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 23:05, 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.

~Andrew