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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

>
>>> 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() :)

Even better!

~Andrew

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