Re: [PATCH] Revert "evtchn: refuse EVTCHNOP_status for Xen-bound event channels"

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.05.2024 22:28, Stefano Stabellini wrote:
> On Fri, 17 May 2024, Jan Beulich wrote:
>> On 17.05.2024 03:21, Stefano Stabellini wrote:
>>> On Thu, 16 May 2024, Jan Beulich wrote:
>>>> 1) In the discussion George claimed that exposing status information in
>>>> an uncontrolled manner is okay. I'm afraid I have to disagree, seeing
>>>> how a similar assumption by CPU designers has led to a flood of
>>>> vulnerabilities over the last 6+ years. Information exposure imo is never
>>>> okay, unless it can be _proven_ that absolutely nothing "useful" can be
>>>> inferred from it. (I'm having difficulty seeing how such a proof might
>>>> look like.)
>>>
>>> Many would agree that it is better not to expose status information in
>>> an uncontrolled manner. Anyway, let's focus on the actionable.
>>>
>>>
>>>> 2) Me pointing out that the XSM hook might similarly get in the way of
>>>> debugging, Andrew suggested that this is not an issue because any sensible
>>>> XSM policy used in such an environment would grant sufficient privilege to
>>>> Dom0. Yet that then still doesn't cover why DomU-s also can obtain status
>>>> for Xen-internal event channels. The debugging argument then becomes weak,
>>>> as in that case the XSM hook is possibly going to get in the way.
>>>>
>>>> 3) In the discussion Andrew further gave the impression that evtchn_send()
>>>> had no XSM check. Yet it has; the difference to evtchn_status() is that
>>>> the latter uses XSM_TARGET while the former uses XSM_HOOK. (Much like
>>>> evtchn_status() may indeed be useful for debugging, evtchn_send() may be
>>>> similarly useful to allow getting a stuck channel unstuck.)
>>>>
>>>> In summary I continue to think that an outright revert was inappropriate.
>>>> DomU-s should continue to be denied status information on Xen-internal
>>>> event channels, unconditionally and independent of whether dummy, silo, or
>>>> Flask is in use.
>>>
>>> I think DomU-s should continue to be denied status information on
>>> Xen-internal event channels *based on the default dummy, silo, or Flask
>>> policy*. It is not up to us to decide the security policy, only to
>>> enforce it and provide sensible defaults.
>>>
>>> In any case, the XSM_TARGET check in evtchn_status seems to do what we
>>> want?
>>
>> No. XSM_TARGET permits the "owning" (not really, but it's its table) domain
>> access. See xsm_default_action() in xsm/dummy.h.
> 
> Sorry I still don't understand. Why is that a problem? It looks like the
> wanted default behavior?

For ordinary event channels - yes. But not for Xen-internal ones, at least
from my pov.

Jan