Re: [PATCH v2 3/10] KVM: MMU: fix direct sp's access corruptted

Avi Kivity
2010-06-29T07:06:38Z

On 06/29/2010 04:17 AM, Xiao Guangrong wrote:
>
>> If B is writeable-and-dirty, then it's D bit is already set, and we
>> don't need to do anything.
>>
>> If B is writeable-and-clean, then we'll have an spte pointing to a
>> read-only sp, so we'll get a write fault on access and an opportunity to
>> set the D bit.
>>
>>
> Sorry, a typo in my reply, i mean mapping A and B both are writable-and-clean,
> while A occurs write-#PF, we should change A's spte map to writable sp, if we
> only update the spte in writable-and-clean sp(form readonly to writable), the B's
> D bit will miss set.
>

Right.

We need to update something to notice this:

- FNAME(fetch)() to replace the spte
- FNAME(walk_addr)() to invalidate the spte

I think FNAME(walk_addr) is the right place, we're updating the gpte, so
we should update the spte at the same time, just like a guest write.
But that will be expensive (there could be many sptes, so we have to
call kvm_mmu_pte_write()), so perhaps FNAME(fetch) is easier.

We have now

if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep))
continue;

So we need to add a check, if sp->role.access doesn't match pt_access &
pte_access, we need to get a new sp with the correct access (can only
change read->write).

>>> Anyway, i think we should re-intall the mapping when the state is
>>> changed. :-(
>>>
>>>
>> When the gpte is changed from read-only to writeable or from clean to
>> dirty, we need to update the spte, yes. But that's true for other sptes
>> as well, not just large gptes.
>>
>>
> I think the indirect sp is not hurt by this bug since for the indirect sp, the access
> just form its upper-level, and the D bit is only in the last level, when we change the
> pte's access, is not affect its sp's access.
>
> But for direct sp, the sp's access is form all level. and different mapping that not share
> the last mapping page will have the same last sp.
>

Right.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.