Open-Source Hangover

“Open-Source Hangover: The feeling you have the morning after staying up way too late hacking on an OSS library.”

No regrets though, had a really productive evening with Karl Trygve, doing pair-programming on our dynamic (runtime) x86 re-compiler, which is part of Frida. It is used for tracing execution entirely in user-space without relying on hardware or kernel support, which means it’s blazingly fast and hard to detect. Last night we made the first successful call-trace deep inside of Spotify, which we like to use as our guinea pig for making sure we cope well with obfuscated code. (Next level up will be Skype, which contains even more nasty anti-RE tricks.)

Previously only got a partial call-trace where we lost control of the thread after about 300 CALLs. Late last night we made it all the way through the anti-RE mine-field and collected 50 000 CALL events in seconds. Next up is call depth tracking and live argument list type inference, this is gonna be even more fun! 🙂

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oSpy and JIT x86 machine code generation

I was faced with a new and interesting challenge the other day, I had to use oSpy2 to intercept calls to DeviceIoControl. Off I went and added the required bits to oSpyAgent’s config.xml so that it essentially contained:

And doing a quick capture resulted in events like this:

And boom, the application being monitored crashed… The reason why is quite obvious; the lpBytesReturned out argument is only filled if the DeviceIoControl succeeds, and because oSpy’s automatic marshaling code used for logging takes the value of this argument as the size of the ByteArray pointed to by lpOutBuffer, it will go horribly wrong if the app doesn’t initialize the value pointed to by lpBytesReturned to zero.

So, guess this means we need some way to say “don’t log this unless the return value is non-zero”. What I came up with is the following:

So the log condition should be fairly self-explanatory; reg is short for CPU register, eax is the name of the x86 CPU register where the return value is placed with the stdcall calling convention, and then there’s the operator and the value to compare against.

The current implementation is really simple and only supports comparing CPU registers for equality and inequality against a value on the right hand side, but this can easily be expanded. What it does is simply allocating a chunk of memory, filling it with the machine code of a function that gets a pointer to a struct with the CPU registers and returns true/false to determine if the argument should be logged. The chunk of memory is then made executable, a function pointer is pointed to it, and oSpy’s logging code just calls it if it’s set.

The machine code is created from a template with the bytes in the left column:

So the byte in orange is filled in depending on the register, the 32 bit value in blue gets filled with the value to compare against, and the byte in green is set depending on the operator, 0x94 for ==, 0x95 for !=, 0x96 for <=, etc.

This log condition feature was something that I remember thinking about doing and that I ended up putting on hold for a few weeks. One late evening I started thinking about it again and suddenly this idea struck me, so I jumped straight in and implemented it. The really scary part is that it actually worked on the first run. Surreal…

The little things in life

This one made my day:

http://www.theonion.com/content/video/warcraft_sequel_lets_gamers_play

Thanks, Håvard!

RTAudio (x-msrta) interop

When WLM got this new feature called voice calls I remember being quite excited. Like, which protocols would they be using this time, which codecs, and so on. I was quite surprised when I learned that the they were using SIP, RTP and ICE this time around, instead of all-proprietary protocols for both signaling and transport like most of the attempts at similar features in the past. There was one disappointment though, if you’re not running Windows and thus not able to use WLM, you won’t get the same experience as those using the official client. The reason is simple; they get to use an adaptive wideband codec which works well over lossy networks, you don’t. If you want to be a first class citizen in the Messenger world you need to be running Windows.

As much fun as it would be to reverse-engineer this codec I sadly don’t have enough sparetime for that these days, I’m no longer a student with virtually infinite amounts of sparetime like I was back in the libmimic days. I still have this passion for reverse-engineering though, and I find it great as a recreational activity in the life part of the work/life balance. So I sat down a couple of hours one late evening with a fresh cup of Yerba Mate tea and poked around with IDA, and worked out the internal API of this codec inside the appropriate binary. I also learned that all the supported audio codecs share the same interfaces. Of course there aren’t any exported functions exposing them, but that’s just esthetics anyway. Just load the DLL, scan through its memory for the signatures of what you need, declare some function pointers, wrap any structures you might need and write some assembly glue, and you’re all set. Use the wineloader approach used by various media players, and you can even run the code on a non-Windows OS given that it’s x86.

So what I did was put together a GStreamer plugin that would wrap all the encoders and decoders dynamically, just like gst-ffmpeg does with the codecs provided by FFmpeg. I first got this plugin working on Windows, and then ported it to Linux by using the wineloader code of gst-pitfdll as a starting point.

Plain encoding and decoding isn’t enough though, you also need error-concealment to work well with lossy networks. Reverse-engineering this interface was more work, as I had to figure out some internals of their RTP stack in order to know more about the data structures and what the different fields meant. I also had to inherit from an internal C++ base class, and this was a quite fun experience. It’s all very simple though, it’s just a matter of figuring out the size of the class, inherit from it the same way as you would with GObject/C and write some wrapper functions to emulate thiscall using stdcall. Meaning that you put the this pointer into ecx and pass the rest of the arguments on the stack.

I implemented it by having a headerfile with for example:

typedef struct _MSEncoder MSEncoder;

…

HRESULT WINAPI ms_encoder_set_bitrate (MSEncoder * encoder, guint bitrate);

Where WINAPI would be defined to the compiler-specific attribute for specifying stdcall calling convention. Now comes the interesting part, the C wrapper, which for MSVC looks like:

__declspec(naked) HRESULT WINAPI
ms_encoder_set_bitrate (MSEncoder * encoder, guint bitrate)
{
INVOKE_VFUNC (16);
}

The naked attribute tells the compiler not to generate any prolog and epilog, meaning that you’re responsible for setting up the stack frame if you want one. In my case I don’t want one, as I want to simply just put the first argument in ecx and shift the return address one level up on the stack.

So the macro INVOKE_VFUNC is simply:

#define INVOKE_VFUNC(func_offset) \
__asm { \
/* fill in ecx with ‘this’ pointer */ \
__asm mov ecx, [esp + 4] \
\
/* put return address where ‘this’ pointer was */ \
__asm pop edx \
__asm mov [esp], edx \
\
/* call the function in the vtable */ \
__asm mov edx, [ecx + 0] \
__asm mov edx, [edx + func_offset] \
__asm jmp edx \
}

As gcc doesn’t support naked functions on x86 I wrote some fairly self-explanatory assembler code for the GNU Assembler. Naked functions are actually quite beautiful, I really hope that gcc will support them on x86 one day.

So, if you want to play with the code you can browse it here (the plugin is in the “ext/mscodecs” subdirectory):

http://bazaar.launchpad.net/~oleavr/oabuild/gst-plugins-farsight/files

Or better:

bzr branch lp:~oleavr/oabuild/gst-plugins-farsight

To build just the plugin and play with it without installing it, simply do:

./configure

cd ext/mscodecs/

make

If you have a Windows installation with Messenger handy, copy RTMPLTFM.dll from the installation directory into /usr/local/lib/win32. However, if you don’t, make sure you accept their EULA and follow the instructions in the helper/README file.

Now to play with it:

export GST_PLUGIN_PATH=./.libs

gst-inspect-0.10 mscodecs

sender:

gst-launch-0.10 audiotestsrc samplesperbuffer=320 is-live=true ! msenc_rta16 bitrate=29000 ! rtprtaudiopay pt=114 ! udpsink

receiver without error concealment:

gst-launch-0.10 udpsrc ! application/x-rtp, payload=114, clock-rate=16000 ! rtprtaudiodepay ! msdec_rta16 ! alsasink

receiver with error concealment (experimental, very alpha):

gst-launch-0.10 udpsrc ! application/x-rtp, payload=114, clock-rate=16000 ! msrtahealer ! alsasink

(Add lossrate=15.0 to the encoder on the sender launch line to enable FEC in case you wanna test it on a lossy network or with “identity drop-probability=x” before the udpsink. Any value greater than 10.0 makes the encoder enable maximum FEC.)

It should now be really easy to set up an RTAudio voice call with a Messenger client using Farsight 2 — any takers? 🙂

Before I forget

For the brave souls out there:

http://people.collabora.co.uk/~oleavr/oSpy2/oSpy2-20080109.zip

\o/ _o/ \o_ /\o/\ \_o/\ /\o_/ \o/

Tokyo here we come!

Not exactly full of awesome, but…

it’s getting there… The new agent has been renamed from oSpyUsbAgent into oSpyDriverAgent as it can easily be extended to other kinds of drivers in the future. Now I think it’s time to get back to the protocol analysis, yay!