Re: [Xen-devel] [PATCH v2 00/25] arm/altp2m: Introducing altp2m to ARM.

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/08/16 17:51, Julien Grall wrote:
>
>
> On 03/08/16 17:42, Tamas K Lengyel wrote:
>> On Wed, Aug 3, 2016 at 10:24 AM, Julien Grall <julien.grall@xxxxxxx>
>> wrote:
>>> Hi Tamas,
>>>
>>>
>>> On 03/08/16 17:01, Tamas K Lengyel wrote:
>>>>
>>>> On Wed, Aug 3, 2016 at 8:08 AM, Julien Grall <julien.grall@xxxxxxx>
>>>> wrote:
>>>>>
>>>>> Hello Sergej,
>>>>>
>>>>> Please try to reply to all when answering on the ML. Otherwise the
>>>>> answer
>>>>> may be delayed/lost.
>>>>>
>>>>> On 03/08/16 13:45, Sergej Proskurin wrote:
>>>>>>
>>>>>>
>>>>>> The interesting part about #VE is that it allows to handle certain
>>>>>> violations (currently limited to EPT violations -- future
>>>>>> implementations might introduce also further violations) inside
>>>>>> of the
>>>>>> guest, without the need to explicitly trap into the VMM. Thus,
>>>>>> #VE allow
>>>>>> switching of different memory views in-guest. Because of this, I
>>>>>> also
>>>>>> agree that event channels would suffice in our case, since we do not
>>>>>> have sufficient hardware support on ARM and would need to trap
>>>>>> into the
>>>>>> VMM anyway.
>>>>>
>>>>>
>>>>>
>>>>> The cost of doing an hypercall on ARM is very small compare to x86
>>>>> (~1/3
>>>>> of
>>>>> the number of x86 cycles) because we don't have to save all the state
>>>>> every
>>>>> time. So I am not convinced by the argument of limiting the number of
>>>>> trap
>>>>> to the hypervisor and allow a guest to play with altp2m on ARM.
>>>>>
>>>>> I will have to see a concrete example before going forward with
>>>>> the event
>>>>> channel.
>>>>
>>>>
>>>> It is out-of-scope for what we are trying to achieve with this series
>>>> at this point. The question at hand is really whether the atp2m switch
>>>> and gfn remapping ops should be exposed to the guest. Without #VE -
>>>> which we are not implementing - setting the mem_access settings from
>>>> within the guest doesn't make sense so restricting access there is
>>>> reasonable.
>>>>
>>>> As I outlined, the switch and gfn remapping can have legitimate
>>>> use-cases by themselves without any mem_access bits involved. However,
>>>> it is not our use-case so we have no problem restricting access there
>>>> either. So the question is whether that's the right path to take here.
>>>> At this point I'm not sure there is agreement about it or not.
>>>
>>>
>>> Could you give a legitimate use case of gfn remapping from the
>>> guest? And
>>> explain how it would work with only this patch series.
>>>
>>> From my perspective, and after the numerous exchange in this thread,
>>> I do
>>> not think it is wise to expose this interface to the guest on ARM.
>>> The usage
>>> is very limited but increase the surface attack. So I will not ack a
>>> such
>>> choice, however I will not nack it.
>>>
>>
>> Since the interface would be available only for domains where they
>> were explicitly created with altp2m=1 flag set I think the exposure is
>> minimal.
>>
>> As for a use-case, I don't have a real world example as it's not how
>> we use the system. But as I pointed out eairlier I could imagine the
>> gfn remapping be used to protect kernel memory areas against
>> information disclosure by only switching to the accessible altp2m view
>> when certain conditions are met. What I mean is that a certain gfn
>> could be remapped to a dummy mfn by default and only switched to the
>> accessible view when necessary. How much extra protection that would
>> add and under what condition is up for debate but IMHO it is a
>> legitimate experimental use - and altp2m is an experimental system.
>
> A such solution may give you a lots of headache with the cache.
>
>>
>> Whether it's worth to have such an interface or not I'm not sure, I'm
>> OK with going either way on this, but since it's available on x86 I
>> think it would make sense to have feature parity - even if only
>> partially for now.
>
> As I mentioned a couple of times, we do not introduce features on ARM
> just because they exists on x86. We introduce them after careful think
> about how they could benefits ARM and the usage.
>
> Nothing prevents a follow-up series to allow the guest accessing
> altp2m operation by default because the interface is already there.
>
> Stefano, do you have any opinions on this?

From my point of view, feature parity with x86 is only important if the
feature is equally capable, and this thread has shown that this is not
the case.

IMO, the choice is between:

1) Don't expose altp2m to guests, or
2) Make a para-virtual version of #VE for ARM guests, and expose the
full guest interface.

I am not fussed either way, but with my Security Team hat on, exposing
half an interface which can't usefully be used had has no current
usecase is a recipe for bugs with an XSA/CVE attached to them, and
therefore extra paperwork for me or someone else to do.

~Andrew

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