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 2:23 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?

>
>>
>> The update_va_mapping hypercall is designed to emulate writing the pte
>> for v, with auditing applied.  As part of this, it does a pagewalk on v
>> to locate and map the l1.  During this walk, Xen it finds the l2 not
>> present, and fails the hypercall.  i.e. v is not reachable from the
>> current cr3.
>>
>> Reading the virtual address immediately before issuing the hypercall
>> causes Linux's memory faulting logic to fault in the l2.  This also
>> explains why vm_unmap_aliases() appears to fix the issue; it is likely
>> to fault in enough of the paging structure for v to be reachable.
>
>
> We've just touched this page (in write_ldt()) in this test so why would it
> not be mapped?

With my patches applied, the LDT is never written via any paravirt
hook -- I write it once (possibly implicitly using kzalloc/vzalloc)
before paravirt_alloc_ldt(), and write_ldt() is never called.  We
could even remove it write_ldt() :)

--Andy

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