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

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Mon, Aug 1, 2016 at 12:15 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
> On 01/08/16 18:10, Sergej Proskurin wrote:
>>
>>
>> Hello all,
>
>
> Hello Sergej,
>
>> The following patch series can be found on Github[0] and is part of my
>> contribution to this year's Google Summer of Code (GSoC)[1]. My project is
>> managed by the organization The Honeynet Project. As part of GSoC, I am
>> being
>> supervised by the Xen developer Tamas K. Lengyel <tamas@xxxxxxxxxxxxx>,
>> George
>> D. Webster, and Steven Maresca.
>>
>> In this patch series, we provide an implementation of the altp2m subsystem
>> for
>> ARM. Our implementation is based on the altp2m subsystem for x86,
>> providing
>> additional --alternate-- views on the guest's physical memory by means of
>> the
>> ARM 2nd stage translation mechanism. The patches introduce new HVMOPs and
>> extend the p2m subsystem. Also, we extend libxl to support altp2m on ARM
>> and
>> modify xen-access to test the suggested functionality.
>>
>> To be more precise, altp2m allows to create and switch to additional p2m
>> views
>> (i.e. gfn to mfn mappings). These views can be manipulated and activated
>> as
>> will through the provided HVMOPs. In this way, the active guest instance
>> in
>> question can seamlessly proceed execution without noticing that anything
>> has
>> changed. The prime scope of application of altp2m is Virtual Machine
>> Introspection, where guest systems are analyzed from the outside of the
>> VM.
>>
>> Altp2m can be activated by means of the guest control parameter "altp2m"
>> on x86
>> and ARM architectures.  The altp2m functionality by default can also be
>> used
>> from within the guest by design. For use-cases requiring purely external
>> access
>> to altp2m, a custom XSM policy is necessary on both x86 and ARM.
>
>
> As said on the previous version, altp2m operation *should not* be exposed to
> ARM guest. Any design written for x86 may not fit exactly for ARM (and vice
> versa), you will need to explain why you think we should follow the same
> pattern.
>
> Speaking about security, I skimmed through the series and noticed that a lot
> of my previous comments have not been addressed. For instance there are
> still no locking on the altp2m operations and a guest could disable altp2m.
>
> I will give a look to the rest of the series once this is fixed.
>

Julien,
we did discuss whether altp2m on ARM should be exposed to guests or
not but we did not agree whether restricting it on ARM is absolutely
necessary. Altp2m was designed even on the x86 to be accessible from
within the guest on all systems irrespective of actual hardware
support for it. Thus, this design fits ARM as well where there is no
dedicated hardware support, from the altp2m perspective there is no
difference. The fact that a guest can disable altp2m by default on
itself is *not* a security issue - it is by design. There are usecases
where the guest is allowed to use altp2m on itself - like having a
security kernel module managing VMM memory permissions from within the
guest. For use-cases where such operations are undesirable - like in a
purely external usecase - the in-guest operations can be readily
restricted by XSM. Thus, it is unreasonable to design a completely
separate altp2m mode for ARM when the current design from x86 works
just as well.

If you consider this design unacceptable for ARM we will need the
other Maintainers' input on the topic as well before we proceed. It
would be possible to introduce another set of domctl's for altp2m and
only implement that for ARM, but unless there is strong consensus
about that being absolutely necessary, changing the design of altp2m
should be avoided. I certainly see no reason to do that right now.

As for locking on altp2m paths, we haven't encountered any issues with
the current version during our tests but it is possible we missed
something. If you have spotted missed locking, please point it out.

Thanks,
Tamas

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