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

From: Avi Kivity
Date: Tue Jun 29 2010 - 03:39:05 EST

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.


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))

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).


- 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.

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