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.

Repository Setup

This repository is meant to be used as part of a Google repo setup. Instead of cloning it directly, please go to the following repository and follow the instructions there:

https://github.com/seL4/verification-manifest

For setting up the theorem prover and other dependencies, please see the section Dependencies below.

Contributing

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

Overview

The repository is organised as follows.

• 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.

Dependencies

Hardware

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.

Software

The proofs in this repository use Isabelle2018. A copy of Isabelle is included in the repository setup.

The dependencies for installing Isabelle in this repository are

• Perl 5.x with libwww-perl
• Python 2.x
• LaTeX, for instance on Ubuntu 14.04 sudo apt-get install texlive-fonts-recommended texlive-latex-extra texlive-metapost texlive-bibtex-extra
• 32-bit C/C++ standard libraries on 64-bit platforms (optional)

For running the standalone version of the C Parser you will additionally need

• MLton ML compiler (package mlton-compiler on Ubuntu)

For building the Haskell kernel model, the Haskell build tool stack is required. The Haskell kernel Makefile will use stack to obtain appropriate versions of ghc and cabal-install. Note that this repository does not contain the QEmu interface for actually running the model.

For running the C proofs, you need a working C preprocessor setup for the seL4 repository.

On Linux: the best way to make sure you have everything is to install the full build environment for seL4:

• seL4 development tool chain on Debian and Ubuntu
• make version 3.81 or higher

You can get away with avoiding a full cross compiler setup form the above, but you will need at least these:

sudo apt-get install python-pip python-dev libxml2-utils
sudo pip install sel4-deps

On MacOS: here it is harder to get a full cross-compiler setup going. For normal proof development, a full setup is not necessary, though. You mostly need a gcc-compatible C pre-processor and python. Try the following steps:

• 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.
• install the misc/scripts/cpp wrapper for clang, by putting it in ~/bin, or somewhere else in your PATH.

Isabelle Setup

After the repository is set up in Google repo, 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 Isabelle2018 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.

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.

License

The files in this repository are released under standard open source licenses. Please see the individual file headers and LICENSE_GPLv2.txt and LICENSE_BSD2.txt files for details.