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SubjectRe: [PATCH V7 2/2] perf/x86: Reset the dirty counter to prevent the leak for an RDPMC task
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On 5/14/2021 10:44 AM, Peter Zijlstra wrote:
> On Thu, May 13, 2021 at 06:14:08PM -0400, Liang, Kan wrote:
>> On 5/13/2021 11:02 AM, Peter Zijlstra wrote:
>>> On Thu, May 13, 2021 at 07:23:02AM -0700, kan.liang@linux.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?
>>
>> That's not enough.
>>
>> I implemented a test case as below:
>> - The main thread A creates a new thread B.
>> - Bind the thread A to CPU 0. Then the thread A opens a event, mmap, enable
>> the event, and sleep.
>> - Bind the thread B to CPU 1. Wait until the event in the thread A is
>> enabled. Then RDPMC can read the counters on CPU 1.
>
> This?

Yes

>
> A B
>
> clone(CLONE_THREAD) --->
> set_affine(0)
> set_affine(1)
> while (!event-enabled)
> ;
> event = perf_event_open()
> mmap(event)
> ioctl(event, IOC_ENABLE); --->
> RDPMC
>
> sleep(n)
> schedule(INTERRUPTIBLE)
> /* idle */
>
>
>> In the x86_pmu_event_mapped(), we do on_each_cpu_mask(mm_cpumask(mm),
>> cr4_update_pce, NULL, 1);
>> The RDPMC from thread B on CPU 1 is not forbidden.
>> Since the counter is not created in thread B, the sched_task() never gets a
>> chance to be invoked. The dirty counter is not cleared.
>
> Per-task counters from CPU1 that ran before B ran?

Yes

>
>> To fix it, I think we have to move the cr4_update_pce() to the context
>> switch, and update it only when the RDPMC task is scheduled. But it probably
>> brings some overhead.
>
> We have CR4:PCE updates in the context switch path, see
> switch_mm_irqs_off() -> cr4_update_pce_mm().
>
> Doing the clear there might actually make sense and avoids this frobbing
> of ->sched_task(). When we call cr4_update_pce_mm(), and @mm has rdpmc
> on, clear dirty or something like that.
>
> Worth a try.
>
>

Looks like a good place. Will try.

Thanks,
Kan

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