## Piston
Piston is the underlying engine for running untrusted and possibly malicious code that originates
from from EMKC contests and challenges. It's also used in the Engineer Man Discord server via
[felix bot](https://github.com/engineer-man/felix).

#### Installation
```
# clone and enter repo
git clone https://github.com/engineer-man/piston
cd piston/lxc

# install dependencies

# centos:
yum install -y epel-release
yum install -y lxc lxc-templates debootstrap libvirt

# everything else:
# not documented, please open pull requests with commands for ubuntu/debian/arch/macos

# start libvirtd
systemctl start libvirtd

# create and start container
lxc-create -t download -n piston -- --dist ubuntu --release bionic --arch amd64
./start

# open a shell to the container
./shell

# install all necessary piston dependencies
echo 'export PATH=/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin' >> /root/.bashrc
echo 'export PATH=$PATH:/root/.cargo/bin' >> /root/.bashrc
sed -i \
    's/http:\/\/archive.ubuntu.com\/ubuntu/http:\/\/mirror.math.princeton.edu\/pub\/ubuntu/' \
    /etc/apt/sources.list
apt-get update
apt-get -y install git tzdata nano \
    dpkg-dev build-essential python python3 \
    ruby nodejs golang php7.2 r-base mono-complete \
    nasm openjdk-8-jdk ubuntu-make bf
umake swift
ln -s /root/.local/share/umake/swift/swift-lang/usr/bin/swift /usr/bin/swift
curl https://sh.rustup.rs -sSf | sh
ln -s /root/.cargo/bin/rustc /usr/bin/rustc
rm -rf /home/ubuntu
chmod 777 /tmp
chmod 777 -R /root

# create runnable users and apply limits
for i in {1..150}; do
    useradd -M runner$i
    usermod -d /tmp runner$i
    echo "runner$i soft nproc 64" >> /etc/security/limits.conf
    echo "runner$i hard nproc 64" >> /etc/security/limits.conf
    echo "runner$i soft nofile 2048" >> /etc/security/limits.conf
    echo "runner$i hard nofile 2048" >> /etc/security/limits.conf
done

# leave container
exit

# optionally run tests
cd ../tests
./test_all_lxc
```

#### CLI Usage
- `lxc/execute [language] [file path] [arg]...`

#### API Usage
The Piston API exposes one endpoint at `http://127.0.0.1:2000/execute`.
This endpoint takes the following JSON payload and expects at least the language and source. If
source is not provided, a blank file is passed as the source.
```json
{
    "language": "js",
    "source": "console.log(process.argv)",
    "args": [
        "1",
        "2",
        "3"
    ]
}
```
A typical response when everything succeeds will be similar to the following:
```json
{
    "ran": true,
    "output": "[ '/usr/bin/node',\n  '/tmp/code.code',\n  '1',\n  '2',\n  '3' ]"
}
```
If an invalid language is supplied, a typical response will look like the following:
```json
{
    "code": "unsupported_language",
    "message": "whatever is not supported by Piston"
}
```

#### Supported Languages
Currently python2, python3, c, c++, go, node, ruby, r, c#, nasm, php, java, swift, brainfuck, rust, and bash is supported.

#### Principle of Operation
Piston utilizes LXC as the primary mechanism for sandboxing. There is a small API written in Go which takes
in execution requests and executes them in the container. High level, the API writes
a temporary source and args file to `/tmp` and that gets mounted read-only along with the execution scripts into the container.
The source file is either ran or compiled and ran (in the case of languages like c, c++, c#, go, etc.).

#### Security
LXC provides a great deal of security out of the box in that it's separate from the system.
Piston takes additional steps to make it resistant to
various privilege escalation, denial-of-service, and resource saturation threats. These steps include:
- Disabling outgoing network interaction
- Capping max processes at 64 (resists `:(){ :|: &}:;`, `while True: os.fork()`, etc.)
- Capping max files at 2048 (resists various file based attacks)
- Mounting all resources read-only (resists `sudo rm -rf --no-preserve-root /`)
- Running as a variety of unprivileged users
- Capping runtime execution at 3 seconds
- Capping stdout to 65536 characters (resists yes/no bombs and runaway output)
- SIGKILLing misbehaving code

#### License
Piston is licensed under the MIT license.