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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan