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 09:18:05 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	mingo@...nel.org, ricardo.neri-calderon@...ux.intel.com,
	tglx@...utronix.de, matt.fleming@...el.com,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/efi] x86/efi: Check for unsafe dealing with FPU state
 in irq ctxt

On Wed, Jun 04, 2014 at 05:19:01PM -0700, Andy Lutomirski wrote:
> On 06/04/2014 03:49 PM, Borislav Petkov wrote:
> > On Wed, Jun 04, 2014 at 03:17:30PM -0700, H. Peter Anvin wrote:
> >> I seem to have lost track of this... does this actually solve
> >> anything, or does it just mean we'll explode hard?
> > 
> > Not that hard - it'll warn once only.
> > 
> > AFAIR, the discussion stalled but we were going in the direction of not
> > calling into efi from pstore in irq context.
> 
> The kernel_fpu_begin thing has annoyed me in the past.  How bad would it
> be to allocate some percpu space and just do a full save/restore when
> kernel_fpu_begin happens in a context where it currently doesn't work?
> 
> I don't know how large the state is these days, but there must be some
> limit to how deeply interrupts and exceptions can nest.  For each IST
> entry, there is a hard limit to how deeply they can nest (once for all
> but debug and four times for debug IIRC), plus one NMI (nested ones
> don't touch FPU).  The most non-IST entries we can have must be bounded,
> too.
> 
> Let's say there are at most 16 levels of nesting.  16 * state size *
> cpus isn't that much.
> 
> Of course, code in interrupts that nests kernel_fpu_begin itself could
> have a problem.  But this can be solved with a little bit of trickery in
> the entry code or something.
> 
> If we did this, then I think the x86 crypto code could get rid of all of
> its ridiculous async code.

How are you going to detect when to save/restore state? Do it
unconditionally would probably be a no-no. Even with all that optimized
XSAVE* fun.

On demand would mean you allow FPU exceptions which probably gravitates
towards a no-no too.

-- 
Regards/Gruss,
    Boris.

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