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

[Will Deacon]

On Thu, Apr 08, 2021 at 01:38:17PM -0500, Rob Herring wrote:
> 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:
> > > 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:
> > > 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.

Well, maybe. It looks the single-PMU case is exposed to the same issue,
but I think a solution needs to take into account the multi-PMU situation.

> > 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/

Going through the archives and talking to Peter, it looks like this is still
an active area of concern:

  - There are patches to clear "dirty" counters on context-switch. They were
    queued for 5.13 but broke -tip on Friday:

    https://lore.kernel.org/lkml/YHm%2FM4za2LpRYePw@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

  - Per-cpu events cannot be protected in software:

    https://lore.kernel.org/lkml/CALCETrVVPzUd_hQ8xoomHn_wWRQJUvROeCt2do4_D4ROZoAVMg@xxxxxxxxxxxxxx/

    so without hardware support, we need a way to disable user access for
    people that care about this leakage

x86 has an "rdpmc" file exposed for the PMU device in sysfs which allows
access to be disabled. I don't think these patches add such a thing, and
that's where the fun with multi-PMU machines would come into play.

Will