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

[Tvrtko Ursulin]

On 28/09/2018 15:02, Thomas Gleixner wrote:
> Tvrtko,
>
> On Fri, 28 Sep 2018, Tvrtko Ursulin wrote:
> > On 28/09/2018 11:26, 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.
> >
> > I accept it was by bad to miss adding Cc's on the cover letter, but my own
> > email addresses hopefully should not bother you. It is simply a question of
> > what I have in .gitconfig vs what I forgot to do manually.
>
> The keyword in the above sentence is 'just'. You can add as many of yours
> as you want as long as everybody else is cc'ed.

Sure, but you also used the word "pile" and I would argue that made the 
rest of your sentence, after and including "instead", sound like it not 
only bothers you I forgot to Cc people on the cover letter, but it also 
bothers you I included a "pile" of my own addresses. If that wasn't your 
intention in the slightest then I apologise for misreading it.

> > > 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..
>
> Sure, and because you don't know you didn't bother to ask around and
> ignored the review request.

No, not because of that. You are assuming my actions and motivations and 
constructing a story.

"did not bother" = negative connotations
"ignored" = negative connotations

Note instead the time lapse between this and previous posting of the 
series, and if you want to assume something, assume things can get 
missed and forgotten without intent or malice.

> I already added Kees and Jann. Please look for the SECCOMP folks in
> MAINTAINERS.

Thanks!

> > > 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?
>
> I did not say at all that this might be diminishing security. And the
> argumentation with 'incompetent sysadmins' is just the wrong attitude.

Wrong attitude what? I was trying to guess your reasoning (cues in 
"presumably" and a lot of question marks) since it wasn't clear to me 
why is your position what it is.

> What I was asking for is proper documentation and this proper documentation
> is meant for _competent_ sysadmins.
>
> That documentation has to clearly describe what kind of information is
> accessible and what potential side effects security wise this might
> have. You cannot expect that even competent sysadmins know offhand what
> which PMU might expose. And telling them 'Use Google' is just not the right
> thing to do.

I did not mention Google.

> If you can't explain and document it, then providing the knob is just
> fulfilling somebodys 'I want a pony' request.

Well it's not a pony, it is mechanism to avoid having to turn off all 
security. We can hopefully discuss it without ponies.

> > 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.
>
> Sure, and this wants to be documented in the cover letter and the
> changelogs.
>
> But this does also require a proper analysis and documentation why it is
> not a security risk to expose the i915 PMU or what potential security
> issues this can create, so that the competent sysadmin can make a
> judgement.
>
> And the same is required for all other PMUs which can be enabled in the
> same way for unprivileged access. And we might as well come to the
> conclusion via analysis that for some PMUs unpriviledged access is just not
> a good idea and exclude them. I surely know a few which qualify for
> exclusion, so the right approach is to provide this knob only when the risk
> is analyzed and documented and the PMU has been flagged as candidate for
> unpriviledged exposure. I.e. opt in and not opt out.

I am happy to work on the mechanics of achieving this once the security 
guys and all PMU owners get involved. Even though I am not convinced the 
bar to allow fine-grained control should be evaluating all possible 
PMUs*, but if the security folks agree that is the case it is fine by me.

Regards,

Tvrtko

*) The part of my reply you did not quote explains how the fine-grained 
control improves security in existing deployments. The documentation I 
added refers to the existing perf_event_paranoid documentation for 
explanation of security concerns involved. Which is not much in itself. 
But essentially we both have a PMU and a knob already. I don't see why 
adding the same knob per-PMU needs much more stringent criteria to be 
accepted. But as said, that's for security people to decide.