Monday, December 30, 2013

Emulating and Debugging Workspace

A grad student emailed me in response to my Netgear auth bypass post.  He's working on a research project and wanted to know if I knew of any resources or techniques to use emulation for executing and debugging the net-cgi binary in the Netgear firmware.  It turns out I've got all the resources to do just that.  I replied with a description of my workspace and some links to resources I use, and, in many cases, have developed.  I thought this might make an interesting blog post, but I don't really have time to write it up all blog-post-like.  Instead I'll just paste in my email.  Maybe it'll be useful to other people as well.


I think the best approach is to describe how I set up my tool chain and environment.  Hopefully that will be helpful for you.

To start with, I do my work in an Ubuntu VM.  Specifically 12.04.  I don't think the exact release matters, but I know 12.04 works with my tools.

I keep a set of cross compilers in my path for various architectures. In my opinion, building with a cross compiler is faster and easier than building with gcc inside QEMU.  I recommend building a set of cross-compiling toolchains using Buildroot.  Buildroot uses a Linux Kernel-style menuconfig build system.  I don't have anything written up on building cross compilers, but I could probably send you my buildroot configuration if you need it, and if I can find it.

You can download the firmware for the router from Netgear's support website.
Here's a link to the firmware:
In order to unpack the firmware, I recommend my colleague, Craig Heffner's tool, Binwalk:
Binwalk will analyze a binary file and describe the subcomponents it finds within, such as filesystems, compressed kernel, etc. Additionally, it can unpack the subcomponents it finds, assuming it knows how.
Install binwalk in your Ubuntu environment using the "" installation script, which will apt-get install a number of dependencies.
Rather than describe binwalk's usage, I'll refer you to the wiki:
Also, in your Ubuntu environment you'll need a Debian MIPS QEMU system that you can use to emulate the firmware's binaries.

I found lots of information about running Debian in QEMU, but most of it was incomplete, and a lot of it was inconsistent, so I've written a blog post describing how I set up my QEMU systems:
This is just personal, but I like to export my workspace to the QEMU machines via NFS.  In fact, I export my workspace from my Mac via NFS, and my Ubuntu VMs and Debian QEMU VMs all mount the same directory. That way I'm not having to copy firmware, scripts and debuggers around.

Once logged into your QEMU VM, you can chroot into the router's firmware and run some of its binaries:

firmware_rootfs # chroot . /bin/sh

The simple ones, such as busybox, will run with no problem.  The web server, upnp server, etc. are more complicated because they make a lot of assumptions about the router's specific hardware being present.

One of the problems you run into has to do with queries to NVRAM for runtime configuration.  Obviously, your Debian MIPS Linux has no NVRAM, so these queries will fail.  For that, I have a project called "nvram-faker":
You build the library for your target and preload it using the LD_PRELOAD environment variable.  It intercepts calls to nvram_get and provides answers based on the contents an nvram.ini file that you provide. It prints all the nvram queries to stdout, and colorizes the ones that it couldn't find in the .ini file.  Obviously it takes some guesswork to provide sane configuration parameters.

Sometimes you can skip running the web server and just run the cgi binaries from a shell script.  Most cgi binaries take their input from the web server as a combination of standard input and environment variables.  They send their response to the web server over standard output.

I hope this helps.  Let me know if I can help any other way.