[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

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.


Julien Grall

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.