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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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@citrix.com> wrote:
>>>>> On 29/07/2015 22:26, Andy Lutomirski wrote:
>>>>>> On Wed, Jul 29, 2015 at 2:23 PM, Boris Ostrovsky
>>>>>> <boris.ostrovsky@oracle.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.

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.

~Andrew