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Date: Thu, 5 Jun 2014 18:08:42 +0200
From: Borislav Petkov <bp@alien8.de>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Matt Fleming <matt.fleming@intel.com>, "H. Peter Anvin" <hpa@zytor.com>,
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Subject: Re: [tip:x86/efi] x86/efi: Check for unsafe dealing with FPU state
 in irq ctxt
Message-ID: <20140605160842.GE16642@pd.tnic>
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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...

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
--