Re: [Xen-devel] [for-4.7 2/2] xen/arm: p2m: Release the p2m lock before undoing the mappings

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano and Wei,

On 17/05/16 12:24, Stefano Stabellini wrote:
> I think you are right. Especially with backports in mind, it would be
> better to introduce an __apply_p2m_changes function which assumes that
> the p2m lock has already been taken by the caller. Then you can base the
> implementation of apply_p2m_changes on it.
> On Tue, 17 May 2016, Wei Chen wrote:
> > Hi Julien,
> >
> > I have some concern about this patch. Because we released the spinlock
> > before remove the mapped memory. If somebody acquires the spinlock
> > before we remove the mapped memory, this mapped memory region can be
> > accessed by guest.
> >
> > The apply_p2m_changes is no longer atomic. Is it a security risk?
Accesses to the page table have never been atomic, as soon as an entry 
is written in the page tables, the guest vCPUs or a prefetcher could 
read it.
The spinlock is only here to protect the page tables against concurrent 
modifications. Releasing the lock is not an issue as Xen does not 
promise any ordering for the p2m changes.
Regards,

--
Julien Grall

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