Re: [PATCH RFC] kvm: write protect memory after slot swap

Avi Kivity
2010-10-25T09:32:40Z

On 10/25/2010 03:21 AM, Michael S. Tsirkin wrote:
> I have observed the following bug trigger:
>
> 1. userspace calls GET_DIRTY_LOG
> 2. kvm_mmu_slot_remove_write_access is called and makes a page ro
> 3. page fault happens and makes the page writeable
> fault is logged in the bitmap appropriately
> 4. kvm_vm_ioctl_get_dirty_log swaps slot pointers
>
> a lot of time passes
>
> 5. guest writes into the page
> 6. userspace calls GET_DIRTY_LOG
>
> At point (5), bitmap is clean and page is writeable,
> thus, guest modification of memory is not logged
> and GET_DIRTY_LOG returns an empty bitmap.
>
> The rule is that all pages are either dirty in the current bitmap,
> or write-protected, which is violated here.
>
> It seems that just moving kvm_mmu_slot_remove_write_access down
> to after the slot pointer swap should fix this bug.
>
> Warning: completely untested.
> Please comment.
> Note: fix will be needed for -stable etc.

Excellent catch, I stared at this code for a while and didn't see the
bug. Patch applied.

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