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

[Mark Rutland]

On Fri, Sep 28, 2018 at 12:26:52PM +0200, Thomas Gleixner wrote:
> On Wed, 19 Sep 2018, Tvrtko Ursulin wrote:
> 
> It would be very helpful if you cc all involved people on the cover letter
> instead of just cc'ing your own pile of email addresses. CC'ed now.
> 
> > 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.
> 
> 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.
> 
> 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.

There's also been prior discussion on these feature in other contexts
(e.g. android expoits resulting from out-of-tree drivers). It would be
nice to see those considered.

IIRC The conclusion from prior discussions (e.g. [1]) was that we wanted
finer granularity of control such that we could limit PMU access to
specific users -- e.g. disallow arbitrary android apps from poking *any*
PMU, while allowing some more trusted apps/users to uses *some* specific
PMUs.

e.g. we could add /sys/bus/event_source/devices/${PMU}/device, protect
this via the usual fs ACLs, and pass the fd to perf_event_open()
somehow. A valid fd would act as a capability, taking precedence over
perf_event_paranoid.

Thanks,
Mark.

[1] https://patchwork.kernel.org/patch/9249919/