linux-kernel - Re: [patch] x64, fpu: fix possible FPU leakage in error conditions

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Date:	Thu, 24 Jul 2008 16:02:43 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"stable@...nel.org" <stable@...nel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch] x64, fpu: fix possible FPU leakage in error conditions

On Thu, Jul 24, 2008 at 03:43:44PM -0700, Linus Torvalds wrote:
> 
> 
> On Thu, 24 Jul 2008, Suresh Siddha wrote:
> > >
> > > but the thing is, the only really valid reason for "restore_i387()" to
> > > fail is because the read failed.
> >
> > Not really. It can cause #GP, if someone sets reserved bits of mxcsr
> > in the memory image.
> 
> Ahh, ok, I can imagine that. And I guess we might copy the data from user
> space into the memory image without validating it at points (signal
> handler restore and/or ptrace). Do we?

Today in ptrace (and in 32bit signal handling), we copy the user data and
then clear the reserved bits blindly again ;)

In 64bit signal handling, we do a fxrstor from the live user buffer and
scream then itself if we find an issue.

Andi being paranoid, added more checks for 64bit math_state_restore().

> > But restore_i387() may be in an insane state (we did clts() but doesn't
> > have the proper state in its live registers etc) when it failed to restore fpu.
> > Ideally we should fix this inside restore_i387(). But restore_i387()
> > is in header file and I have to re-arrange some of the code
> > in the header file, to call clear_fpu() from restore_i387().
> 
> Ok, how about we just move restore_i387() out of the header file? I
> realize that the x86 code plays some games with this whole thing (that
> whole '#define restore_i387_ia32 restore_i387'), but that is 32-bit
> specific, and the restore_i387() in the header file is 64-bit specific.
> 
> Hmm. In fact, I think that x86-64 version is actually only used in a
> single place - arch/x86/kernel/signal_64.c. So it's actively *wrong* to
> have that thing in a header file to begin with!
> 
> So how about this patch as a starting point? This is the RightThing(tm) to
> do regardless, and if it then makes it easier to do some other cleanups,
> we should do it first. What do you think?

Sure. You have my Ack. I will request -stable folks to pickup multiple patches
(second patch, I will post shortly on top of yours).

thanks,
suresh
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