Re: [PATCH v6 02/10] arm64: perf: Enable PMU counter direct access for perf event

[Rob Herring]

On Thu, Apr 8, 2021 at 6:08 AM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Wed, Apr 07, 2021 at 01:44:37PM +0100, Will Deacon wrote:
> > [Moving Mark to To: since I'd like his view on this]
> >
> > On Thu, Apr 01, 2021 at 02:45:21PM -0500, Rob Herring wrote:
> > > On Wed, Mar 31, 2021 at 11:01 AM Will Deacon <will@xxxxxxxxxx> wrote:
> > > >
> > > > On Tue, Mar 30, 2021 at 12:09:38PM -0500, Rob Herring wrote:
> > > > > On Tue, Mar 30, 2021 at 10:31 AM Will Deacon <will@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > On Wed, Mar 10, 2021 at 05:08:29PM -0700, Rob Herring wrote:
> > > > > > > From: Raphael Gault <raphael.gault@xxxxxxx>
>
> > > > > > > +static void armv8pmu_event_unmapped(struct perf_event *event, struct mm_struct *mm)
> > > > > > > +{
> > > > > > > +     struct arm_pmu *armpmu = to_arm_pmu(event->pmu);
> > > > > > > +
> > > > > > > +     if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
> > > > > > > +             return;
> > > > > > > +
> > > > > > > +     if (atomic_dec_and_test(&mm->context.pmu_direct_access))
> > > > > > > +             on_each_cpu_mask(&armpmu->supported_cpus, refresh_pmuserenr, mm, 1);
> > > > > >
> > > > > > Given that the pmu_direct_access field is global per-mm, won't this go
> > > > > > wrong if multiple PMUs are opened by the same process but only a subset
> > > > > > are exposed to EL0? Perhaps pmu_direct_access should be treated as a mask
> > > > > > rather than a counter, so that we can 'and' it with the supported_cpus for
> > > > > > the PMU we're dealing with.
> > > > >
> > > > > It needs to be a count to support multiple events on the same PMU. If
> > > > > the event is not enabled for EL0, then we'd exit out on the
> > > > > ARMPMU_EL0_RD_CNTR check. So I think we're fine.
> > > >
> > > > I'm still not convinced; pmu_direct_access is shared between PMUs, so
> > > > testing the result of atomic_dec_and_test() just doesn't make sense to
> > > > me, as another PMU could be playing with the count.
> > >
> > > How is that a problem? Let's make a concrete example:
> > >
> > > map PMU1:event2 -> pmu_direct_access = 1 -> enable access
> > > map PMU2:event3 -> pmu_direct_access = 2
> > > map PMU1:event4 -> pmu_direct_access = 3
> > > unmap PMU2:event3 -> pmu_direct_access = 2
> > > unmap PMU1:event2 -> pmu_direct_access = 1
> > > unmap PMU1:event4 -> pmu_direct_access = 0 -> disable access
> > >
> > > The only issue I can see is PMU2 remains enabled for user access until
> > > the last unmap. But we're sharing the mm, so who cares? Also, in this
> > > scenario it is the user's problem to pin themselves to cores sharing a
> > > PMU. If the user doesn't do that, they get to keep the pieces.
> >
> > I guess I'm just worried about exposing the counters to userspace after
> > the PMU driver (or perf core?) thinks that they're no longer exposed in
> > case we leak other events.
>
> IMO that's not practically different from the single-PMU case (i.e.
> multi-PMU isn't material, either we have a concern with leaking or we
> don't); more on that below.
>
> While it looks odd to place this on the mm, I don't think it's the end
> of the world.
>
> > However, I'm not sure how this is supposed to work normally: what
> > happens if e.g. a privileged user has a per-cpu counter for a kernel
> > event while a task has a counter with direct access -- can that task
> > read the kernel event out of the PMU registers from userspace?
>
> Yes -- userspace could go read any counters even though it isn't
> supposed to, and could potentially infer information from those. It
> won't have access to the config registers or kernel data structures, so
> it isn't guaranteed to know what the even is or when it is
> context-switched/reprogrammed/etc.
>
> If we believe that's a problem, then it's difficult to do anything
> robust other than denying userspace access entirely, since disabling
> userspace access while in use would surprise applications, and denying
> privileged events would need some global state that we consult at event
> creation time (in addition to being an inversion of privilege).
>
> IIRC there was some fuss about this a while back on x86; I'll go dig and
> see what I can find, unless Peter has a memory...

Maybe this one[1].

Rob

[1] https://lore.kernel.org/lkml/20200730123815.18518-1-kan.liang@xxxxxxxxxxxxxxx/