The main goal of today's post is to show a bit more of what is now possible with the latest Windbg (currently branded "WinDbg Preview" in the Microsoft store) and the time travel debugging tools that Microsoft released a few months ago. When these finally got released, a bit after cppcon2017 this year, I expected a massive pick-up from the security / reverse-engineering industry with a bunch of posts, tools, scripts, etc. To my surprise, this has not happened yet so I have waited patiently for my vacation to write a little something about it myself. So, here goes!
So grab yourself a cup of fine coffee and read on :).more ...
Yesterday afternoon, I was peacefully coding some stuff you know but I couldn't make my code working. As usual, in those type of situations you fire up your debugger in order to understand what is going on under the hood. That was a bit weird, to give you a bit of context I was doing some inline x86 assembly, and I've put on purpose an int3 just before the piece of assembly code I thought was buggy. Once my file loaded in OllyDbg2, I hit F9 in order to reach quickly the int3 I've slipped into the inline assembly code. A bit of single-stepping, and BOOM I got a nasty crash. It happens sometimes, and that's uncool. Then, I relaunch my binary and try to reproduce the bug: same actions and BOOM again. OK, this time it's cool, I got a reproducible crash in OllyDbg2.
I like when things like that happens to me (remember the crashes I've found in OllyDbg/IDA here: PDB Ain't PDD), it's always a nice exercise for me where I've to:
- pinpoint the bug in the application: usually not trivial when it's a real/big application
- reverse-engineer the codes involved in the bug in order to figure out why it's happening (sometimes I got the sources, sometimes I don't like this time)
In this post, I will show you how I've manage to pinpoint where the bug was, using GFlags, PageHeap and WinDbg. Then, we will reverse-engineer the buggy code in order to understand why the bug is happening, and how we can code a clean trigger.more ...