However, stuff like WebAssembly and other sandboxing methods can be used to leverage two processes into the same address space. Your filesystem driver then simply lives as a module in address space and it's a normal process. The IPC turns into a simple jump using a pointer value provided by the kernel (which depending on the trust level can provide parameter validation or can be a plain pointer to the correct function).
The cost of switching processes are not a problem of address space, the MMU makes sure that is not a problem as shared memory is a reasonable tradeoff for certain domains where performance is necessary. Normally you'd rather want use messages but that is a different topic.
In either case, the cost of a process switch is handling the registers and cache - that will not go away no matter how you do it, which is why multicore implementation with messages can actually turn out being faster. Less switching and more locality.
[0] https://en.wikipedia.org/wiki/Singularity_%28operating_syste...
[1] https://courses.cs.washington.edu/courses/cse551/15sp/papers...
The cost of switching processes is significantly reduced when you don't need to switch privilege levels and not having to invalidate the TLB as well as not having to change address space at all will make a context switch not significantly more expensive than a function call.
You get compile-time isolation and can still take advantage of the MMU when needed.
And using such an implementation does not prevent you from implementing your driver such that it can run on each core and take advantage of it or even passes messages. An ethernet driver could still, for example, pass a message when the "send data to tcp socket" function is called while allowing another program to use the same function without any message passing, depending on what is better for your use case.
If a driver runs in user mode, an exploit needs to exploit the hardware as well - and that is for all intents and purposes something that we see very rarely.
If the same driver runs in "software user mode", but executing as supervisor (basically inside an VM environment), we need constant security checks in software, and an exploit now have the VM code to further exploit, if successful that will automatically grant it supervisor access.
In both cases it's assumed that neither implementation has access to more interfaces than necessary for it to do it's work. For instance, a driver for a mouse does not need access to the disks.
From my experience, a lot of hardware is terribly insecure against exploits. Not necessarily the CPU but stuff like your GPU or HBAs, ethernet cards, etc.
With software containment, the advantage is that you can set it up that drivers need to declare their interfaces and privileges beforehand. In an ELF or WASM you have to declare imported and linked functions, it should not be difficult to leverage that to determine what a driver can effectively do. With WASM you get the added benefit that doing anything but using declared interfaces results in a compile-time error.
A driver can be written so that a minimal, audited interface exists to talk to the hardware almost directly with some security checks and then the WASM part that handles the larger logic parts and provides the actual functionality.
WASM isn't a supervisor, so exploits on VM code aren't that relevant. Exploiting the WASM compiler/interpreter/JIT is more interesting but those are exposed to the daily internet shitstorm exploits, so I think they are fairly safe.
> it should not be difficult to ...
Famous last words.
Page 135.
http://sigops.org/s/conferences/sosp/2013/papers/p133-elphin...
So the trusted core part of the OS can run without any spectre prevention, though you can still enable the various hardware protections available in the chicken bits.
And if it's necessary to protect against spectre attacks, you can use shim layers or even isolation into ring3 to take preventative measures. This allows leveraging performance were important and security where necessary.
If it's in webassembly, you can even run two versions of a driver; one with spectre-mitigations compiled in and one without, sharing one memory space and the kernel can choose to invoke either one depending on the call chain.
As far as I remember they weren't able to defend from side channel attacks within the same process completely and decided to rely on process isolation instead, estimating it would be too much work to address all known spectre class vulnerabilities on their existing compilers and too hard to ensure for defenses not to be broken later by compiler developers.