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

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Date:	Sun, 29 Dec 2013 20:44:28 +0000
From:	halfdog <me@...fdog.net>
To:	"H. Peter Anvin" <hpa@...or.com>,
	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

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H. Peter Anvin wrote:
> On 12/28/2013 02:02 PM, halfdog wrote:
>> It seems that missing CPU-state sanitation during task switching
>>  triggers kernel-panic. This might be related to unhandled 
>> FPU-errors. See [1] for POC and serial console log of OOPs. Due
>> to missing real 32-bit x86-hardware it is not clear, if this
>> issue might be related to subtle differences in virtual-8086
>> mode handling when inside a virtualbox guest.
>> 
> 
> This oops happens inside the guest?  Either way, I would be *very* 
> skeptical of Virtualbox in this case.
> 
> You can run a 32-bit kernel on 64-bit hardware, you know...

I know, but hardware was occupied with long-running simulation.

With the initial POC, there might be a timing issue involved, with
different process layout, exception does not occur in swith_to but
sometimes on other locations.

I created a new random-code testcase [1] , which works around that
problem. When booted a Debian initrd and tried id, OOPSes are fired
like wild but at least system does not lock up immediately.

hd

[1]
http://www.halfdog.net/Security/2013/Vm86SyscallTaskSwitchKernelPanic/Virtual86RandomCode.c

- -- 
http://www.halfdog.net/
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