Messages in this thread | | | From | Jann Horn <> | Date | Fri, 28 Sep 2018 17:12:00 +0200 | Subject | Re: [RFC 0/5] perf: Per PMU access controls (paranoid setting) |
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On Fri, Sep 28, 2018 at 3:22 PM Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> 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@lists.openwall.com on the next version.
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