Re: [PATCH v8 3/5] arm64: perf: Enable PMU counter userspace access for perf event

On Tue, Jun 1, 2021 at 12:12 PM Mark Rutland [off-list ref] wrote:

On Tue, Jun 01, 2021 at 10:00:53AM -0500, Rob Herring wrote:

quoted

On Tue, Jun 1, 2021 at 8:55 AM Mark Rutland [off-list ref] wrote:

quoted

On Mon, May 17, 2021 at 02:54:03PM -0500, Rob Herring wrote:

quoted

+static void armv8pmu_enable_user_access(struct arm_pmu *cpu_pmu)
+{
+     struct pmu_hw_events *cpuc = this_cpu_ptr(cpu_pmu->hw_events);
+
+     if (!bitmap_empty(cpuc->dirty_mask, ARMPMU_MAX_HWEVENTS)) {
+             int i;
+             /* Don't need to clear assigned counters. */
+             bitmap_xor(cpuc->dirty_mask, cpuc->dirty_mask, cpuc->used_mask, ARMPMU_MAX_HWEVENTS);
+
+             for_each_set_bit(i, cpuc->dirty_mask, ARMPMU_MAX_HWEVENTS) {
+                     if (i == ARMV8_IDX_CYCLE_COUNTER)
+                             write_sysreg(0, pmccntr_el0);
+                     else
+                             armv8pmu_write_evcntr(i, 0);
+             }
+             bitmap_zero(cpuc->dirty_mask, ARMPMU_MAX_HWEVENTS);
+     }
+
+     write_sysreg(ARMV8_PMU_USERENR_ER | ARMV8_PMU_USERENR_CR, pmuserenr_el0);
+}

This still leaks the values of CPU-bound events, or task-bound events
owned by others, right?

For CPU-bound events, yes. There's not any way to prevent that without
per counter access controls.

It is clearing other's task-bound events.

Sorry, I misspoke. When I said "task-bound events owned by others", I
had meant events bounds to this task, but owned by someone else (e.g.
the system administrator).

Ah yeah, those would still be exposed, but that would only tell the
task someone is watching them. Though I guess the task could write
those counters and corrupt the data.

Thanks for confirming!

quoted

quoted

quoted

+static void armv8pmu_event_mapped(struct perf_event *event, struct mm_struct *mm)
+{
+     if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR) || (atomic_read(&event->mmap_count) != 1))
+             return;
+
+     if (atomic_inc_return(&event->ctx->nr_user) == 1) {
+             unsigned long flags;
+             atomic_inc(&event->pmu->sched_cb_usage);
+             local_irq_save(flags);
+             armv8pmu_enable_user_access(to_arm_pmu(event->pmu));
+             local_irq_restore(flags);
+     }
+}
+
+static void armv8pmu_event_unmapped(struct perf_event *event, struct mm_struct *mm)
+{
+     if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR) || (atomic_read(&event->mmap_count) != 1))
+             return;
+
+     if (atomic_dec_and_test(&event->ctx->nr_user)) {
+             atomic_dec(&event->pmu->sched_cb_usage);
+             armv8pmu_disable_user_access();
+     }
 }

We can open an event for task A, but call mmap()/munmap() for that event
from task B, which will do the enable/disable on task B rather than task
A. The core doesn't enforce that the mmap is performed on the same core,
so I don't think this is quite right, unfortunately.

Why do we care and who wants to do that? It all seems like a
convoluted scenario that isn't really going to happen. I prefer to not
support that until someone asks for it. Maybe we should check for the
condition (event->ctx->task != current) though.

My reason for caring is that it means our accounting structures aren't
aligned with the actual CPU state, and it's going to be very easy for
this to go wrong as things get reworked in future.

If we're saying someone shouldn't do this, then we should enforce that
they can't. If they can, then I'd prefer to have deterministic behaviour
that isn't going to cause us issues in future.

I think that we should treat this like changing other event properties
(e.g. the period, or enabling/disabling the event), with an
event_function_call, since that will do the right thing regardless. I
also expect that people will want to do setup/teardown in a single
thread, and this would make that work too.

Looks like that will take some work in the core to get
event_function_call to be called in this case.

A much more simple solution is simply not enabling/disabling user
access on mmap/unmap. That was added along the way for x86, but I
don't think that buys us much given the departure from x86
implementation. We already have to enable the feature via sysctl (as
Will wants it default off) and the open has to request user access. If
a task succeeded in opening the event, is there any scenario where it
can't mmap its event? If so, could that condition not also be checked
in open?

Rob

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