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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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?