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

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Message-ID: <CALCETrXH5_PMqfH1en_5c+5gUpq8SjCnQ3Xaz-K6ej6FgBgLDQ@mail.gmail.com>
Date:	Tue, 28 Jul 2015 17:21:22 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:	Andrew Cooper <andrew.cooper3@...rix.com>,
	"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 Tue, Jul 28, 2015 at 10:10 AM, Boris Ostrovsky
<boris.ostrovsky@...cle.com> wrote:
> 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

Looking at map_ldt_shadow_page: what keeps shadow_ldt_mapcnt from
getting incremented once on each CPU at the same time if both CPUs
fault in the same shadow LDT page at the same time?  Similarly, what
keeps both CPUs from calling get_page_type at the same time and
therefore losing track of the page type reference count?

I don't see why vmalloc or vm_unmap_aliases would have anything to do
with this, though.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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