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 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.

--Andy

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