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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Wed, Jul 29, 2015 at 4:02 PM, Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
> 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@xxxxxxxxxx> wrote:
>>>>>> On 29/07/2015 22:26, Andy Lutomirski wrote:
>>>>>>> On Wed, Jul 29, 2015 at 2:23 PM, Boris Ostrovsky
>>>>>>> <boris.ostrovsky@xxxxxxxxxx> 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.
>
> Batching is absolutely not necessary to alter update_va_mapping to
> mmu_update_normal_pt.  After all, update_va_mapping isn't batched.
>
> However this isn't the first issue issue we have had lazy mmu faulting,
> and I doubt it is the last.  There are not many callsites of
> update_va_mapping - I will audit them tomorrow and see if any similar
> issues are lurking elsewhere.

One thing I should add: nothing flushes old aliases in xen_alloc_ldt,
yet I haven't been able to get xen_alloc_ldt to fail or subsequent LDT
access to fault.  Is this something we should be worried about?

--Andy

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