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SubjectRe: virtio-balloon: VIRTIO_BALLOON_F_PAGE_POISON discussion
On 11/07/2018 11:27 PM, Michael S. Tsirkin wrote:


> 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
- 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.


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