The most used OS by far

Intel's Management Engine is based on MINIX3, so it is the most widely used OS in the world.

Every modern Intel CPU has Intel’s Management Engine (ME) built in. You actually get another OS completely free with your CPU: MINIX.

Static analysis of Intel’s ME by Positive Technologies has dicovered some strings that clearly originate from MINIX3 code. This strongly suggests that the ME is based on MINIX3, developed by Andrew Tanenbaum. In an open letter to Intel Andrew Tanenbaum expressed his surprise to find out that MINIX was used in Intel’s ME and that it would make MINIX the most widely used OS in the world. The open letter contains a few confirming facts that Intel has been working with MINIX for several years and even requested some changes to MINIX, for example, reducing the memory footprint.

MINIX

MINIX is a OS with a microkernel. A tiny kernel, responsible only for the utmost core functions of the OS. Device drivers and file and process servers are actually running in separate user-mode processes, making it virtually imposible to crash the OS. Monolithic kernels, like Windows and Linux, are likely to crash when an issue with a driver occurs.

The creator of MINIX, Andrew Tanenbaum, is a great professor. As teaching a course on operating systems, a conflict with AT&T restricted him to teach any of the UNIX internals. This made him decide to write a minimal UNIX clone and MINIX was born! The full source code was even published as an appendix to the book: “Operating Systems: Design and Implementation

I was fortunate enough to have attended a few of his lectures. You might know him from the flamefest debate with Linus Torvalds, calling Linux obsolete…

Be thankful you are not my student. You would not get a high grade for such a design :-)

– Andrew Tanenbaum

CPU Protection Rings

A CPU has a protection ring architecture to restrict what programs in different levels can do. Each ring is restricted by its lower level ring and cannot do anything that is not allowed by any lower level ring. This mechanism protects three main resources: memory, I/O ports, and the ability to execute certain machine instructions. Ring 3 typically executes programs in so called user mode. The operating system executes in ring 0, formerly thought of as the most privileged level.

Deeper than ring 0

Some things are so important that even programs running in ring 0 should not have access to them. So ring -1 was added for virtual machine hypervisors. This allows the virtual machine hypervisor to control what the OS sees and can do. The hypervisor actually multiplex several OSs and control what each OS can see and do.

… but still, there are things that even the hypervisor should not be able to access.

Deepest depth of the CPU

MINIX is running on “Ring -3”, ie. “negative 3”. That is even untouchable by your computers kernel. Research shows that some features of ring 3 are:

  • Full networking stack
  • File systems
  • Many drivers (including USB, networking, etc.)
  • … a web server

What on earth does ring -3 need a web server for?! I’ll leave the rest as an exercise for the reader.

Wrap up

Ring -3 actually reminds me of The Matrix. It is a bit frightning to know that some people are able to control your Intel powered device like they see fit, without anyone knowing or seeing…

A great presentation on Black Hat 2015 about running your own code in more privileged rings, so it is hidden from everything, i.e. the OS/kernel/AV.

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