Re: virtio-balloon: VIRTIO_BALLOON_F_PAGE_POISON discussion

[Wei Wang <wei.w.wang@intel.com>]

On 11/08/2018 10:50 AM, Michael S. Tsirkin wrote:
> On Thu, Nov 08, 2018 at 10:49:20AM +0800, Wei Wang wrote:
>> On 11/07/2018 11:27 PM, Michael S. Tsirkin wrote:
>>
>> + LKML
>>
>>> On Wed, Nov 07, 2018 at 02:29:02PM +0000, Wang, Wei W wrote:
>>>> Hi Michael,
>>>>
>>>>
>>>> Thanks again for reviewing so many versions of patches, and I learnt a lot from
>>>> your comments.
>>>>
>>>>
>>>> While I’m writing the virtio-balloon spec patches, I’m thinking probably we
>>>> don’t need VIRTIO_BALLOON_F_PAGE_POISON to limit
>>>> VIRTIO_BALLOON_F_FREE_PAGE_HINT, because now the guest frees the allocated
>>>> pages after the migration is done (that is, the skipped free pages will be
>>>> poisoned when the guest is already on the destination machine).
>>> The concern was this:
>>>
>>> guest poisons the page by writing a non-0 pattern there
>>> guest sends page to host
>>> VM is migrated, page is unmapped
>>> guest reads page, zero page is mapped
>> Not sure about this one: I think guest wouldn't read the page,
>> since they are held by balloon (balloon itself will also
>> not read it, the page just stays on a list waiting to be freed).
>> Please see the below example.
>>
>>> guest sees 0 in page and detects it as use after free
>>   - balloon collects (i.e. alloc) a free page X (now it
>>     has 0xaa poison value) and reports X to host to be skipped in
>>     migration;
>>   -  Now VM is migrated to the destination, and on the destination
>>      side, X is not mapped initially.
>>   -  Nobody will access X since it has been taken by balloon
>>      and stays on a list waiting to be freed. So the first chance
>>      that will get X mapped will be the moment that balloon
>>      returns X to mm via free(), as free() writes the
>>      poison value to X.
>>
>>
>> Best,
>> Wei
>
> Oh I see, that was with the previous design where we bypassed alloc.
> I think you are right, but better stress-test it.
>

Sure, will do.

Best,
Wei