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

[Julien Grall <julien.grall@arm.com>]

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
> it.

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.

Regards,

-- 
Julien Grall

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