GSoC Reports: Enhancing Syzkaller support for NetBSD, Part 2


August 05, 2020 posted by Kamil Rytarowski

This report was prepared by Ayushi Sharma as a part of Google Summer of Code 2020

As a part of Google summer code 2020, I have been working on Enhance the Syzkaller support for NetBSD. This post summarises the work done in the past month.

For work done in the first coding period, you can take a look at the previous post.

Automation for enhancement

With an aim of increasing the number of syscalls fuzzed, we have decided to automate the addition of descriptions for syscalls as well as ioctl device drivers in a customised way for NetBSD.

Design

All the ioctl commands for a device driver in NetBSD are stored inside the /src/sys/dev/<driver_name>/ folder. The idea is to get information related to a particular ioctl command by extracting required information from the source code of drivers. To achieve the same, we have broken down our project into majorly three phases.

  1. Generating preprocessed files
  2. Extracting information required for generating descriptions
  3. Conversion to syzkaller’s grammar
Design

Generating Preprocessed files

For a given preprocessed file, c2xml tool outputs the preprocessed C code in xml format. Further, the intermediate xml format descriptions would help us to smoothly transform the c code to syzkaller specific descriptions, in the last stage of this tool. We have used Bear as an aid for fetching commands to preprocess files for a particular driver. Bear generates a file called compile_commands.json which stores the commands used for compiling a file in json format. We then run these commands with ‘-E’ gcc flag to fetch the preprocessed files.These preprocessed files then serve as an input to the c2xml program.

Extractor

Definition of ioctl calls defined in header files of device driver in NetBSD can be broken down to:

ioctl

When we see it from syzkaller’s perspective, there are basically three significant parts we need to extract for adding description to syzkaller.

Description of a particular ioctl command acc to syzkaller’s grammar:

ioctl$FOOIOCTL(fd <fd_driver>, cmd const[FOOIOCTL], pt ptr[DIR, <ptr_type>])
ioctl description
ioctl description

These definitions can be grepped from a device’s header files. The type information or description for pointer can then be extracted from the output files generated by c2xml. If the third argument is a struct, the direction of the pointer is determined with the help of fun() macros.

To-Do

The extracted descriptions have to be converted into syzkaller-friendly grammer. We plan to add support for syscalls too , which would ease the addition of complex compat syscalls. This would help us to increase the syzkaller’s coverage significantly.

Stats

Along with this, We have continued to add support for few more syscalls these include:
  • ksem(2) family
  • mount(2) family
Syscalls related to sockets have also been added. This has increased syscall coverage percentage to 50.35.

Atlast, I would like to thank my mentors - Cryo, Siddharth Muralee and Santhosh along with Kamil for their guidance and support. I am thankful to NetBSD community too along with Google for providing me such an amazing opportunity. [0 comments]

 



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