Re: [RFC 0/5] perf: Per PMU access controls (paranoid setting)

[Jann Horn]

On Fri, Sep 28, 2018 at 3:22 PM Tvrtko Ursulin
<tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
> On 28/09/2018 11:26, Thomas Gleixner wrote:
> > On Wed, 19 Sep 2018, Tvrtko Ursulin wrote:
> >> For situations where sysadmins might want to allow different level of
> >> access control for different PMUs, we start creating per-PMU
> >> perf_event_paranoid controls in sysfs.
> >>
> >> These work in equivalent fashion as the existing perf_event_paranoid
> >> sysctl, which now becomes the parent control for each PMU.
> >>
> >> On PMU registration the global/parent value will be inherited by each PMU,
> >> as it will be propagated to all registered PMUs when the sysctl is
> >> updated.
> >>
> >> At any later point individual PMU access controls, located in
> >> <sysfs>/device/<pmu-name>/perf_event_paranoid, can be adjusted to achieve
> >> fine grained access control.
> >>
> >> Discussion from previous posting:
> >> https://lkml.org/lkml/2018/5/21/156
> >
> > This is really not helpful. The cover letter and the change logs should
> > contain a summary of that discussion and a proper justification of the
> > proposed change. Just saying 'sysadmins might want to allow' is not useful
> > at all, it's yet another 'I want a pony' thing.
>
> Okay, for the next round I will expand the cover letter with at least
> one concrete example on how it is usable and summarize the discussion a bit.
>
> > I read through the previous thread and there was a clear request to involve
> > security people into this. Especially those who are deeply involved with
> > hardware side channels. I don't see anyone Cc'ed on the whole series.
>
> Who would you recommend I add? Because I really don't know..
>
> > For the record, I'm not buying the handwavy 'more noise' argument at
> > all. It wants a proper analysis and we need to come up with criteria which
> > PMUs can be exposed at all.
> >
> > All of this want's a proper documentation clearly explaining the risks and
> > scope of these knobs per PMU. Just throwing magic knobs at sysadmins and
> > then saying 'its their problem to figure it out' is not acceptable.
>
> Presumably you see adding fine grained control as diminishing the
> overall security rather than raising it? Could you explain why? Because
> incompetent sysadmin will turn it off for some PMU, while without having
> the fine-grained control they wouldn't turn it off globally?
>
> This feature was requested by the exact opposite concern, that in order
> to access the i915 PMU, one has to compromise the security of the entire
> system by allowing access to *all* PMU's.
>
> Making this ability fine-grained sounds like a logical solution for
> solving this weakening of security controls.
>
> Concrete example was that on video transcoding farms users want to
> monitor the utilization of GPU engines (like CPU cores) and they can do
> that via the i915 PMU. But for that to work today they have to dial down
> the global perf_event_paranoid setting. Obvious improvement was to allow
> them to only dial down the i915.perf_event_paranoid setting. As such,
> for this specific use case at least, the security is increased.

Which paranoia level would be used for the i915.perf_event_paranoid
setting in such a case?

Perhaps also CC kernel-hardening@xxxxxxxxxxxxxxxxxx on the next version.