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

Avi Kivity
2010-06-29T07:38:50Z

On 06/29/2010 10:06 AM, Avi Kivity wrote:
> 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).

Note:

- modifying walk_addr() to call kvm_mmu_pte_write() is probably not so
bad. It's rare that a large pte walk sets the dirty bit, and it's
probably rare to share those large ptes. Still, I think the fetch()
change is better since it's more local.

- there was once talk that instead of folding pt_access and pte_access
together into the leaf sp->role.access, each sp level would have its own
access permissions. In this case we don't even have to get a new direct
sp, only change the PT_DIRECTORY_LEVEL spte to add write permissions
(all direct sp's would be writeable and permissions would be controlled
at their parent_pte level). Of course that's a much bigger change than
this bug fix.

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