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

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Date:   Fri, 28 Sep 2018 18:40:17 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Tvrtko Ursulin <tursulin@...ulin.net>,
        LKML <linux-kernel@...r.kernel.org>,
        tvrtko.ursulin@...ux.intel.com,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com>
Subject: Re: [RFC 0/5] perf: Per PMU access controls (paranoid setting)

On Fri, Sep 28, 2018 at 10:23:40AM -0700, Andi Kleen wrote:
> > 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.
> 
> That sounds like an orthogonal feature. I don't think the original
> patchkit would need to be hold up for this. It would be something
> in addition.

I have to say that I disagree -- these controls will have to interact
somehow, and the fewer of them we have, the less complexity we'll have
to deal with longer-term.

> BTW can't you already do that with the syscall filter? I assume
> the Android sandboxes already use that. Just forbid perf_event_open
> for the apps.

Note that this was about providing access to *some* PMUs in some cases.

IIUC, if that can be done today via a syscall filter, the same is true
of per-pmu paranoid settings.

Thanks,
Mark.