linux-kernel - Re: [PATCH V2 3/3] perf/x86: Reset the dirty counter to prevent the leak for an RDPMC task

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Message-ID: <20200907160115.GS2674@hirez.programming.kicks-ass.net>
Date:   Mon, 7 Sep 2020 18:01:15 +0200
From:   peterz@...radead.org
To:     kan.liang@...ux.intel.com
Cc:     mingo@...hat.com, acme@...nel.org, linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, mark.rutland@....com, luto@...capital.net
Subject: Re: [PATCH V2 3/3] perf/x86: Reset the dirty counter to prevent the
 leak for an RDPMC task

On Fri, Aug 21, 2020 at 12:57:54PM -0700, kan.liang@...ux.intel.com wrote:
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 0f3d01562ded..fa08d810dcd2 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -1440,7 +1440,10 @@ static void x86_pmu_start(struct perf_event *event, int flags)
>  
>  	cpuc->events[idx] = event;
>  	__set_bit(idx, cpuc->active_mask);
> -	__set_bit(idx, cpuc->running);
> +	/* The cpuc->running is only used by the P4 PMU */
> +	if (!cpu_has(&boot_cpu_data, X86_FEATURE_ARCH_PERFMON) &&
> +	    (boot_cpu_data.x86 == 0xf))
> +		__set_bit(idx, cpuc->running);
>  	x86_pmu.enable(event);
>  	perf_event_update_userpage(event);
>  }

Yuck! Use a static_branch() or something. This is a gnarly nest of code
that runs 99.9% of the time to conclude a negative. IOW a complete waste
of cycles for everybody not running a P4 space heater.

> @@ -1544,6 +1547,9 @@ static void x86_pmu_del(struct perf_event *event, int flags)
>  	if (cpuc->txn_flags & PERF_PMU_TXN_ADD)
>  		goto do_del;
>  
> +	if (READ_ONCE(x86_pmu.attr_rdpmc) && x86_pmu.sched_task &&
> +	    test_bit(event->hw.idx, cpuc->active_mask))
> +		__set_bit(event->hw.idx, cpuc->dirty);

And that too seems like an overly complicated set of tests and branches.
This should be effectivly true for the 99% common case.

> @@ -2219,11 +2225,45 @@ static int x86_pmu_event_init(struct perf_event *event)
>  	return err;
>  }
>  
> +void x86_pmu_clear_dirty_counters(void)
> +{
> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> +	int i;
> +
> +	if (bitmap_empty(cpuc->dirty, X86_PMC_IDX_MAX))
> +		return;
> +
> +	 /* Don't need to clear the assigned counter. */
> +	for (i = 0; i < cpuc->n_events; i++)
> +		__clear_bit(cpuc->assign[i], cpuc->dirty);
> +
> +	for_each_set_bit(i, cpuc->dirty, X86_PMC_IDX_MAX) {
> +		/* Metrics events don't have corresponding HW counters. */
> +		if (is_metric_idx(i))
> +			continue;
> +		else if (i >= INTEL_PMC_IDX_FIXED)
> +			wrmsrl(MSR_ARCH_PERFMON_FIXED_CTR0 + (i - INTEL_PMC_IDX_FIXED), 0);
> +		else
> +			wrmsrl(x86_pmu_event_addr(i), 0);
> +	}
> +
> +	bitmap_zero(cpuc->dirty, X86_PMC_IDX_MAX);
> +}

The bitmap_{empty,zero}() do indeed compile into single instructions,
neat!

>  static void x86_pmu_event_mapped(struct perf_event *event, struct mm_struct *mm)
>  {
>  	if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
>  		return;
>  
> +	/*
> +	 * Enable sched_task() for the RDPMC task,
> +	 * and clear the existing dirty counters.
> +	 */
> +	if (x86_pmu.sched_task && event->hw.target && !is_sampling_event(event)) {
> +		perf_sched_cb_inc(event->ctx->pmu);
> +		x86_pmu_clear_dirty_counters();
> +	}

I'm failing to see the correctness of the !is_sampling_event() part
there.

>  	/*
>  	 * This function relies on not being called concurrently in two
>  	 * tasks in the same mm.  Otherwise one task could observe
> @@ -2246,6 +2286,9 @@ static void x86_pmu_event_unmapped(struct perf_event *event, struct mm_struct *m
>  	if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
>  		return;
>  
> +	if (x86_pmu.sched_task && event->hw.target && !is_sampling_event(event))
> +		perf_sched_cb_dec(event->ctx->pmu);
> +

Idem.

>  	if (atomic_dec_and_test(&mm->context.perf_rdpmc_allowed))
>  		on_each_cpu_mask(mm_cpumask(mm), cr4_update_pce, NULL, 1);
>  }
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index c72e4904e056..e67713bfa33a 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -4166,11 +4166,39 @@ static void intel_pmu_cpu_dead(int cpu)
>  	intel_cpuc_finish(&per_cpu(cpu_hw_events, cpu));
>  }
>  
> +static void intel_pmu_rdpmc_sched_task(struct perf_event_context *ctx)
> +{
> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> +	struct perf_event *event;
> +
> +	if (bitmap_empty(cpuc->dirty, X86_PMC_IDX_MAX))
> +		return;
> +
> +	/*
> +	 * If the new task has the RDPMC enabled, clear the dirty counters to
> +	 * prevent the potential leak. If the new task doesn't have the RDPMC
> +	 * enabled, do nothing.
> +	 */
> +	list_for_each_entry(event, &ctx->event_list, event_entry) {
> +		if (event->hw.target &&
> +		    (event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED) &&
> +		    !is_sampling_event(event) &&
> +		    atomic_read(&event->mmap_count))
> +			break;
> +	}
> +	if (&event->event_entry == &ctx->event_list)
> +		return;

That's horrific, what's wrong with something like:

	if (!atomic_read(&current->mm->context.perf_rdpmc_allowed))
		return;

> +
> +	x86_pmu_clear_dirty_counters();
> +}

How is this Intel specific code? IIRC AMD has RDPMC too.