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Operating System and Security

A project log for F-CPU

The Freedom CPU project has a log here too now :-)

Yann Guidon / YGDES 11/16/2016 at 23:260 Comments

Lately, I've discussed with @★ STMAN ★who is very knowldgeable about computer security and I'm starting to see old problems with a fresh, new and promising angle. Cue opening theme.

The trivial.

Code is not data and vice versa. FC1 is a Harvard system that does not allow self-modifying code. You can't execute data, so there is no risk from the stack, for example.

Each globule might contain enough space to store registers for 2 or 4 different threads so context switching is pretty immediate. Upon context switching, you can choose to completely switch the context or to keep the current register set (good if you need to provide informations). An instruction provides a mask to clear/wipe any register containing unneeded information for the callee.

The bad news.

Forget about Linux, forget about POSIX. FC1 is aiming at speed and security, which Linux can't garantee. In turn, this frees us from the tyrany of "having to port GCC".

My favorite approach borrows ideas from microkernels. Thank you Hurd people :-) But STMAN convinced me to go even further. Actually, the most critical code (which handles vital parts) should be as short as possible (for speed AND security audit AND non-modifiability in a small ROM). To this end, hardware must support fast and safe support for all the necessary primitives. This is implemented, for example, with specific instructions with inherent checks against user-unaccessible task properties.

Even worse news

You'll hate me, I know, but I swear it's for your own good ;-)

I'll enforce an idea I have tinkered with while thinking about the YASEP: the executable code size of a thread is limited to 64KB.

This does not apply to other things, like data sizes. But honestly, when your executable code reaches 64KB (or 16K instructions), you're definitely not doing a trivial program. The chances of having a bug is not nil. An exploit can hide.

The F-CPU forces you to compartiment your functions and make your application modular. The modules can communicate faster than with a classic architecture so there is no excuse to not enforce safe coding practices.

For example, I borrow other ideas from the YASEP: the InterProcess Call/Entry/Return trinity of instructions lets you call any publicly accessible code entry point safely, for the caller and the callee. With almost no penalty compared to a classicl call/return, and much less risks. Use it, though you won't be able to abuse it ;-)

Transfering data

An old riddle might get finally solved...

How do you transmit data from one thread to another ? Safely yet fast ? I've been researching about this for 14 years now. STMAN offered a clue : avoid aliasing by design. It sounds weird at first but this actually solves many things, since aliasing creates more problems than it solves. Usually, aliasing lets us upload executable code or share a data segment between two or more threads. But if these two cases are solved, there is no need to maintain a risky "feature".

Loading executable code in the instruction space can be achieved with special instructions that can only run in a restricted more. Data sharing is a more complex case but might be solved at last.

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