wasn't available for download. They said to e-mail them for information on it. So I did. This was the response: It is free to registered users of IDA Pro. Have you registered your copy ? Well, no, I was planning to crack my copy instead. I went out in search of more information on IDA. Maybe there was some out of the way web site containing more info. There was and still is. IDA is written by a brilliant Russian man named Ilfak Guilfanov and Mr. Guilfanov has his own IDA web site on a server in Russia ( http://www.unibest.ru/~ig/index.html ). Go there now and download everything you can, because it has, among other things, the IDA SDK. The IDA SDK has very well commented C++ header files for most of the program. This was an unimaginable boon. Even better, it has a Borland lib file for accessing the exported functions in ida.wll. This lib file conatins the real names of all those 500+ functions/global variables. To get at this information, you need a program that dumps out the contents of Borland lib files (which are a proprietary format). tdump.exe, which is included in most Borland development products, does it, and you can easily find that or a freeware equivalent on the web. Now you can go into IDA and start renaming the exports to their real names instead of those meaningless numbers. Between the headers and the lib file I had more than enough to finish the job. Sure enough, export _637 is called _segs (this made me feel pretty good). In the header files you can find a complete desription of the class object that resides there (it's an area control block (areacb_t)). Furthermore, looking through the segment.hpp and area.hpp headers you'll see some very interesting functions, including the following: // Link area control block to Btree. Allocate cache, etc. // Btree should contain information about the specified areas. // After calling this function you may work with areas. .. some comments deleted ... int link(const char *file, // Access to existing areas const char *name, int useva, int infosize); // Initializa work with segments // Called by the kernel itself. // file - name of input file void initSegment (const char *file); Btree is the database. Calling one of these two functions seemed like the thing to do. However, neither of them are exported by ida.wll, so we've got to find them. Finding them took a while, but I realized an interested fact about executable files in the course of doing it. What determines where a particular function is put inside of an exe/dll/etc.? When a programmer compiles a project, each source file is compiled into .obj files, which contain the machine code to be processed by the linker. The linker then combines all the obj files into the finished product, changing the addresses appropriately so that everything works out. What does this mean? It means that all functions in the same source file will be adjacent to one another. Now,of course, different programmers arrange their source files in different ways, but we still know that adjacent functions tend to be conceptually linked in some way. Of course, when we have the header files, we have a very good idea where to look for functions. To make a long story short, here's how I found the link and initsegment functions. First, we know in general where to look. Second, we know what parameters each function takes and that they, like just about every function in ida.wll, were compiled with the __fastcall option (see the appendix to my last essay). Borland implements __fastcall in the following way: arg1: eax arg2: edx arg3: ecx arg4: last thing pushed on stack arg5: second to last pushed etc. I looked for link first because it has more arguments and ought to be easier to find. Well, I found what I'm pretty sure is it at sub_4399AC, but more importantly, in the course of looking, I found the right function which is initSegment (with a name like that and given our problem, you may be wondering how I could have thought that any other function could possibly be the right one---well, it was late and I'd been looking at this program for days and managed to get myself to believe all sorts of crazy things about it). initSegment is at sub_456D70. The first thing it does is call areacb_t::create to create the _segs area control block. It then calls another function which in turn calls link. Ok, what we need to try now is to rewrite the function at sub_403520 to call initSegment. However, we need to pass it the name of the EXE file that was saved in the database. However, eax comes in sub_403520 with a pointer to the name of DATABASE file. So, how do we get a pointer to the right filename? Well, in the course of studying IDA, I discovered that there is a very easy way to do this. Look at this code snippet which is straight out of my ida.wll database: 00403DB0 mov eax, offset _RootNode ; idb specific 00403DB5 call @netnode@value$xqqrv ; netnode::value(void) 00403DBA push eax ; pointer to exe filename from dbase 00403DBB push 244h ; Database for file '%s' is loaded. 00403DC0 call @Message$qie ; Message(int,...) 00403DC5 add esp, 8 ; end idb specific To get the filename pointer into eax, all we have to do is call netnode::value and pass it the address of _RootNode (4998B0). So, sub_403520 needs to be this: mov eax, 4998B0h call 425F5C ; netnode::value call 456D70 ; initSegment ret Unfortunately, we've got two problems. (1) This code takes 10h bytes and we've only got 0Fh in the area of sub_403520. (2) We're referencing a global variable in a program that is inevitably going to be relocated. That means that _RootNode is never actually going to be at 4998B0. Windows deals with this little issue in the .reloc section of PE files. This section contains all the addresses of places in the program that make absolute reference to an address (note the most jmp and call instructions use relative offsets and are therefore not affected by reloctaion). The first problem is easy to get around. Take a look at PNA's essay on adding a save function to the demo of w32dasm. We'll just stick the code at the end of the CODE segment where there are about 190h free bytes. The second problem involves patching the relocation table. I won't describe the details of this table, because they are somewhat hairy and you can find many good descriptions other places. The best I have seen is in the ESSENTIAL and INVALUABLE book "Windows 95 System Programming Secrets" by Matt Pietrek (a NuMega employee no less), but descriptions of the PE file format are a dime a dozen on the web (I think there is even one on the site). Let's start patching. First thing is change the value assigned to ebx at 403D3F to point to our new routine. We're going to put the routine at 488875, right after the dll import jump stubs. So, patch 403D3F mov ebx, offset loc_403520 to 403D3F BB 75 88 48 00 mov ebx, offset loc_488875 Notice that we don't have to worry about relocations here because there already was an absolute address reference at the location where we've stuck the new one, so the loader already knows to fix it up. Now, let's insert our new routine: 488875 B8 B0 98 49 00 mov eax, 4998B0h 48887A E8 DD D6 F9 FF call 425F5C ; netnode::value 48887F E8 EC E4 FC FF call 456D70 ; initSegment 488884 C3 ret The last thing left to do is patch the relocation table. We need the dword at 488876 to be adjusted. The necessary patch is to change the two bytes at offset 96DA4 in ida.wll from 00 00 to 76 38. I'll leave it as an exercise to figure out exactly how this works if you don't already know. Here a small correction: the relocation table patch must be applied to locations 9EDA4 and 9EDA5, not 96DA4 and 96DA5, as Quine says. It may be a Tipo. greets as usual zeezee Now, the moment of truth. Run it. Load a database. It works. That's it. I really didn't think it would work, to be honest. I assumed that all the other global areacb_t's (_funcs, etc.) would have to be initialized also. That, however, gets done eventually in the call to sub_408100. Could I have done it without the header files and the export names? Who knows. If I could have, it's not entirely clear that I wouldn't have given up in frustration after weeks of trying before I ever got it. I was glad to know that I was at least on the right track. Demo function enabling is what I suppose that I find most enjoyable in cracking and I have a word of advice to demo writers: TAKE AS MANY FUNCTIONS AS YOU CAN OUT OF THE DEMO. Mr. Guilfanov took one very small function out and left in a ton of code that he never intended the demo to execute. With those functions gone, it is simply impossible barring an act of God to re-enable the function. I don't care how good a cracker you are. There would be no concievable way to reconstruct what happens in the call to 408100. Future Plans for IDA Pro First, enable the saving of ASM files. This code really is gone from the demo, but with the information I have about the program, I'm half way there. It's going to involve inserting rather a lot of code in the target, so hopefully I'll come up with tricks for that. Second, and more interesting, adding features. This can be done in part through the IDC macro language, but also through code patching. To get an idea of the features I have in mind, check out http://www.cs.uq.edu.au/groups/csm/dcc.html and anything you can find written by Cristina Cifuentes (she is truly BRILLIANT). DCC is a full fledged DOS deCOMPILER! That's right, it kicks out real C code. Admittedly this can be done only to a limited extent with large and complex programs, the concepts she discusses are very deep and important for understanding how to reverse engineer. Ilfak Guilfanov has certainly read her work (see his web site). Well, I'm tired and I am very behind in my work. Good Night.

And you better don't joke with this request of Quine either, because I'll find out if you do.

Fravia+, I was wondering if you could add to my essay(s) the note that I would very much like for no one to release the cracks to IDA as crack programs (those vulgar little .com files) on the web or for anyone to publish the cracks without the full essay. I have too much respect for the author of the program to have the demo crack tossed about the web for people who are not serious about reverse engineering. He has written such a beautiful program that those of us who really cannot afford to buy it ought to -at least- earn the right to use it. Thanks, Quine

(c) Quine 1997. All rights reversed

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