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

[David Vrabel <dvrabel@cantab.net>]

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.

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

David