Messages in this thread | | | Date | Fri, 28 Sep 2018 17:41:11 +0100 | From | Mark Rutland <> | Subject | Re: [RFC 0/5] perf: Per PMU access controls (paranoid setting) |
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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/
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