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

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Date:	Wed, 29 Jul 2015 23:55:51 +0100
From:	David Vrabel <dvrabel@...tab.net>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Andy Lutomirski <luto@...capital.net>
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>,
	David Vrabel <david.vrabel@...rix.com>,
	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 29/07/2015 23:49, Boris Ostrovsky wrote:
> On 07/29/2015 06:46 PM, David Vrabel wrote:
>>
>> On 29/07/2015 23:11, Andrew Cooper wrote:
>>> On 29/07/2015 23:05, Andy Lutomirski wrote:
>>>> On Wed, Jul 29, 2015 at 2:37 PM, Andrew Cooper
>>>> <andrew.cooper3@...rix.com> wrote:
>>>>> On 29/07/2015 22:26, Andy Lutomirski wrote:
>>>>>> On Wed, Jul 29, 2015 at 2:23 PM, Boris Ostrovsky
>>>>>> <boris.ostrovsky@...cle.com> wrote:
>>>>>>> On 07/29/2015 03:03 PM, Andrew Cooper wrote:
>>>>>>>> On 29/07/15 15:43, Boris Ostrovsky wrote:
>>>>>>>>> FYI, I have got a repro now and am investigating.
>>>>>>>> Good and bad news.  This bug has nothing to do with LDTs
>>>>>>>> themselves.
>>>>>>>>
>>>>>>>> I have worked out what is going on, but this:
>>>>>>>>
>>>>>>>> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
>>>>>>>> index 5abeaac..7e1a82e 100644
>>>>>>>> --- a/arch/x86/xen/enlighten.c
>>>>>>>> +++ b/arch/x86/xen/enlighten.c
>>>>>>>> @@ -493,6 +493,7 @@ static void set_aliased_prot(void *v,
>>>>>>>> pgprot_t prot)
>>>>>>>>             pte = pfn_pte(pfn, prot);
>>>>>>>>    +       (void)*(volatile int*)v;
>>>>>>>>           if (HYPERVISOR_update_va_mapping((unsigned long)v,
>>>>>>>> pte, 0)) {
>>>>>>>>                   pr_err("set_aliased_prot va update failed w/
>>>>>>>> lazy mode
>>>>>>>> %u\n", paravirt_get_lazy_mode());
>>>>>>>>                   BUG();
>>>>>>>>
>>>>>>>> Is perhaps not the fix we are looking for, and every use of
>>>>>>>> HYPERVISOR_update_va_mapping() is susceptible to the same problem.
>>>>>>> I think in most cases we know that page is mapped so hopefully
>>>>>>> this is the
>>>>>>> only site that we need to be careful about.
>>>>>> Is there any chance we can get some kind of quick-and-dirty fix that
>>>>>> can go to x86/urgent in the next few days even if a clean fix isn't
>>>>>> available yet?
>>>>> Quick and dirty?
>>>>>
>>>>> Reading from v is the most obvious and quick way, for areas where
>>>>> we are
>>>>> certain v exists, is kernel memory and is expected to have a backing
>>>>> page.  I don't know offhand how many of current
>>>>> HYPERVISOR_update_va_mapping() callsites this applies to.
>>>> __get_user((char *)v, tmp), perhaps, unless there's something better
>>>> in the wings.  Keep in mind that we need this for -stable, and it's
>>>> likely to get backported quite quickly due to CVE-2015-5157.
>>> Hmm - something like that tucked inside HYPERVISOR_update_va_mapping()
>>> would probably work, and certainly be minimal hassle for -stable.
>>>
>>> Altering the hypercall used is certainly not something to backport, nor
>>> are we sure it is a viable fix at this time.
>> Changing this one use of update_va_mapping to use mmu_update_normal_pt
>> is the correct fix to unblock this LDT series.  I see no reason why this
>> cannot be backported.
> 
> To properly fix it should include batching and that is not something
> that I think we should target for stable.

The original call isn't batched, so it's replacement doesn't need to be,

David
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