Re: [Xen-devel] [PATCH] x86/altp2m: Add a subop for obtaining the mem access of a page

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 07/06/2018 05:52 PM, Tamas K Lengyel wrote:
> On Fri, Jul 6, 2018 at 2:56 AM Razvan Cojocaru
> <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>
>> On 07/05/2018 07:45 PM, Tamas K Lengyel wrote:
>>> On Thu, Jul 5, 2018 at 9:22 AM Razvan Cojocaru
>>> <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>>> However, our particular application is only interested in setting (and
>>>> querying) page restrictions from userspace (from the dom0 agent). It
>>>> will also need to be able to set the convertible bit of guest pages from
>>>> the dom0 agent as well (patches pending). So we're also fine with a
>>>> "DOMCTL if nobody wants it as a HVMOP" policy, if polluting the DOMCTLs
>>>> (possibly temporarily) is an option.
>>>>
>>>> We could also (at least between Tamas and us) come up with current /
>>>> likely use-cases and downgrade all altp2m HVMOPs that could be DOMCTLs
>>>> in all the scenarios to DOMCTLs.
>>>
>>> Aye. There is really just one HVMOP that the guest absolutely needs
>>> access to so that it can use #VE, and that's
>>> HVMOP_altp2m_vcpu_enable_notify. AFAIU everything else could be just a
>>> DOMCTL.
>>
>> We need even less than that - we want to modify
>> HVMOP_altp2m_vcpu_enable_notify to be able to call it from dom0 as well,
>> and we don't call it from the in-guest agent ever. Because we agree that
>> the smallest attack surface is a requirement, all we ever call that's
>> #VE / altp2m related is actually from the privileged domain doing
>> introspection. The in-guest driver only needs to do VMFUNC and be able
>> to communicate with the dom0 introspection agent.

For some reason my impression was that Intel was hoping to be able to
enable a guest-only usage as well -- that basically a guest which had
been booted (say) with measured boot, and then wrote its own enclave
using #VE and altp2ms, should be able to allow an in-guest agent to be
reasonably secure and also keep tabs on the operating system.  Was this
not your impression?

>> In that case, if everyone agrees, I propose that we make all the others
>> DOMCTLs. This would also have several maintenance benefits:
>>
>> 1. We can then get rid of the ugly compat code that was required for
>> upstreaming xc_altp2m_set_mem_access_multi() (and clean up the
>> hypervisor code corresponding to it).
>>
>> 2. We can probably remove Tamas' patch that controls if dom0, the guest,
>> or both can call altp2m operations (although maybe we should keep it for
>> the one remaining HVMOP? I'm not sure).
>>
>> So to my mind, it's less, cleaner, safer code. I don't see how the
>> original designers of the code would object, since their goal I would
>> assume was helping introspection, and Tamas and us are the ones trying
>> to use it - furthermore these changes address the security objections of
>> the Xen community.
>>
>> Does the plan sound reasonable?
> 
> +1 from me!

I don't have much skin in this particular game -- my main goal in these
discussions (other than keeping Xen well-designed and reasonably secure)
has been to keep contributors happy.  If you guys are happy, and Intel
doesn't express any opinions, then I have no objections.

 -George

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