linux-kernel - Re: [Xen-devel] [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option

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Date:	Tue, 28 Jul 2015 13:10:41 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Andrew Cooper <andrew.cooper3@...rix.com>
CC:	"security@...nel.org" <security@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	xen-devel <xen-devel@...ts.xen.org>,
	Borislav Petkov <bp@...en8.de>,
	Jan Beulich <jbeulich@...e.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [Xen-devel] [PATCH v4 0/3] x86: modify_ldt improvement, test,
 and config option

On 07/28/2015 01:07 PM, Andy Lutomirski wrote:
> On Tue, Jul 28, 2015 at 9:30 AM, Andrew Cooper
> <andrew.cooper3@...rix.com> wrote:
>> I suspect that the set_ldt(NULL, 0) call hasn't reached Xen before
>> xen_free_ldt() is attempting to nab back the pages which Xen still has
>> mapped as an LDT.
>>
> I just instrumented it with yet more LSL instructions.  I'm pretty
> sure that set_ldt really is clearing at least LDT entry zero.
> Nonetheless the free_ldt call still oopses.
>

Yes, I added some instrumentation to the hypervisor and we definitely 
set LDT to NULL before failing.

-boris
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