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

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Date:	Tue, 31 Dec 2013 10:42:47 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	halfdog <me@...fdog.net>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
CC:	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: Sanitize CPU-state when switching from virtual-8086 mode to other
 task

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.)

	-hpa

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