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

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Date:	Wed, 04 Jun 2014 17:19:01 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...e.de>, "H. Peter Anvin" <hpa@...or.com>
CC:	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 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.

--Andy
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