linux-kernel - Re: [Xen-devel] [PATCH v5 0/4] x86: modify_ldt improvement, test, and config option

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Date:	Thu, 30 Jul 2015 13:18:20 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Andrew Cooper <andrew.cooper3@...rix.com>,
	Borislav Petkov <bp@...en8.de>
CC:	"security@...nel.org" <security@...nel.org>,
	Jan Beulich <jbeulich@...e.com>,
	Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	xen-devel <xen-devel@...ts.xen.org>,
	Andy Lutomirski <luto@...nel.org>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [Xen-devel] [PATCH v5 0/4] x86: modify_ldt improvement, test,
 and config option

On 07/30/2015 01:06 PM, Andrew Cooper wrote:
> On 30/07/15 17:31, Boris Ostrovsky wrote:
>> On 07/30/2015 12:12 PM, Andrew Cooper wrote:
>>> On 30/07/15 17:05, Borislav Petkov wrote:
>>>> On Thu, Jul 30, 2015 at 11:53:34AM -0400, Boris Ostrovsky wrote:
>>>>> As far as Xen guests are concerned,
>>>>>
>>>>> Tested-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>>> Does that mean, this patch 1/4 fixes the 32bit issue you guys are still
>>>> debugging on the v4 thread? Or does that need more fixing?
>>>>
>>> I was going to say... This v5 pre-dates figuring out what was wrong with
>>> 32bit Xen.  v5 1/4 is still susceptible.
>>>
>>> Boris: does your Tested-by cover v5 + proposed fix?
>>>
>> Only V5, no extra changes.
> Including running the ldt_gdt test?

Yes, except that 32-on-64 doesn't work, but that's not Xen-specific.

Still, user-visible behavior changes.

>
>> And perhaps dropping aliases in xen_alloc_ldt() may be sufficient
>> since with that done we will only have one mapping so a subsequent
>> fault will have "correct" cr2 provided by the hypervisor (from your
>> earlier email it sounded that hypervisor may have been providing
>> incorrect cr2 if alias exists)
> They are sufficient to fix the first of the two bugs, but the free side
> still has no protection against a missing l2, unless I am missing
> something in the rest of the series?

Without aliases a subsequent fault *will* fill correct l2, won't it?

-boris
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