The L4.verified Proofs

This is the L4.verified git repository with formal specifications and proofs for the seL4 microkernel.

Most proofs in this repository are conducted in the interactive proof assistant Isabelle/HOL. For an introduction to Isabelle, see its official website and documentation.

Setup

This repository is meant to be used as part of a Google repo setup. Instead of cloning it directly, follow the instructions at the manifest git repo.

Linux Packages - Debian

On Buster or Bullseye, to run all the tests against the ARMv7-A architecture you will need to install the following packages:

sudo apt-get install \
    python3 python3-pip python3-dev \
    gcc-arm-none-eabi build-essential libxml2-utils ccache \
    ncurses-dev librsvg2-bin device-tree-compiler cmake \
    ninja-build curl zlib1g-dev texlive-fonts-recommended \
    texlive-latex-extra texlive-metapost texlive-bibtex-extra \
    rsync

There is no package for the MLton compiler on Buster or Bullseye, so you will need to install it from the MLton website.

The Haskell Stack package is unavailable on Bullseye and out-of-date on Buster, so you will need to install it from the Haskell Stack website.

Linux Packages - Ubuntu

On Ubuntu 18.04, to run all the tests against the ARMv7-A architecture you will need to install the following packages:

sudo apt-get install \
    python3 python3-pip python3-dev \
    gcc-arm-none-eabi build-essential libxml2-utils ccache \
    ncurses-dev librsvg2-bin device-tree-compiler cmake \
    ninja-build curl zlib1g-dev texlive-fonts-recommended \
    texlive-latex-extra texlive-metapost texlive-bibtex-extra \
    mlton-compiler haskell-stack

Python

The build system for the seL4 kernel requires several python packages:

sudo pip3 install --upgrade pip
sudo pip3 install sel4-deps

Haskell Stack

After installing haskell-stack, make sure you've adjusted your PATH to include $HOME/.local/bin, and that you're running an up-to-date version:

stack upgrade --binary-only
which stack # should be $HOME/.local/bin/stack

MacOS

Other than the cross-compiler gcc toolchain, setup on MacOS should be similar to that on Ubuntu. To set up a cross-compiler, try the following:

• Install XCode from the AppStore and its command line tools. If you are running MacPorts, you have these already. Otherwise, after you have XCode installed, run gcc --version in a terminal window. If it reports a version, you're set. Otherwise it should pop up a window and prompt for installation of the command line tools.
• Install the seL4 Python dependencies, for instance using sudo easy_install sel4-deps. easy_install is part of Python's [setuptools][9].
• Install the misc/scripts/cpp wrapper for clang, by putting it in ~/bin, or somewhere else in your PATH.

Contributing

Contributions to this repository are welcome. Please read CONTRIBUTING.md for details.

Overview

The repository is organised as follows.

• docs: documentation on conventions, style, etc.

• spec: a number of different formal specifications of seL4

  • abstract: the functional abstract specification of seL4

  • sep-abstract: an abstract specification for a reduced version of seL4 that is configured as a separation kernel

  • haskell: Haskell model of the seL4 kernel, kept in sync with the C code

  • machine: the machine interface of these two specifications

  • cspec: the entry point for automatically translating the seL4 C code into Isabelle

  • capDL: a specification of seL4 that abstracts from memory content and concrete execution behaviour, modelling the protection state of the system in terms of capabilities. This specification corresponds to the capability distribution language capDL that can be used to initialise user-level systems on top of seL4.

  • take-grant: a formalisation of the classical take-grant security model, applied to seL4, but not connected to the code of seL4.

  • There are additional specifications that are not tracked in this repository, but are generated from other files:

    • design: the design-level specification of seL4, generated from the Haskell model.
    • c: the C code of the seL4 kernel, preprocessed into a form that can be read into Isabelle. This is generated from the seL4 repository.

• proof: the seL4 proofs

  • invariant-abstract: invariants of the seL4 abstract specification
  • refine: refinement between abstract and design specifications
  • crefine: refinement between design specification and C semantics
  • access-control: integrity and authority confinement proofs
  • infoflow: confidentiality and intransitive non-interference proofs
  • asmrefine: Isabelle/HOL part of the seL4 binary verification
  • drefine: refinement between capDL and abstract specification
  • sep-capDL: a separation logic instance on capDL
  • capDL-api: separation logic specifications of selected seL4 APIs

• lib: generic proof libraries, proof methods and tools. Among these, further libraries for fixed-size machine words, a formalisation of state monads with nondeterminism and exceptions, a generic verification condition generator for monads, a recursive invariant prover for these (crunch), an abstract separation logic formalisation, a prototype of the Eisbach proof method language, a prototype levity refactoring tool, and others.

• tools: larger, self-contained proof tools

  • asmrefine: the generic Isabelle/HOL part of the binary verification tool
  • c-parser: a parser from C into the Simpl language in Isabelle/HOL. Includes a C memory model.
  • autocorres: an automated, proof-producing abstraction tool from C into higher-level Isabelle/HOL functions, based on the C parser above
  • haskell-translator: a basic python script for converting the Haskell prototype of seL4 into the executable design specification in Isabelle/HOL.

• misc: miscellaneous scripts and build tools

• camkes: an initial formalisation of the CAmkES component platform on seL4. Work in progress.

• sys-init: specification of a capDL-based, user-level system initialiser for seL4, with proof that the specification leads to correctly initialised systems.

Hardware requirements

Almost all proofs in this repository should work within 4GB of RAM. Proofs involving the C refinement, will usually need the 64bit mode of polyml and about 16GB of RAM.

The proofs distribute reasonably well over multiple cores, up to about 8 cores are useful.

Isabelle Setup

After the repository is set up using repo (as per the setup section above), you should have following directory structure, where l4v is the repository you are currently looking at:

verification/
    isabelle/
    l4v/
    seL4/

To set up Isabelle for use in l4v/, assuming you have no previous installation of Isabelle, run the following commands in the directory verification/l4v/:

mkdir -p ~/.isabelle/etc
cp -i misc/etc/settings ~/.isabelle/etc/settings
./isabelle/bin/isabelle components -a
./isabelle/bin/isabelle jedit -bf
./isabelle/bin/isabelle build -bv HOL-Word

These commands perform the following steps:

• create an Isabelle user settings directory.
• install L4.verified Isabelle settings. These settings initialise the Isabelle installation to use the standard Isabelle contrib tools from the Munich Isabelle repository and set up paths such that multiple Isabelle repository installations can be used side by side without interfering with each other.
• download contrib components from the Munich repository. This includes Scala, a Java JDK, PolyML, and multiple external provers. You should download these, even if you have these tools previously installed elsewhere to make sure you have the right versions. Depending on your internet connection, this may take some time.
• compile and build the Isabelle PIDE jEdit interface.
• build basic Isabelle images, including HOL-Word to ensure that the installation works. This may take a few minutes.

Alternatively, it is possible to use the official Isabelle2020 release bundle for your platform from the Isabelle website. In this case, the installation steps above can be skipped, and you would replace the directory verification/isabelle/ with a symbolic link to the Isabelle home directory of the release version. Note that this is not recommended for development, since Google repo will overwrite this link when you synchronise repositories and Isabelle upgrades will have to be performed manually as development progresses.

JEdit

We provide a JEdit macro that is very useful when working with large theory files, goto-error, which moves the cursor to the first error in the file.

To install the macro, run the following commands in the directory verification/l4v/:

mkdir -p ~/.isabelle/jedit/macros
cp misc/jedit/macros/goto-error.bsh ~/.isabelle/jedit/macros/.

You can add keybindings for this macro in the usual way, by going to Utilities -> Global Options -> jEdit -> Shortcuts.

Additionally, our fork of Isabelle/jEdit has an updated indenter which is more proof-context aware than the 'original' indenter. Pressing ctrl+i while some apply-script text is selected should auto-indent the script while respecting subgoal depth and maintaining the relative indentation of multi-line apply statements.

Running the Proofs

If Isabelle is set up correctly, a full test for the proofs in this repository can be run with the command

./run_tests

from the directory l4v/.

Not all of the proof sessions can be built directly with the isabelle build command. The seL4 verification proofs depend on Isabelle specifications that are generated from the C source code and Haskell model. Therefore, it's recommended to always build using the supplied makefiles, which will ensure that these generated specs are up to date.

To do this, enter one level under the l4v/ directory and run make <session-name>. For example, to build the C refinement proof session, do

cd l4v/proof
make CRefine

As another example, to build the session for the Haskell model, do

cd l4v/spec
make ExecSpec

See the HEAPS variable in the corresponding Makefile for available targets.

Proof sessions that do not depend on generated inputs can be built directly with

./isabelle/bin/isabelle build -d . -v -b <session name>

from the directory l4v/. For available sessions, see the corresponding ROOT files in this repository. There is roughly one session corresponding to each major directory in the repository.

For interactively exploring, say the invariant proof of the abstract specification with a pre-built logic image for the abstract specification, run

./isabelle/bin/isabelle jedit -d . -l ASpec

in l4v/ and open one of the files in proof/invariant-abstract.