Re: FSGSBASE ABI considerations

[Linus Torvalds]

On Mon, Aug 7, 2017 at 9:20 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> Windows does something sort of like this (I think), but I don't like
> this solution.  I fully expect that someone will write a program that
> does:
>
> old = rdgsbase();
> wrgsbase(new);
> call_very_fast_function();
> wrgsbase(old);
>
> This will work if GS == 0, which is fine.  The problem is that it will
> *also* work if GS != 0 with very high probability, especially if this
> code sequence is right after some operation that sleeps.  And then
> we'll get random crashes with very low probability, depending on where
> the scheduler hits.

It will work reliably if you just make the scheduler save/restore the
base rather than the selector.

I really think you need to walk away from the "selector is meaningful"
model. Yes, yes, it's the legacy model, but it's the *insane* model.

So screw the selector. It doesn't matter. We'll need to save/restore
the value, but that's it. What we *really* save and restore is just
the base pointer.

Why do you care so much about the selector? If people *don't* use the
fsgsbase, then the selector and the base of the segment will always
match anyway (modulo the system calls that actually change the
gdt/ldt, and we can just sat that *then* selectors matter).

And if people *do* use fsgsbase, then the selector is by definition
not important.

So just make the scheduler save the base first, and restore it last.
End of problem. Your user-space code above just works. There is no
race, i doesn't matter one whit whether GS is 0 ir not, there simply
is no problem.

So just what is the problem you're trying to solve?

               Linus