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

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Date:	Thu, 5 Jun 2014 09:01:02 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Matt Fleming <matt.fleming@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/efi] x86/efi: Check for unsafe dealing with FPU state in
 irq ctxt

On Thu, Jun 5, 2014 at 8:53 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Thu, Jun 05, 2014 at 08:44:20AM -0700, Andy Lutomirski wrote:
>> Are there weird contexts from which EFI calls can happen? It looks
>> like the current code isn't necessarily safe in things that aren't
>> normal process context but aren't interrupts either (e.g. debug traps,
>> #GP, etc).
>
> The efi-pstore thing registers as a kmsg dumper which can be run in NMI
> context and efi can be called there.

NMI might be okay.  I haven't checked.

>
>> I wonder if it would make sense at some point to maintain an explicit
>> stack of kernel entries. There doesn't seem to be a reliable way to
>> answer the question of "what context am I in" from C code right now.
>
> So that you can ask int ctxt = what_context_Im_in() and then that
> context can go and change right underneath you. :-)
>

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
// kprobe changes the context back

use the FPU.  Life is good.

put_back_the_fpu(ctxt);

--Andy
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