linux-kernel - Re: [PATCH V7 2/2] perf/x86: Reset the dirty counter to prevent the leak for an RDPMC task

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Date:   Thu, 13 May 2021 17:02:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org, robh@...nel.org,
        ak@...ux.intel.com, acme@...nel.org, mark.rutland@....com,
        luto@...capital.net, eranian@...gle.com, namhyung@...nel.org
Subject: Re: [PATCH V7 2/2] perf/x86: Reset the dirty counter to prevent the
 leak for an RDPMC task

On Thu, May 13, 2021 at 07:23:02AM -0700, kan.liang@...ux.intel.com wrote:

> +	if (x86_pmu.sched_task && event->hw.target) {
> +		atomic_inc(&event->pmu->sched_cb_usage);
> +		local_irq_save(flags);
> +		x86_pmu_clear_dirty_counters();
> +		local_irq_restore(flags);
> +	}

So what happens if our mmap() happens after we've already created two
(or more) threads in the process, all of who already have a counter (or
more) on?

Shouldn't this be something like?

--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2474,7 +2474,7 @@ static int x86_pmu_event_init(struct per
 	return err;
 }
 
-static void x86_pmu_clear_dirty_counters(void)
+static void x86_pmu_clear_dirty_counters(void *unused)
 {
 	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
 	int i;
@@ -2512,9 +2512,9 @@ static void x86_pmu_event_mapped(struct
 	 */
 	if (x86_pmu.sched_task && event->hw.target) {
 		atomic_inc(&event->pmu->sched_cb_usage);
-		local_irq_save(flags);
-		x86_pmu_clear_dirty_counters();
-		local_irq_restore(flags);
+		on_each_cpu_mask(mm_cpumask(mm),
+				 x86_pmu_clear_dirty_counters,
+				 NULL, true);
 	}
 
 	/*
@@ -2653,7 +2653,7 @@ static void x86_pmu_sched_task(struct pe
 	 */
 	if (sched_in && ctx && READ_ONCE(x86_pmu.attr_rdpmc) &&
 	    current->mm && atomic_read(&current->mm->context.perf_rdpmc_allowed))
-		x86_pmu_clear_dirty_counters();
+		x86_pmu_clear_dirty_counters(NULL);
 }
 
 static void x86_pmu_swap_task_ctx(struct perf_event_context *prev,