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

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Message-ID: <55B95863.2000102@oracle.com>
Date:	Wed, 29 Jul 2015 18:49:07 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	David Vrabel <dvrabel@...tab.net>,
	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 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.

-boris


>
> We can address any other potential update_va_mapping calls at a later
> date (if they are shown to be problematic).
>
> David

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