21 Nov

Running WinDbgX on Windows 7

Motivation

Main reason for writing this blog-post is the extremely crappy article by Vallejo named "Installation and First Contact With the New WinDbg". I read it, cried for a few minutes and decided to fix it.

Mandatory XKCD reference:
Someone is wrong on the Internet

Having said that, let's go through some of the most "brilliant" Vallejo's statements!

Installation

We execute WinDbg from installation shortcut and we search the main process.

Dude, when your article is called "Installation and first steps..." shouldn't you start at the beginning and tell us where to get this app and how to install it?

You need to get the app from the Windows Store: https://www.microsoft.com/store/apps/9pgjgd53tn86.

No, there is no real technical reason for that, just another attempt of Microsoft to convert you to Windows 10 and make you use their Windows Store.

To make matters worse, you need to have installed the latest and greatest update of Windows 10 to do that. There is no technical reason for that, either.

After you've jumped through all those hoops, you get this nice and shiny Windows Store app. Windows Store apps get installed under "C:\Program Files\WindowsApps\" and this one is no different. At the moment of writing the application version was 1.0.16, so it got installed into "C:\Program Files\WindowsApps\Microsoft.WinDbg_1.0.16.0_x86__8wekyb3d8bbwe".

Reparse point

The installation creates another exe here: C:\Users\<user>\AppData\Local\Microsoft\WindowsApps\WinDbgX.exe. It is zero bytes, and if you try, for example, to copy it, you can’t.

Because it's a reparse point, not an EXE file.

Windows 10 processes bundled Windows Store application's AppxManifest.xml and creates appropriate appExecutionAlias'es:

Same thing goes for all other applications Vallejo "found" under C:\Users\<user>\AppData\Local\Microsoft\WindowsApps\, like dbgsrv64.exe:

You can read more about aliases on MSDN: Start your app by using an alias.

Fsutil

I have not found a tool or way to manage or get information about these files.

Ever tried fsutil? It's been part of Windows since Windows7.. bigsmile

Here's output of fsutil reparsepoint query WinDbgX.exe:

As you can see, it's a reparse point with a tag 0x8000001b (IO_REPARSE_TAG_APPEXECLINK).

Sure, you can Google for that - but it won't tell you much, except that it's not really documented and should be left alone.

Command-line

Old windbg.exe accepted parameters with “-“, for example -k. New Windbg needs /k parameter to pass the connection configuration

Bullshit! In fact, WinDbgX accepts any of 4 different delimiters: "-", "–", "—", "/" and combinations of those..

Ok, that was enough criticism for one day. Let's do something more constructive!

Running WinDbgX on Windows 7

Remember how I said that there is no technical reason why WinDbgX should be available only on Windows 10 and only as a store app? There really isn't. smile

Here's WinDbgX running on my Windows 7 and debugging one of the FLARE2017 crackmes:

It's actually a really simple fix.

  1. You need to copy all the files from your Windows 10 machine to your other machine (Windows 7 in my case). It's as simple as selecting all files in "C:\Program Files\WindowsApps\Microsoft.WinDbg_1.0.16.0_x86__8wekyb3d8bbwe" and copy-pasting them. Don't worry about the reparse points in C:\Users\<user>\AppData\Local\Microsoft\WindowsApps\, we'll fix that later.
  2. If you try to run DbgX.Shell.exe on Windows 7, it will fail with Exception:
  3. Let's look at that code in DbgX.dll using dnSpy:

    It's crashing on the Package.Current.Id.FamilyName, as this function is available only for Windows Store apps.

    As a simple hack, we can replace this call with an empty string. Better hack would be to use the proper folder based on the actual WinDbgX path. But the simple way will do for our demo..

  4. Using "Edit IL instructions" function in dnSpy, replace first 4 instructions with ldstr and nops:
  5. Save the module. If saving fails, remove read-only attribute from DbgX.dll and try again.
  6. Since we chose a simple hack, create folder C:\Users\<user>\AppData\Local\Microsoft\WindowsApps\
  7. Depending on your OS (32- or 64-bit), copy files from WinDbgX\X86 or WinDbgX\amd64 folder to C:\Users\<user>\AppData\Local\Microsoft\WindowsApps\. Rename dbgsrv.exe to dbgsrv32.exe or dbgsrv64.exe accordingly.
  8. Run the DbgX.Shell.exe shell now. It will work just fine!

There are 3 more places in DbgXUI.dll, DbgX.Util.dll and Extensions\DbgX.External.dll that might need similar fixes. But that's behind the scope of this article.

Conclusion

It is true that classic WinDbg looks really dated. So, I can totally understand why Microsoft would want to create a replacement with a better UI. However, WinDbgX falls short on everything - its installation via Windows Store is brain-dead stupid, its user interface is confusing (who would look at Model->Change Query to change debugger settings?!) and severely limited (no multiple Memory windows, seriously?). If it was a school project, it wouldn't get even a B-. But for some reason, Microsoft insists that this is the only way forward. Oh, well.. sad

At least, the DLLs are not obfuscated, so someone can take them and make a much better UI.. wink

Have fun!
kao.

05 Dec

Since you asked.. How to obtain value of a field using WinDbg?

My friends occasionally ask me tricky questions - about PE file format, about .NET internals, usage of dnlib or WinDbg and what not. Even if I'm not an expert in some areas, I usually can figure out stuff pretty fast.

So, here's a question that li0nsar3c00l asked me few days ago:

How to get the value of a static field using WinDbg?

To put the question in context - he is trying to debug a .NET assembly which is obfuscated and all names are unprintable. When all names look like "□", it's quite hard to find out which is which - and I doubt you can use those names when setting breakpoints.. So, we need to figure out a way that avoids using object names.

I will assume that you have very basic knowledge of using WinDbg with .NET applications. If you don't, I suggest that you start by reading introductory tutorials, for example, Getting started with Windbg - part I and part II. You could also check WinDbg tutorial by netmatze. Or just go through the entire amazing collection of .NET Debugging Demos by Tess Ferrandez.

Quick answer

You can use !mx command to locate the class and field you need. Then you can use !mdt -e command to display values you need.

But if you have no idea how those commands work, please continue reading..

Long answer

Dealing with static classes and static fields is easier. After the corresponding .cctor is executed, you can get the values you need. But for non-static classes and fields you need to stop debugger at a place where an instance of the object is already created and values initialized.

So, we'll take a scenic route from the very beginning till the end. Here's a demo app you can test your skills with: https://www.mediafire.com/?wavmalk6sqhdcy6

  1. Load sos and sosex extensions. I'm using this one-liner for .NET 4.0 - you can use separate commands, if you prefer.
  2. Examine domain, find module
  3. Find the type you want
  4. Find the method you need
  5. Put breakpoint on the method and run
  6. Disassemble code, locate place where the interesting object will be already initialized.
  7. You need to put a breakpoint after the object is created and initialized.

    Unfortunately !U doesn't show IL code. So, let's use !mu output to confirm our findings:

    Command !mu doesn't show call targets but interleaves IL code with x86 code. Combining both outputs we know that we should get to address 003f00cd.

  8. Execute code to the place where data are already initialized
  9. Use command !mx
  10. This command is little known but it's bloody awesome!

    Usage: !sosex.mx <Filter String>

    Displays any matching type, method or field for <Filter String>, where <Filter String> is a string in module!metadataname format. If module! is not specified, all modules are searched for the specified metadata. Searched info includes types, methods and fields.

    In order to search globals, do not precede the field or method filter with a ".". To enumerate all globals for a given module filter, use "globals" as the type filter. eg: "globals" "*!globals" "mymod!globals", etc...

    So, let's ask to show us all objects from our test application:

  11. Get the address of class
  12. Class Bla contains static field key1 which we're interested in. Click on Bla hyperlink. You actually don't care about the output, just the address of class.

  13. Use !mdt command to get address of the field
  14. Use provided hyperlinks to access data
  15. Clicking on "02722340" and then on "Expand" will get you the good stuff. Or you can enter "!mdt" command manually:

    Congratulations, this is the value of static field. Task #1 done! smile

  16. Locate instance of the class
  17. Remember, we put a breakpoint just after the class initialization. You can either use your x86 knowledge and check the proper register values, or just display managed stack objects:

  18. Use !mdt command to get the stuff just like before
  19. Congrats, this is value of non-static field. Task #2 done!

This concludes our journey. Hopefully you learned something new today!