Marcelo Tosatti wrote:
> On Mon, Dec 10, 2007 at 07:07:54PM +0200, Avi Kivity wrote:
>
>> Marcelo Tosatti wrote:
>>
>>> There is a race where VCPU0 is shadowing a pagetable entry while VCPU1
>>> is updating it, which results in a stale shadow copy.
>>>
>>> Fix that by comparing the contents of the cached guest pte with the
>>> current guest pte after write-protecting the guest pagetable.
>>>
>>> Attached program kvm_shadow_race.c demonstrates the problem.
>>>
>>>
>>>
>> Where is it?
>>
>
> Attached.
>
>
Can you explain what it does? I get the same results on both host and
guest (successful completion).
>>> diff --git a/drivers/kvm/paging_tmpl.h b/drivers/kvm/paging_tmpl.h
>>> index 72d4816..4fece01 100644
>>> --- a/drivers/kvm/paging_tmpl.h
>>> +++ b/drivers/kvm/paging_tmpl.h
>>> @@ -66,6 +66,7 @@ struct guest_walker {
>>> int level;
>>> gfn_t table_gfn[PT_MAX_FULL_LEVELS];
>>> pt_element_t pte;
>>> + gpa_t pte_gpa;
>>>
>>>
>> I think this needs to be an array like table_gfn[]. The guest may play
>> with the pde (and upper entries) as well as the pte.
>>
>
> I was working under the assumption that the only significant bits of
> upper entries (WRITEABLE and PRESENT) that can be changed by the guest
> must be reflected first in the lower level pte's.
>
> Isnt that a fair assumption to make?
>
>
The other bits (including the physical addresses) may change too. There
is no requirement that the changes to pde write/present bits be
reflected on pte write/present bits.
Consider a unix kernel implementing fork() by write-protecting the pud
tables. It can write protect the entire user address space by clearing
the write bit on the first 256 pgd entries.
(I don't think Linux does that; maybe that's a worthwhile optimization)
--
Any sufficiently difficult bug is indistinguishable from a feature.
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