Re: [PATCH V5] perf/x86: Reset the dirty counter to prevent the leak for an RDPMC task

Liang, Kan
2021-04-22T18:47:32Z

On 4/22/2021 1:52 PM, Rob Herring wrote:
> On Wed, Apr 21, 2021 at 11:12:06AM -0400, Liang, Kan wrote:
>>
>>
>> On 4/21/2021 4:11 AM, Peter Zijlstra wrote:
>>>> @@ -2327,10 +2367,17 @@ static void x86_pmu_event_mapped(struct perf_event *event, struct mm_struct *mm)
>>>> static void x86_pmu_event_unmapped(struct perf_event *event, struct mm_struct *mm)
>>>> {
>>>> + unsigned long flags;
>>>> if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
>>>> return;
>>>> + if (x86_pmu.sched_task && event->hw.target) {
>>>> + local_irq_save(flags);
>>>> + perf_sched_cb_dec(event->ctx->pmu);
>>>> + local_irq_restore(flags);
>>>> + }
>>>> +
>>>> if (atomic_dec_and_test(&mm->context.perf_rdpmc_allowed))
>>>> on_each_cpu_mask(mm_cpumask(mm), cr4_update_pce, NULL, 1);
>>>> }
>>> I don't understand how this can possibly be correct. Both
>>> perf_sched_cb_{inc,dec} modify strict per-cpu state, but the mapped
>>> functions happen on whatever random CPU of the moment whenever the task
>>> memory map changes.
>>>
>>> Suppose we mmap() on CPU0 and then exit on CPU1. Suppose the task does
>>> mmap() on CPU0 but then creates threads and runs on CPU1-4 concurrently
>>> before existing on CPU5.
>>>
>>> Could be I'm not seeing it due to having a snot-brain, please explain.
>>
>> You are right.
>> I implemented a new test case which mmap() on CPU 3, run and exit on CPU 0.
>> It can still read the counter values from other tasks on CPU 0.
>
> Can you publish your tests? I'm working on arm64 user access support.

Does Arm support PDPMC?

Not sure whether I can post the codes. I guess I need to check and do
some modification before sharing the code.

The test case is very simple. The pseudo code is as below.

BindToCpu(3);
perf_open_and_enable()
rdpmc_read(); #read all counters in CPU 3.
BindToCpu(0);
rdpmc_read(); #read all counters in CPU 0.
perf_close();

>
>> Actually, I don't think we need perf_sched_cb_{inc,dec} and sched_task().
>>
>> The mm->context.perf_rdpmc_allowed will tell us if it's a RDPMC task.
>> I think a clean way should be to add a new check_leakage() method. When perf
>> schedules in a RDPMC task, we invoke the method and clear the dirty
>> counters.
>> (I use a generic name check_leakage for the method so it can be reused by
>> other ARCHs if possible.)
>>
>> The patch is as below. The new and old test cases are all passed. I will do
>> more tests.
>>
>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>> index c7fcc8d..229dd48 100644
>> --- a/arch/x86/events/core.c
>> +++ b/arch/x86/events/core.c
>> @@ -1624,6 +1624,8 @@ static void x86_pmu_del(struct perf_event *event, int
>> flags)
>> if (cpuc->txn_flags & PERF_PMU_TXN_ADD)
>> goto do_del;
>>
>> + __set_bit(event->hw.idx, cpuc->dirty);
>> +
>> /*
>> * Not a TXN, therefore cleanup properly.
>> */
>> @@ -2631,6 +2633,37 @@ static int x86_pmu_check_period(struct perf_event
>> *event, u64 value)
>> return 0;
>> }
>>
>> +static void x86_pmu_clear_dirty_counters(void)
>> +{
>> + struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>> + int i;
>> +
>> + /* Don't need to clear the assigned counter. */
>> + for (i = 0; i < cpuc->n_events; i++)
>> + __clear_bit(cpuc->assign[i], cpuc->dirty);
>> +
>> + if (bitmap_empty(cpuc->dirty, X86_PMC_IDX_MAX))
>> + return;
>> +
>> + for_each_set_bit(i, cpuc->dirty, X86_PMC_IDX_MAX) {
>> + /* Metrics and fake events don't have corresponding HW counters. */
>> + if (is_metric_idx(i) || (i == INTEL_PMC_IDX_FIXED_VLBR))
>> + 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);
>> +}
>> +
>> +static void x86_pmu_check_leakage(void)
>> +{
>> + if (READ_ONCE(x86_pmu.attr_rdpmc))
>> + x86_pmu_clear_dirty_counters();
>> +}
>> +
>> static int x86_pmu_aux_output_match(struct perf_event *event)
>> {
>> if (!(pmu.capabilities & PERF_PMU_CAP_AUX_OUTPUT))
>> @@ -2675,6 +2708,7 @@ static struct pmu pmu = {
>> .sched_task = x86_pmu_sched_task,
>> .swap_task_ctx = x86_pmu_swap_task_ctx,
>> .check_period = x86_pmu_check_period,
>> + .check_leakage = x86_pmu_check_leakage,
>>
>> .aux_output_match = x86_pmu_aux_output_match,
>>
>> diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
>> index 27fa85e..d6003e0 100644
>> --- a/arch/x86/events/perf_event.h
>> +++ b/arch/x86/events/perf_event.h
>> @@ -229,6 +229,7 @@ struct cpu_hw_events {
>> */
>> struct perf_event *events[X86_PMC_IDX_MAX]; /* in counter order */
>> unsigned long active_mask[BITS_TO_LONGS(X86_PMC_IDX_MAX)];
>> + unsigned long dirty[BITS_TO_LONGS(X86_PMC_IDX_MAX)];
>> int enabled;
>>
>> int n_events; /* the # of events in the below arrays */
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index a763928..bcf3964 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -514,6 +514,11 @@ struct pmu {
>> * Check period value for PERF_EVENT_IOC_PERIOD ioctl.
>> */
>> int (*check_period) (struct perf_event *event, u64 value); /* optional */
>> +
>> + /*
>> + * Check and clear dirty counters to prevent potential leakage
>> + */
>> + void (*check_leakage) (void); /* optional */
>> };
>>
>> enum perf_addr_filter_action_t {
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 928b166..b496113 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -3822,6 +3822,12 @@ static void cpu_ctx_sched_in(struct perf_cpu_context
>> *cpuctx,
>> ctx_sched_in(ctx, cpuctx, event_type, task);
>> }
>>
>> +static bool has_check_leakage(struct pmu *pmu)
>> +{
>> + return pmu->check_leakage && current->mm &&
>> + atomic_read(&current->mm->context.perf_rdpmc_allowed);
>
> context.perf_rdpmc_allowed is arch specific.

Thanks. It's addressed in V6.

>
>> +}
>> +
>> static void perf_event_context_sched_in(struct perf_event_context *ctx,
>> struct task_struct *task)
>> {
>> @@ -3832,6 +3838,8 @@ static void perf_event_context_sched_in(struct
>> perf_event_context *ctx,
>> if (cpuctx->task_ctx == ctx) {
>> if (cpuctx->sched_cb_usage)
>> __perf_pmu_sched_task(cpuctx, true);
>> + if (has_check_leakage(pmu))
>> + pmu->check_leakage();
>> return;
>> }

The above check is also removed in V6, because it's unnecessary for the
identical contexts.

Thanks,
Kan