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SubjectRe: [PATCH v6 04/12] Add memory slot versioning and use it to provide fast guest write interface
On Thu, Oct 07, 2010 at 12:42:48PM -0300, Marcelo Tosatti wrote:
> On Thu, Oct 07, 2010 at 12:00:13PM +0200, Avi Kivity wrote:
> > On 10/06/2010 10:08 PM, Gleb Natapov wrote:
> > >> Malicious userspace can cause entry to be cached, ioctl
> > >> SET_USER_MEMORY_REGION 2^32 times, generation number will match,
> > >> mark_page_dirty_in_slot will be called with pointer to freed memory.
> > >>
> > >Hmm. To zap all cached entires on overflow we need to track them. If we
> > >will track then we can zap them on each slot update and drop "generation"
> > >entirely.
> >
> > To track them you need locking.
> >
> > Isn't SET_USER_MEMORY_REGION so slow that calling it 2^32 times
> > isn't really feasible?
>
> Assuming it takes 1ms, it would take 49 days.
>
We may fail ioctl when max value is reached. The question is how much slot
changes can we expect from real guest during its lifetime.

> > In any case, can use u64 generation count.
>
> Agree.
Yes, 64 bit ought to be enough for anybody.

--
Gleb.


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