The Day I Fell in Love with Fuzzing

nullprogram.com/blog/2019/01/25/

In 2007 I wrote a pair of modding tools, binitools, for a space
trading and combat simulation game named Freelancer. The game
stores its non-art assets in the format of “binary INI” files, or “BINI”
files. The motivation for the binary format over traditional INI files
was probably performance: it’s faster to load and read these files than
it is to parse arbitrary text in INI format.

Much of the in-game content can be changed simply by modifying these
files — changing time names, editing commodity prices, tweaking ship
statistics, or even adding new ships to the game. The binary nature
makes them unsuitable to in-place modification, so the natural approach
is to convert them to text INI files, make the desired modifications
using a text editor, then convert back to the BINI format and replace
the file in the game’s installation.

I didn’t reverse engineer the BINI format, nor was I the first person
the create tools to edit them. The existing tools weren’t to my tastes,
and I had my own vision for how they should work — an interface more
closely following the Unix tradition despite the target being a
Windows game.

When I got started, I had just learned how to use yacc (really
Bison) and lex (really flex), as well as
Autoconf, so I went all-in with these newly-discovered tools. It was
exciting to try them out in a real-world situation, though I slavishly
aped the practices of other open source projects without really
understanding why things were they way they were. Due to the use of
yacc/lex and the configure script build, compiling the project required
a full, Unix-like environment. This is all visible in the original
version of the source
.

The project was moderately successful in two ways. First, I was able to
use the tools to modify the game. Second, other people were using the
tools, since the binaries I built show up in various collections of
Freelancer modding tools online.

The Rewrite

That’s the way things were until mid-2018 when I revisited the project.
Ever look at your own old code and wonder what they heck you were
thinking? My INI format was far more rigid and strict than necessary, I
was doing questionable things when writing out binary data, and the
build wasn’t even working correctly.

With an additional decade of experience under my belt, I knew I could do
way better if I were to rewrite these tools today. So, over the course
of a few days, I did, from scratch. That’s what’s visible in the master
branch today.

I like to keep things simple which meant no more Autoconf, and
instead a simple, portable Makefile. No more yacc or lex, and
instead a hand-coded parser. Using only conforming, portable C. The
result was so simple that I can build using Visual Studio in a
single, short command, so the Makefile isn’t all that necessary. With
one small tweak (replace stdint.h with a typedef), I can even build
and run binitools in DOS
.

The new version is faster, leaner, cleaner, and simpler. It’s far more
flexible about its INI input, so its easier to use. But is it more
correct?

Fuzzing

I’ve been interested in fuzzing for years, especially
american fuzzy lop, or afl. However, I wasn’t having success
with it. I’d fuzz some of the tools I use regularly, and it wouldn’t
find anything of note, at least not before I gave up. I fuzzed my
JSON library
, and somehow it turned up nothing. Surely my
JSON parser couldn’t be that robust already, could it? Fuzzing just
wasn’t accomplishing anything for me. (As it turns out, my JSON
library is quite robust, thanks in large part to various
contributors!)

So I’ve got this relatively new INI parser, and while it can
successfully parse and correctly re-assemble the game’s original set of
BINI files, it hasn’t really been exercised that much. Surely there’s
something in here for a fuzzer to find. Plus I don’t even have to write
a line of code in order to run afl against it. The tools already read
from standard input by default, which is perfect.

Assuming you’ve got the necessary tools installed (make, gcc, afl),
here’s how easy it is to start fuzzing binitools:

$ make CC=afl-gcc
$ mkdir in out
$ echo '[x]' > in/empty
$ afl-fuzz -i in -o out -- ./bini

The bini utility takes INI as input and produces BINI as output, so
it’s far more interesting to fuzz than its inverse, unbini. Since
unbini parses relatively simple binary data, there are (probably) no
bugs for the fuzzer to find. I did try anyway just in case.

In my example above, I swapped out the default compiler for afl’s GCC
wrapper (CC=afl-gcc). It calls GCC in the background, but in doing so
adds its own instrumentation to the binary. When fuzzing, afl-fuzz
uses that instrumentation to monitor the program’s execution path. The
afl whitepaper explains the technical details.

I also created input and output directories, placing a minimal, working
example into the input directory, which gives afl a starting point. As
afl runs, it mutates a queue of inputs and observes the changes on the
program’s execution. The output directory contains the results and, more
importantly, a corpus of inputs that cause unique execution paths. In
other words, the fuzzer output will be lots of inputs that exercise many
different edge cases.

The most exciting and dreaded result is a crash. The first time I ran it
against binitools, bini had many such crashes. Within minutes, afl
was finding a number of subtle and interesting bugs in my program, which
was incredibly useful. It even discovered an unlikely stale pointer
bug
by exercising different orderings for various memory
allocations. This particular bug was the turning point that made me
realize the value of fuzzing.

Not all the bugs it found led to crashes. I also combed through the
outputs to see what sorts of inputs were succeeding, what was failing,
and observe how my program handled various edge cases. It was rejecting
some inputs I thought should be valid, accepting some I thought should
be invalid, and interpreting some in ways I hadn’t intended. So even
after I fixed the crashing inputs, I still made tweaks to the parser to
fix each of these troublesome inputs.

Building a test suite

Once I combed out all the fuzzer-discovered bugs, and I agreed with the
parser on how all the various edge cases should be handled, I turned the
fuzzer’s corpus into a test suite — though not directly.

I had run the fuzzer in parallel — a process that is explained in the
afl documentation — so I had lots of redundant inputs. By redundant I
mean that the inputs are different but have the same execution path.
Fortunately afl has a tool to deal with this: afl-cmin, the corpus
minimization tool. It eliminates all the redundant inputs.

Second, many of these inputs were longer than necessary in order to
invoke their unique execution path. There’s afl-tmin, the test case
minimizer, which I used to further shrink my test corpus.

I sorted the valid from invalid inputs and checked them into the
repository. Have a look at all the wacky inputs invented by the
fuzzer starting from my single, minimal input:

This essentially locks down the parser, and the test suite ensures a
particular build behaves in a very specific way. This is most useful
for ensuring that builds on other platforms and by other compilers are
indeed behaving identically with respect to their outputs. My test suite
even revealed a bug in diet libc, as binitools doesn’t pass the tests
when linked against it. If I were to make non-trivial changes to the
parser, I’d essentially need to scrap the current test suite and start
over, having afl generate an entire new corpus for the new parser.

Fuzzing has certainly proven itself to be a powerful technique. It found
a number of bugs that I likely wouldn’t have otherwise discovered on my
own. I’ve since gotten more savvy on its use and have used it on other
software — not just software I’ve written myself — and discovered more
bugs. It’s got a permanent slot on my software developer toolbelt.

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