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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Apr 05, 2022 at 08:06:31AM -0400, Daniel P. Smith wrote:
> On 4/5/22 03:42, Roger Pau Monné wrote:
> > On Mon, Apr 04, 2022 at 12:08:25PM -0400, Daniel P. Smith wrote:
> >> On 4/4/22 11:12, Roger Pau Monné wrote:
> >>> On Mon, Apr 04, 2022 at 10:21:18AM -0400, Daniel P. Smith wrote:
> >>>> On 3/31/22 08:36, Roger Pau Monné wrote:
> >>>>> On Wed, Mar 30, 2022 at 07:05:48PM -0400, Daniel P. Smith wrote:
> >>>>>> diff --git a/xen/include/xsm/xsm.h b/xen/include/xsm/xsm.h
> >>>>>> index e22d6160b5..157e57151e 100644
> >>>>>> --- a/xen/include/xsm/xsm.h
> >>>>>> +++ b/xen/include/xsm/xsm.h
> >>>>>> @@ -189,6 +189,28 @@ struct xsm_operations {
> >>>>>>  #endif
> >>>>>>  };
> >>>>>>  
> >>>>>> +static always_inline int xsm_elevate_priv(struct domain *d)
> >>>>>
> >>>>> I don't think it needs to be always_inline, using just inline would be
> >>>>> fine IMO.
> >>>>>
> >>>>> Also this needs to be __init.
> >>>>
> >>>> AIUI always_inline is likely the best way to preserve the speculation
> >>>> safety brought in by the call to is_system_domain().
> >>>
> >>> There's nothing related to speculation safety in is_system_domain()
> >>> AFAICT. It's just a plain check against d->domain_id. It's my
> >>> understanding there's no need for any speculation barrier there
> >>> because d->domain_id is not an external input.
> >>
> >> Hmmm, this actually raises a good question. Why is is_control_domain(),
> >> is_hardware_domain, and others all have evaluate_nospec() wrapping the
> >> check of a struct domain element while is_system_domain() does not?
> > 
> > Jan replied to this regard, see:
> > 
> > https://lore.kernel.org/xen-devel/54272d08-7ce1-b162-c8e9-1955b780ca11@xxxxxxxx/
> 
> Jan can correct me if I misunderstood, but his point is with respect to
> where the inline function will be expanded into and I would think you
> would want to ensure that if anyone were to use is_system_domain(), then
> the inline expansion of this new location could create a potential
> speculation-able branch. Basically my concern is not putting the guards
> in place today just because there is not currently any location where
> is_system_domain() is expanded to create a speculation opportunity does
> not mean there is not an opening for the opportunity down the road for a
> future unprotected use.
> 
> >>> In any case this function should be __init only, at which point there
> >>> are no untrusted inputs to Xen.
> >>
> >> I thought it was agreed that __init on inline functions in headers had
> >> no meaning?
> > 
> > In a different reply I already noted my preference would be for the
> > function to not reside in a header and not be inline, simply because
> > it would be gone after initialization and we won't have to worry about
> > any stray calls when the system is active.
> 
> If an inline function is only used by __init code, how would be
> available for stray calls when the system is active? I would concede
> that it is possible for someone to explicitly use in not __init code but
> I would like to believe any usage in a submitted code change would be
> questioned by the reviewers.

Right, it's IMO easier when things just explode when not used
correctly, hence my suggestion to make it __init.

> With that said, if we consider Jason's suggestion would this remove your
> concern since that would only introduce a de-privilege function and
> there would be no piv escalation that could be erroneously called at
> anytime?

Indeed.  IMO everything that happens before the system switches to the
active state should be considered to be running in a privileged
context anyway.  Maybe others have different opinions.  Or maybe there
are use-cases I'm not aware of where this is not true.

Thanks, Roger.