Our work is based on MirageOS, a suite to develop operating systems (developed since 2009 at University of Cambridge, UK) written in the programming language OCaml (developed since 1996 at INRIA in Paris, France).
Most libraries are developed under permissive licenses (MIT/ISC/BSD2/Apache2). The OCaml compiler is dual-licensed: LGPLv2 or BSD for consortium members (3000 EUR/year).
MirageOS is a library operating system. It composes OCaml libraries into a bespoke operating system, called a unikernel. A unikernel can be a compiled as a UNIX binary, or a standalone virtual machine image. To build exactly the right unikernel for each purpose, we can pick from hundreds of libraries which implement protocols, storage on block devices, or interfaces to network devices provided by the hypervisor.
On top of the hypervisor, a small layer of C code (developed at IBM research) is used to unify the interface between the different hypervisors, on which the OCaml runtime system is executed.
OCaml is a functional programming language with automated memory management, preventing manual memory management errors.
The strong and expressive type system of OCaml catches most programming errors already at compile time.
We use a declarative style with immutable data structures and memory to avoid side effects that are hard to reason about. Errors are expressed as part of the type signature. IO is contained on top of the protocol logic. An implementation of a protocol can be used both as executable code, and as a test oracle.
Most security problems for network services arise in parsers of received network data.
OCaml allows us to write strict parsers, which return success and error on the type system level to ensure that the caller handles all cases.
If a parser contains an error, in our system the impact is local, it cannot access memory beyond the the network data.
Errors in parsers written in unsafe languages often lead to buffer overflows, which can lead to remote code execution.
OCaml code is compiled to native code running in the OCaml runtime, which is very performant, on par with C++ code.
The OCaml runtime is just used for memory management, and very small compared to a JVM or Python runtime.
As example, our TLS library has up to 85% of the bulk throughput of OpenSSL (using AES128-CBC). The TLS handshake performance is equal with OpenSSL.
OBWOHL AUF BESTIMMTE KONTEXTE BESCHRAENKT/NICHT ALLERWELTSPRACHE DA LERNINTENSIV? OCaml is known as a mature and safe programming language that is used in both industry (facebook for compilers, jane street for trading, docker, ahrefs, simcorp, lexifi) and academia (coq, compcert, ...).
2. Module system & Compilation
OCaml has a unique module system, in which a module specifies abstract datatypes and functions, and each module can have multiple implementations. Modules can take other modules as parameters, the module system is a complete programming language, evaluated at compile time.
MirageOS uses this module system as a powerful abstraction mechanism to adapt the unikernel to the compilation target.
It defines modules for all operating system services, such as the console, the network stack, the random number generator.
For each service, an implementation can be provided depending on the compilation target (UNIX process or virtual machine).
On UNIX, the sockets API is used as the networking stack. On a virtual machine, the TCP/IP stack in OCaml is being used.
By leveraging the module system of OCaml for MirageOS, we get module separation and dependency analysis from the well-tested module system of OCaml and avoid reimplementing these error-prone and important parts.
A MirageOS unikernel developer does not need to reason about compilation targets, just about the module interface.
3. MirageOS: Running a unikernel & system security
3.0 Simple config management model with localized reasoning
There are three ways to feed a virtual machine with configuration data, such as network configuration or TLS certificate and key.
- Compile the information into the virtual machine image, which requires recompilation on configuration change.
- Pass the information as boot parameters, which requires reboot on configuration change.
- Store this information in a virtual block device which is attached to the virtual machine.
For example, logs can be exfiltrated using syslog with UDP, TCP, or TLS as transport. The transport to use has to be chosen at compile time because TLS requires the TLS library to be linked in, but the log destination can be passed as boot parameter to the unikernel.
3.1 Simple concurrency model with localized reasoning
MirageOS is an event based operating system with asynchronous tasks. A task yields the CPU once its execution is finished, or if it has to wait for IO.
This concurrency model leads to a cooperative multitasking programming style, rather than the error prone preemptive multitasking where each code block needs to be hardened to allow reentrant execution.
For example ..
3.2 Simple process memory model with localized reasoning
The virtual memory subsystem in contemporary operating systems provides an address mapping for each process. Since a unikernel is only a single service, it uses a single address space, avoiding the need for complex address mapping code altogether.
For example ..
3.3 Simple library model with localized reasoning
As stated above, a MirageOS unikernel is much smaller than a comparable UNIX virtual machine, since only the required libraries are linked into the virtual machine image. By avoiding superfluous code we decrease the attack surface immensly.
Example: lines of code / binary size fuer zb dns server / dhcp server
The choice of programming language avoids several attack vectors. Immutable data structures, type checking and the OCaml runtime memory manager ensure memory safety in OCaml.
By minimising each unikernel to its minimal footprint, security breaches are contained to the information the unikernel contains.
3.4 Secure updates
If an OCaml library introduces security flaws or information leakage, all unikernels depending on that library need to be updated.
Updating an OCaml library can safely be done via its package manager opam, which uses signed repositories.
Many libraries developed in the MirageOS project are deployed by Docker for Mac and Docker for Windows, which have more than 100000 active users.
Available libraries include an IPv4 stack (TCP, UDP, ARP), DHCP client and server, DNS server and resolver (both recursive and forwarding), HTTP (including webmachine for request routing, and sessions), syslog, git (both client and server, with mutliple storage backends: block device, in-memory), prometheus integration.
A TLS library, including random number generator (Fortuna), cryptographic primitives (RSA, DSA, DH, AES), X.509 (using ASN.1), was developed 3 years ago and is in production serving websites, plus some applications using the client side.
A prototype implementation for managing unikernels on the host system is already deployed and actively used, similar to libvirt, but with a minimised code base, and written in OCaml. Monitoring is done with prometheus. <- das bedeutet structured data und cloud ready / scalable / funktioniert in distributed system?
More libraries are under active development, this includes an OpenPGP implementation, an ssh implementation, structured syslog, Cap'n proto (RPC with support for capabilities).
OCaml can be compiled to JavaScript, which means projects can developed in a single language to ensure consistency and avoid errors, but code can be executed on the client, or on the server.
The idea of unikernels is not limited to MirageOS, other projects apply the same methodology in different programming languages.
HalVM - the Haskell ligthweight virtual machine - was developed by Galois Inc., and is based on Haskell. It is used for network services such as honeypots and secure IPSec gateways.
IncludeOS is a C++ based unikernel which was initially developed at University of Oslo, and now further developed in a startup.
Other unikernels are listed on Wikipedia https://en.wikipedia.org/wiki/Unikernel and there is an exchange of ideas between different approaches. Among all unikernels, MirageOS has been around the longest, has the most libraries available and deployed to production, has an active developer community, and the safe programming language makes it suitable for secure systems.
MirageOS has a small trusted code base compared to other operating systems. Apart from the CPU (and its virtualisation extensions, VT-x, VT-d, EPT), and the hypervisor implementation.Dieser satzkein verb
On top of the hypervisor in the host system a tiny virtual machine monitor (solo5) is executed. It does not rely on qemu or other emulation code, but only contains drivers needed for the actual unikernel (block and network devices).
The unikernel itself consists of roughly 2000 lines of C code with basic functions such as malloc, memcopy, memcmp, on which the vanilla OCaml runtime is executed.
On top of that, only OCaml code is executed, which includes an asynchronous task engine, the mentioned TCP/IP stack, and the concrete services.
The security of MirageOS unikernels is planned to be improved even more in several areas:
- data segments will be be mapped read/write, code segments execute-only
- private key material will be zeroed before destruction
- the address space layout will be randomised to make exploitation harder
- MirageOS will be ported to (se)L4 as hypervisor to minimize the trusted code running on the host system
- once open hardware (RISC-V) is widely available, MirageOS will use this as target. There is already a RISC-V backend for OCaml
- OCaml will be compilable to Coq (an interactive theorem prover) definitions, within which theorems about the code can be proven
- Coq code will also be extractable to OCaml.
## Conclusion
MirageOS started as a research project, and has matured to a full suite for building secure operating systems, with libraries that work well in production and cover a variety of application needs.
MirageOS is a game changer for secure network services, since the attack surface is minimised to 1% of the size of other contemporary operating systems. In addition, common attack vectors are avoided by the usage of a programming language with memory safety.
A unikernel boots within tens of milliseconds, and services can be spawned on demand. When a request (e.g. a DNS request) for a unikernel comes in, the kernel boots up, handles the request, and is destroyed after an inactivity period. Only the necessary services need to run, and they can be short-lived to minimize state in the system.
The choice of a high-level programming language also allows for rapid prototyping, new features can be developed quickly.
In contrast to scripting languages, the code does not need to be re-implemented for production use (but nevertheless can be fine-tuned for performance).
