Fuzzing Perl/XS modules with AFL

This blog post will walk through the steps required to fuzz a Perl XS module with American Fuzzy Lop, the coverage-based fuzzing tool.

At Booking.com, we use Perl. A lot. Perl has a lot of upsides for us (which I won’t go into), and it has been a large factor in our success. For many, though, the biggest downside can be its speed. Being an interpreted language, it runs 10x-100x slower for CPU intensive tasks. When Perl isn’t fast enough, the most common thing to do is write XS modules. But XS modules are in C, and writing safe, secure C is hard. (Side note: one of my coworkers gave a presentation on writing XS modules in Rust ) Most of the time we can get away with unit tests and code review. But for parsers, you need to go one step further: fuzzing.

A few years ago, we decided to create our own serialization format. Sereal grew out of our dissatisfaction with existing Perl serialization options. (More details can be found in the original announcement .) While Sereal is geared towards dynamic languages (and Perl in particular), there are now high-quality Go and Java implementations to match our more diverse tech stack.

Fuzzing is mostly used in computer security industry to find exploits. I wasn’t expecting our Sereal decoders to be attack vectors, but rather I wanted to be able to handle errors from corrupted data. Errors happen all the time at scale. We joke that a “one in a million” problem is happening 20 times a day. And there are real reasons why our deserialization code will need to handle bad data. Maybe a data packet will get truncated in a database column because it’s too big. Maybe it will pass through a system which adds utf8 encoding. In all these cases, it’s much better to return an error to the user than to segfault.

So, how do you get bad data? Well, the easiest way is to start with a corpus of good data and change it slightly. Then, you see if your change had any effect. If the new input causes a crash, great! You just found a bug. If the new input caused the program to hit new code paths, it’s marked as “interesting” and added back to the corpus.

This is called “coverage guided fuzzing”. For the Go implementation, I was able to use the excellent go-fuzz package from Dmitry Vyukov. The Perl module, being an XS module which is loaded into the Perl executable at runtime was a bit trickier. However, it can be done using American Fuzzy Lop. (The strange name comes from a species of particularly fuzzy bunnies.)

AFL is normally used for C libraries where it’s easy to write a small program to feed corrupted input directly into the function you want to test. Fuzzing an interpreter is a bit more complicated. First, we have to build the perl binary with AFL. Then we need to build the Sereal XS module with AFL. Then we need to write a perl script that the instrumented perl binary runs that will load the instrumented XS module and pass in our fuzzed input. Phew.

This took me a few tries to get every detail correctly sorted out.

First, build the latest release of AFL. We’re going to be using the LLVM mode, which means you need to have a recent LLVM install on your machine. My Ubuntu install has 3.8, so that’s what I used.

curl http://lcamtuf.coredump.cx/afl/releases/afl-latest.tgz
tar xf afl-latest.tgz
cd afl-2.32b
cd llvm_mode
LLVM_CONFIG=llvm-config-3.8 make

Next, clone the Perl source code and build that with AFL

git clone git://perl5.git.perl.org/perl.git perl
OR from GitHub
git clone https://github.com/Perl/perl5

Next, configure Perl to use AFL as the compiler and install it in a custom perl-afl directory.

sh Configure -des -Dusedevel \
-Dcc=/home/dgryski/src/afl-2.32b/afl-clang-fast \
-Dprefix=/home/dgryski/src/perl-afl \
-Dld=/home/dgryski/src/afl-2.32b/afl-clang-fast \
-Dloclibpth=' '

In order to AFL to recognize that we want to use persistent mode while fuzzing (more below), we need to insert a tiny patch at the start of Perl’s main function. The perl build system is pretty hairy, but edit ext/ExtUtils-Miniperl/lib/ExtUtils/Miniperl.pm and add the following line as a declaration before main(): This magic constant is detected by AFL.

volatile char *__afl_persistent_sig = "##SIG_AFL_PERSISTENT##";

This will add sufficient magic for AFL’s detection to work.

Now, build and install perl. The perl binaries that are created will have the version number on the end. By the time you read this, that might be something other than what I have here (5.25.5).

make && make install

Next we need a target to fuzz. I’ve created a small XS module to make this easier. All it does it check if the input starts with ABCD and crashes if it does.

First, generate all the boilerplate we need for the module:

~/src/perl-afl/bin/h2xs5.25.5 -A -n Fuzz

Then add the following to Fuzz.xs

char *input
if (strlen(input) < 4) { return ; }
if (input[0] == 'A') {
if (input[1] == 'B') {
if (input[2] == 'C') {
if (input[3] == 'D') {

Finally, we need to add a routine to our fuzzing module to call back into AFL’s persistent mode handler:

unsigned int count;
extern int __afl_persistent_loop(unsigned int);
RETVAL = __afl_persistent_loop(count);

Now, build the module. Perl will automatically use the compiler it was built with, which in our case is AFL.

~/src/perl-afl/bin/perl5.25.5 Makefile.PL

Next, our perl script that will feed the input from STDIN (where AFL will put it ) to our buggy XS module. Note this script is using AFL’s persistent mode to avoid spawning the perl interpreter for every test case.

Here’s our test script, afl.pl:

use blib "Fuzz/blib";
use Fuzz;
while(Fuzz::afl_persistent_loop(1000)) {
my $input;
sysread(STDIN, $input, 1024);

And we’ll test that we have a correctly broken module:

bash$ echo "ABCD" | ~/src/perl-afl/bin/perl5.25.4 afl.pl
Aborted (core dumped)
bash$ echo "1234" | ~/src/perl-afl/bin/perl5.25.4 afl.pl

Create input corpus:

mkdir corpus
echo 1234 > corpus/input

Finally, start the fuzzing!

~/src/afl-2.32b/afl-fuzz \
-i ./corpus/ \
-o ./crashers/ \
-- \
~/src/perl-afl/bin/perl5.25.5 afl.pl

Now we wait. On my laptop I get ~17k execs per second per core, which is not that bad.

Another environment variable we can set when building perl is AFL_USE_ASAN=1. This links the perl binary against Google’s Address Sanitizer plugin. This will unfortunately slow down AFL to about half of what it’s running now, but it catches a larger class of bugs that don’t necessarily trigger a crash, only an out-of-bounds memory read. You will also need to build the XS module with AFL_USE_ASAN=1 set.

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