linux-kernel - Re: Sanitize CPU-state when switching from virtual-8086 mode to other task

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Message-ID: <20131231192106.GA22535@phenom.dumpdata.com>
Date:	Tue, 31 Dec 2013 14:21:06 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	halfdog <me@...fdog.net>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Sanitize CPU-state when switching from virtual-8086 mode to
 other task

On Tue, Dec 31, 2013 at 10:42:47AM -0800, H. Peter Anvin wrote:
> On 12/30/2013 07:52 AM, halfdog wrote:
> >>
> >> Still in VirtualBox?
> > 
> > Yes, again: after comparing the results from initrd on real hardware
> > with Vbox, I'm getting to understand the timing problem involved and why
> > timing in VBox is different: The test program usually OOPSes when
> > touching FPU multiple times, otherwise, when terminated before second
> > FPU-interacation, it OOPSes on next invocation, stumbling over invalid
> > CPU state from prior invocation. With improved code, I can rather
> > reliably bring CPU into that state, so that next process invoked and
> > touching FPU/MMX-state is OOPSed. Currently searching SUID-binaries and
> > running UID=0 daemons, that might show interesting reaction on that
> > event, but only on DOS level yet, e.g. after running V2 test program
> > once and then connecting via SSH, this currently kills the ssh daemon
> > nicely.
> > 
> > It seems that machine lockup occurs when e.g. switch to idle task
> > happens at exactly the right moment, which I currently cannot trigger on
> > real hardware, but still working on that.
> > 
> 
> I'm still wondering if this is a VirtualBox-specific problem or if it is
> something that *could* occur on hardware, or in other virtualization
> environments (KVM, Xen HVM, Hy-perV, VMware etc.)

So, I am wondering if this is related to " x86/fpu: CR0.TS should be set before trap
into PV guest's #NM exception handle" which does have a similar pattern - you
do enough of the task switches and the FPU is screwed.

See http://mid.gmane.org/1383720072-6242-1-git-send-email-gaoyang.zyh@taobao.com

(I thought there was a thread about this on LKML too but I can't
find it).
> 
> 	-hpa
> 
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