Re: [PATCH 1/2] xsm: add ability to elevate a domain to privileged

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.04.2022 11:09, Roger Pau Monné wrote:
> On Wed, Apr 06, 2022 at 10:48:23AM +0200, Jan Beulich wrote:
>> On 06.04.2022 10:46, Roger Pau Monné wrote:
>>> On Wed, Apr 06, 2022 at 09:06:59AM +0200, Jan Beulich wrote:
>>>> On 05.04.2022 19:17, Jason Andryuk wrote:
>>>>> On Mon, Apr 4, 2022 at 11:34 AM Daniel P. Smith 
>>>>> <dpsmith@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>>>> On 3/31/22 09:16, Jason Andryuk wrote:
>>>>>>> For the default policy, you could start by creating the system domains
>>>>>>> as privileged and just have a single hook to drop privs.  Then you
>>>>>>> don't have to worry about the "elevate" hook existing.  The patch 2
>>>>>>> asserts could instead become the location of xsm_drop_privs calls to
>>>>>>> have a clear demarcation point.  That expands the window with
>>>>>>> privileges though.  It's a little simpler, but maybe you don't want
>>>>>>> that.  However, it seems like you can only depriv once for the Flask
>>>>>>> case since you want it to be one-way.
>>>>>>
>>>>>> This does simplify the solution and since today we cannot differentiate
>>>>>> between hypervisor setup and hypervisor initiated domain construction
>>>>>> contexts, it does not run counter to what I have proposed. As for flask,
>>>>>> again I do not believe codifying a domain transition bound to a new XSM
>>>>>> op is the appropriate approach.
>>>>>
>>>>> This hard coded domain transition does feel a little weird.  But it
>>>>> seems like a natural consequence of trying to use Flask to
>>>>> deprivilege.  I guess the transition could be behind a
>>>>> dom0less/hyperlaunch Kconfig option.  I just don't see a way around it
>>>>> in some fashion with Flask enforcing.
>>>>>
>>>>> Another idea: Flask could start in permissive and only transition to
>>>>> enforcing at the deprivilege point.  Kinda gross, but it works without
>>>>> needing a transition.
>>>>
>>>> I don't think that would be right. Logically such behavior ought to be
>>>> mirrored to SILO, and I'll take that for the example for being the
>>>> simpler model: Suppose an admin wants to disallow communication
>>>> between DomU-s created by Xen. Such would want enforcing when creating
>>>> those DomU-s, despite the creator (Xen) being all powerful. If the
>>>> device tree information said something different (e.g. directing for
>>>> an event channel to be established between two such DomU-s), this
>>>> should be flagged as an error, not be silently permitted.
>>>
>>> I could also see this argument the other way around: what if an admin
>>> wants to disallow domUs freely communicating between them, but would
>>> still like some controlled domU communication to be possible by
>>> setting up those channels at domain creation?
>>
>> Well, imo that would require a proper (Flask) policy then, not SILO mode.
> 
> But when creating such domains in SILO mode from dom0, dom0 would be
> allowed to create and bind event channels to the created domains, even
> if the domain themselves are not allowed the operation.
> 
> That's because the check in evtchn_bind_interdomain() is done against
> 'current' and not the domain where the event channel will be bound.

Yes and no - the check is against current, but that's because
communication is established between current ( == ld) and rd. The
function in its present shape simply can't establish a channel
between two arbitrary domains.

Jan

> Maybe such check should instead take 3 parameters: current context
> domain, domain to bind the event channel to and remote domain on the
> other end of the event channel.
> 
> Thanks, Roger.
>