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Aquinas is an interactive learning system that aims to teach computer programming and exploit development. Teachers define programming projects, and students complete the projects and submit their work using Git. Aquinas provides a website which lists the projects and provides a summary of each student’s progress.

A number of goals drove the design of Aquinas:

  1. Allow for projects which involve network programming and exploit development.

  2. Facilitate easy-to-define projects with a consistent specification language.

  3. Ease reuse of projects across many programming languages.

  4. Allow for high-quality assignment instructions.

  5. Provide a web- and Git-based interface to students.

  6. Provide for automated grading and student feedback.

  7. Apply the principle of least privilege and use a type-safe language.

Installing Aquinas

Building and deploying Aquinas requires the following software:

* make
* The Go programming language
* The jq JSON parser
* Git
* An ssh client
* LaTeXML
* A less (to '../../flyn.org.css') compiler

Quick check

  1. Run “make deps all”.

  2. Run “(sudo) ./httpd -dummy -root ./www/”.

You should now be able to browse to http://localhost/. The dynamic aspects of Aquinas will not work without a full installation, but you should be able to review most of Aquinas' look and feel using this dummy mode.

Full installation

  1. Edit aquinas.json and aquinas-build.json to facilitate installing Aquinas at your site.

  2. Run “make deps all vms”. This will build some utilities and the Aquinas VMs.

  3. Install each disk image (e.g., aquinas-git-openwrt-x86-64-combined-ext4.img) and each domain configuration (e.g., vm-aquinas-git.cfg) so that they can be run by your hypervisor.

  4. Update your DHCP service to provide the correct IP address to each domain. Refer to each domain configuration for the host’s MAC address.

  5. Update your DNS service to provide an appropriate A record for each domain. Refer to each domain configuration for the host’s name.

  6. Configure syslog-ng on each VM. The aquinas-user and aquinas-target VMs must be configured to reference a log server by IP address, as their firewall prohibits DNS queries.

  7. Start each configured VM.

  8. On your development computer, run “./aquinas-setup-ssh”.

  9. On aquinas-www, run “sudo -u http aquinas-add-student test PASSWORD ‘"Test Account”’“.

  10. On your development computer, run “./aquinas-add-teacher ~/.ssh/id_rsa.pub”.

  11. Push a projects repository to aquinas-git:/mnt/xvdb/teacher/projects.

  12. Push a records repository to aquinas-git:/mnt/xvdb/teacher/records.

  13. Push the HTML documents to aquinas-www.

  14. On your development computer and from directory test/, run “./test-all.”

Writing a project

Writing a project is a matter of creating a machine-readable JSON file to define the project and a LaTeX (or Markdown) fragment to instruct students in how to complete the project.

Project definition

Here is the definition of a very simple project named unix. The absence of the checks keyword means that Aquinas will not grade this project. Because languages is set to none, Aquinas will generate no language-specific variants of this project. Perhaps this project could guide the student through an introduction to UNIX without requiring a graded deliverable.

{
    "name": "unix",
    "summary": "An introduction to Unix and the Bourne shell",
    "languages": [ "none" ]
}

Here is another language-agnostic project. This project, git, assumes the completion of unix. Aquinas will take this into account when ordering the list of projects presented to a student. This project provides a check (checks); running the command cat NOTES from the root of the student’s Git repository should print “In case of fire: git commit, git push, and leave the building!” to standard out. (I.e., the file NOTES should exist in the Git repository and it should contain “In case of fire: …”)

The value in the stdout field is this string, base64 encoded (but not depicted in its entirety here). The base64 encoding is to allow such values to contain binary data.

{
    "name": "git",
    "summary": "An introduction to the Git version control system",
    "languages": [ "none" ],
    "prerequisites": [ "unix" ],
    "checks": [{ "command": "cat NOTES",
                 "stdin": null,
                 "stdout": "SW4gY2Fz...",
                 "stderr": null,
                 "exitCode": 0
              }]
}

A student may submit the following project in C or Python. In the case of C, the submission should contain hello.c, and this file should compile to a program which prints “Hello, world\n”. A Python submission should take the form of hello as an executable script (i.e., with shebang). As with the previous example, the value of stdout is base64 encoded to support binary data.

{
    "name": "hello",
    "summary": "Printing to the screen",
    "languages": [ "C", "Python" ],
    "prerequisites": [ "git" ],
    "checks": [{ "command": "./hello",
                 "stdin": null,
                 "stdout": "SGVsbG8sIHdvcmxkIQo=",
                 "stderr": null,
                 "exitCode": 0
              }]
}

The following network project installs a service, defined in network.go, on Aquinas' target host. A correct solution will interact with this service as the project instructions prescribe to produce the output required by the project’s checks. The value of service_port minus its extension must match the value of name, and every project must bear a unique service_port.

{
    "name": "network",
    "summary": "Programming a TCP/IPv4 client",
    "languages": [ "C", "Go", "Python" ],
    "prerequisites": [ "hello" ],
    "checks": [{ "command": "./network",
                     "stdin": null,
                     "stdout": "...",
                     "stderr": null,
                     "exitCode": 0
                  }],
    "services": [{ "source": "service-network.go",
                   "port": 1025
    }]
}

Adding the following within a particular service’s definition makes the service binary available for download.

    "publish_binary": true

A teacher can also specify flags in a service definition that Aquinas will pass to the compiler when building the service binary:

    "compiler_flags": "-fstack-protector"

When presenting lists of projects, Aquinas will make use of tags for organization. Tags augment the language categories:

    "tags": [ "Exploitation" ]

Sometimes it might be useful to directly call functions in a project solution when testing. An altmain statement allows this. For example, if altmain is defined as true for C, Aquinas will compile the submitted program such that the function main2 in the teacher-defined file main2.c serves as the program’s main function.

{
    "name": "functions",
    "summary": "Abstraction and code reuse using functions",
    "languages": [ "C", "Go", "Python" ],
    "prerequisites": [ "hello" ],
    "altmain": { "C": true },
    "checks": [{ "command": "./functions",
                     "stdin": null,
                     "stdout": "...",
                     "stderr": null,
                     "exitCode": 0
                  }]
}

Aquinas supports solution templates, which provide students with a starting point for their solution. In this example, the values of templates should correspond with template files for C, Go, and Python. These files must exists in the same Git repository as this description, and Aquinas will copy them to the students' variables repositories.

{
    "name": "variables",
    "summary": "Programming language variables and arithmetic operators",
    "languages": [ "C", "Go", "Python" ],
    "prerequisites": [ "hello" ],
    "checks": [{ "command": "./variables",
                     "stdin": "...",
                     "stdout": "...",
                     "stderr": null,
                     "exitCode": 0
                  }],
    "templates": {
        "C": "template-variablesC.c",
        "Go": "template-variablesGo.go",
        "Python": "template-variablesPython"
    }
}

Project instructions

Teachers write project instructions in the form of a LaTeX fragment, which Aquinas combines with a template before processing into a HTML document. It is a good practice to use \section* to provide three sections: Command (or Function) summary, Lesson, and Assignment. Aquinas will provide the prelude and epilog material; here it is sufficient to begin with the first \section*.

Aquinas provides the following LaTeX commands for use in a project’s instructions:

\cmd
Typeset the argument as a command.
\project
Typeset the argument as a project name.
\conf
Typeset the argument as a configuration file.
\file
Typeset the argument as a file name.
\dir
Typeset the argument as a directory name.
\fn
Typeset the argument as a function name.
\host
Typeset the argument as a host name.
\keypress
Typeset the argument as if it were a key to be pressed.
\unix
Typeset the work UNIX.
\shprompt
Typeset a Bourne shell prompt.
\cmddesc
Define a command within a LaTeX description list. Like \item, except Aquinas notes occurences of \cmddesc to produce a command reference page.
\fncdesc
Define a C function within a LaTeX description list. Like \item, except Aquinas notes occurences of \fncdesc to produce a C function reference page. Variants exist for other languages such as Go (\fngodesc) and Python (\fnpythondesc).
\ifC
If the language associated with the current project variant matches (here the C variant), then include the subsequent text up until the next \fi. Otherwise print nothing.
\ifshebang
If the language can make use of a shebang, then print the second argument. Otherwise print nothing.
\clone
Provide instructions on how to clone the current project using Git.
\submission
Print instructions on how to submit a project solution.
\submissionNoLanguage
Print instructions on how to submit a language-agnostic project solution.

Alternatively, a teacher can describe a project using Markdown.

The Aquinas VMs

aquinas-www
The HTTP server which allows users to read project assignments and view submission results.
aquinas-git
The Git server to which users make project submissions.
aquinas-user
The host which runs project submissions during the grading process.
aquinas-target
The host which runs network services with which user programs might interact.

System inputs

(unauthenticated) http://aquinas-www/landing.html
Allows a user to either log in or register.
(unauthenticated) http://aquinas-www/login.html
Accepts student email and password. Allows a student to log in to the web interface. Transitions state of HTTP session to authenticated.
(unauthenticated) http://aquinas-www/register.html
Accepts an email address. Allows a registering student to initiate the registration process. Sends an email to the registering student which allows him to complete the registration.
(unauthenticated) http://aquinas-www/register3.html
Accepts an email address, nonce, hashed token, and password. Allows a registering student to prove ownership of an email address and thus complete the registration process.
(authenticated) http://aquinas-www/index.html
Allows a student to select a project page.
(authenticated) http://aquinas-www/p.html, where p is a project
Allows a student to view information which describes project p.
ssh://s@aquinas-git/mnt/xvdb/s/p, where s is a student and p is a project
Interact with student s’s project p submission using Git/git-shell. Git hook invokes grader, run as teacher, with s and p as its input.
ssh://t@aquinas-git/mnt/xvdb/teacher/projects, where t is a teacher
Interact with the project definitions using Git/git-shell. Git hook invokes aquinas-initialize-projects, run as root.
ssh://t@aquinas-git/mnt/xvdb/teacher/records, where t is a teacher
Interact with the project submission records using Git/git-shell.
ssh://root@*
Allows developers shell access.

Sudo permissions

aquinas-git

SSH keys

root@aquinas-git
generated by openwrt-build
teacher@aquinas-git
generated by openwrt-build
http@aquinas-www
generated by openwrt-build
Developers
installed by hand
Teachers
installed by aquinas-add-teacher
Students
installed by ssh/ssh2.html

Permitted SSH connections

From To Installed by Purpose
Developers root@aquinas-git openwrt-build/manual Development/administration
http@aquinas-www http@aquinas-git setup-ssh Httpd uses to run httpsh:
check for student
check for SSH key
add a new student
deploy SSH key
remove a student
Developers test@aquinas-git test case Pushing solutions during
tests
Teachers teacher@aquinas-git add-teacher Project deployment
Developers root@aquinas-user openwrt-build/manual Development
teacher@aquinas-git root@aquinas-user setup-ssh Used by initialize-project
to pull host key and user
key from aquinas-user
(Cannot use test@, because
test has shell set to
buildrunsh)
Also used by grader to place
user-submitted code on
aquinas-user
root@aquinas-git root@aquinas-user setup-ssh Add or remove user.
root@aquinas-git teacher@aquinas-user setup-ssh Compile network project services.
teacher@aquinas-git test@aquinas-user add-student Grader uses to run buildrunsh
on user VM; Git hook runs as
user, and uses sudo to run
grader as teacher
Developers root@aquinas-www openwrt-build/manual Development/administration
Teachers teacher@aquinas-www add-teacher Updating HTML documents
teacher@aquinas-git teacher@aquinas-www setup-ssh Used by grader to update
records on aquinas-www
Students STUDENT@aquinas-git ssh/ssh2.html Commit submissions to Git
root@aquinas-git root@aquinas-target setup-ssh Deploy network project services.

Component isolation

The design of Aquinas isolates its components to prevent student program from exfiltrating data available to them during the grading process. This prevents student knowledge of the checks which grade their submissions. Furthermore, student programs cannot interact beyond the aquinas-user host with the exception of allowed connections to aquinas-target.

Firewall rules

The host firewalls on aquinas-user and aquinas-target prohibit all outgoing connections with the exception of a connection to a syslog server. All interaction with these hosts is by way of incoming SSH connections or project-specific services. The aim of this is to prevent a user-written program from exfiltrating data from either host while executing for the purpose of grading.

Chroot jails

Services on aquinas-target run as the user nobody and within a chroot jail.

Email: www@flyn.org — ✉ 6110 Campfire Court; Columbia, Maryland 21045; USA