[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,

On 17/05/16 13:27, Stefano Stabellini wrote:
On Tue, 17 May 2016, Julien Grall wrote:
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.

I understand that. However I am wondering whether it might be possible
for the guest to run commands which cause concurrent p2m change requests
on purpose, inserting something else between the first phase and the
second phase of apply_p2m_changes, causing problems to the hypervisor.

Removing and inserting entries are 2 distinct steps.

Or maybe not even on purpose, but causing problem to itself nonetheless.

Each vCPU can only trigger one command at the time. So concurrent p2m changes would involve 2 vCPUs.

Even if vCPU A send the command before vCPU B, nothing prevents Xen to serve B before A.

The only way a guest could harm itself would be to have the 2 requests modifying the same regions in the page tables. However, per-above this behavior is undefined no matter the implementation of apply_p2m_changes.

Honestly it is true that it doesn't look like Xen could run into
troubles. But still this is a change in behaviour compared to the
current code (which I know doesn't actually work) and I wanted to flag

This code has always been buggy, and I suspect the goal was to call back without the lock.

There is no reason to keep the lock more than necessary. Releasing the lock allow other p2m changes to be executed rather than spinning while the long execution (INSERTION + REMOVAL) is done.


Julien Grall

Xen-devel mailing list



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