Re: [tip:x86/efi] x86/efi: Check for unsafe dealing with FPU state in irq ctxt

From: Borislav Petkov
Date: Thu Jun 05 2014 - 12:11:13 EST

On Thu, Jun 05, 2014 at 09:01:02AM -0700, Andy Lutomirski wrote:
> NMI might be okay. I haven't checked.

Well, if efi decides to do FPU math and it happens in NMI, we will have
to provide for proper contexts handling.

> It has to change back, though. Completely unrealistic and useless example:
> int ctxt = what_context_im_in();
> set_up_the_fpu(ctxt);
> // kprobe fires and changes the context
> // kprobe does something

And since we're being completely unrealistic, kprobe decides to use the
fpu too and uses it...

> // kprobe changes the context back
> use the FPU. Life is good.
> put_back_the_fpu(ctxt);

So you probably need some way of mapping preallocated, per-cpu FPU
contexts to their users which can get and put them.

It's a whole different question whether that makes sense though, at all
or we simply remain conservative and don't do any efi in NMI context...


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