Re: [Patch v2 3/6] perf/x86: Check if cpuc->events[*] pointer exists before accessing it

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 8/13/2025 2:16 AM, Liang, Kan wrote:
>
> On 2025-08-11 7:33 p.m., Mi, Dapeng wrote:
>> On 8/12/2025 7:32 AM, Liang, Kan wrote:
>>> On 2025-08-11 2:00 a.m., Dapeng Mi wrote:
>>>> The PMI handler could disable some events as the interrupt throttling
>>>> and clear the corresponding items in cpuc->events[] array.
>>>>
>>>> perf_event_overflow()
>>>>   -> __perf_event_overflow()
>>>>     ->__perf_event_account_interrupt()
>>>>       -> perf_event_throttle_group()
>>>>         -> perf_event_throttle()
>>>>           -> event->pmu->stop()
>>>>             -> x86_pmu_stop()
>>>>
>>>> Moreover PMI is NMI on x86 platform and it could interrupt other perf
>>>> code like setup_pebs_adaptive_sample_data(). 
>>> The PMU is disabled when draining the PEBS records. I don't think a PMI
>>> can be triggered in the setup_pebs_adaptive_sample_data().
>> Besides in NMI handler, the drain_pebs helper intel_pmu_drain_pebs_buffer()
>> could be called in many places, like context switch and PEBS event
>> disabling. 
> Yes
>
>> All these places could be interrupted by the NMI handler, and
> No. Before draining the buffer, the PMU must be stopped. No NMI could be
> triggered.
>
>> then the trigger this NULL pointer access issue.
>>
>>
>>>> So once PMI handling
>>>> finishes and returns into setup_pebs_adaptive_sample_data() and it could
>>>> find the cpuc->events[*] becomes NULL and accessing this NULL pointer
>>>> triggers an invalid memory access and leads to kernel crashes eventually.
>>> The commit 9734e25fbf5a stops all events in a group when processing the
>>> last records of the leader event. For large PEBS, it's possible that
>>> there are still some records for member events left. It should be the
>>> root cause of the NULL pointer. If so, we should drain those records as
>>> well.
>> The left PEBS record would always be cleared by
>> intel_pmu_drain_large_pebs() when disabling PEBS event.
> When stopping the event.
>
> Then you should only need the check in intel_pmu_drain_pebs_icl(), since
> the stop only happens when handling the last event.

Yes, it's enough to only add NULL check in intel_pmu_drain_pebs_icl(). Thanks.


>
> Thanks,
> Kan>
>>> Thanks,
>>> Kan>
>>>> Thus add NULL check before accessing cpuc->events[*] pointer.
>>>>
>>>> Reported-by: kernel test robot <oliver.sang@intel.com>
>>>> Closes: https://lore.kernel.org/oe-lkp/202507042103.a15d2923-lkp@intel.com
>>>> Fixes: 9734e25fbf5a ("perf: Fix the throttle logic for a group")
>>>> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
>>>> Tested-by: kernel test robot <oliver.sang@intel.com>
>>>> ---
>>>>  arch/x86/events/core.c       |  3 +++
>>>>  arch/x86/events/intel/core.c |  6 +++++-
>>>>  arch/x86/events/intel/ds.c   | 13 ++++++-------
>>>>  3 files changed, 14 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>>>> index 7610f26dfbd9..f0a3bc57157d 100644
>>>> --- a/arch/x86/events/core.c
>>>> +++ b/arch/x86/events/core.c
>>>> @@ -1711,6 +1711,9 @@ int x86_pmu_handle_irq(struct pt_regs *regs)
>>>>  			continue;
>>>>  
>>>>  		event = cpuc->events[idx];
>>>> +		if (!event)
>>>> +			continue;
>>>> +
>>>>  		last_period = event->hw.last_period;
>>>>  
>>>>  		val = static_call(x86_pmu_update)(event);
>>>> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
>>>> index 15da60cf69f2..386717b75a09 100644
>>>> --- a/arch/x86/events/intel/core.c
>>>> +++ b/arch/x86/events/intel/core.c
>>>> @@ -2718,6 +2718,8 @@ static void update_saved_topdown_regs(struct perf_event *event, u64 slots,
>>>>  		if (!is_topdown_idx(idx))
>>>>  			continue;
>>>>  		other = cpuc->events[idx];
>>>> +		if (!other)
>>>> +			continue;
>>>>  		other->hw.saved_slots = slots;
>>>>  		other->hw.saved_metric = metrics;
>>>>  	}
>>>> @@ -2761,6 +2763,8 @@ static u64 intel_update_topdown_event(struct perf_event *event, int metric_end,
>>>>  		if (!is_topdown_idx(idx))
>>>>  			continue;
>>>>  		other = cpuc->events[idx];
>>>> +		if (!other)
>>>> +			continue;
>>>>  		__icl_update_topdown_event(other, slots, metrics,
>>>>  					   event ? event->hw.saved_slots : 0,
>>>>  					   event ? event->hw.saved_metric : 0);
>>>> @@ -3138,7 +3142,7 @@ static void x86_pmu_handle_guest_pebs(struct pt_regs *regs,
>>>>  
>>>>  	for_each_set_bit(bit, (unsigned long *)&guest_pebs_idxs, X86_PMC_IDX_MAX) {
>>>>  		event = cpuc->events[bit];
>>>> -		if (!event->attr.precise_ip)
>>>> +		if (!event || !event->attr.precise_ip)
>>>>  			continue;
>>>>  
>>>>  		perf_sample_data_init(data, 0, event->hw.last_period);
>>>> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
>>>> index c0b7ac1c7594..b23c49e2e06f 100644
>>>> --- a/arch/x86/events/intel/ds.c
>>>> +++ b/arch/x86/events/intel/ds.c
>>>> @@ -2480,6 +2480,8 @@ static void intel_pmu_pebs_event_update_no_drain(struct cpu_hw_events *cpuc, u64
>>>>  	 */
>>>>  	for_each_set_bit(bit, (unsigned long *)&pebs_enabled, X86_PMC_IDX_MAX) {
>>>>  		event = cpuc->events[bit];
>>>> +		if (!event)
>>>> +			continue;
>>>>  		if (event->hw.flags & PERF_X86_EVENT_AUTO_RELOAD)
>>>>  			intel_pmu_save_and_restart_reload(event, 0);
>>>>  	}
>>>> @@ -2579,10 +2581,7 @@ static void intel_pmu_drain_pebs_nhm(struct pt_regs *iregs, struct perf_sample_d
>>>>  			continue;
>>>>  
>>>>  		event = cpuc->events[bit];
>>>> -		if (WARN_ON_ONCE(!event))
>>>> -			continue;
>>>> -
>>>> -		if (WARN_ON_ONCE(!event->attr.precise_ip))
>>>> +		if (!event || WARN_ON_ONCE(!event->attr.precise_ip))
>>>>  			continue;
>>>>  
>>>>  		/* log dropped samples number */
>>>> @@ -2645,9 +2644,7 @@ static void intel_pmu_drain_pebs_icl(struct pt_regs *iregs, struct perf_sample_d
>>>>  		pebs_status = basic->applicable_counters & cpuc->pebs_enabled & mask;
>>>>  		for_each_set_bit(bit, (unsigned long *)&pebs_status, X86_PMC_IDX_MAX) {
>>>>  			event = cpuc->events[bit];
>>>> -
>>>> -			if (WARN_ON_ONCE(!event) ||
>>>> -			    WARN_ON_ONCE(!event->attr.precise_ip))
>>>> +			if (!event || WARN_ON_ONCE(!event->attr.precise_ip))
>>>>  				continue;
>>>>  
>>>>  			if (counts[bit]++) {
>>>> @@ -2663,6 +2660,8 @@ static void intel_pmu_drain_pebs_icl(struct pt_regs *iregs, struct perf_sample_d
>>>>  			continue;
>>>>  
>>>>  		event = cpuc->events[bit];
>>>> +		if (!event)
>>>> +			continue;
>>>>  
>>>>  		__intel_pmu_pebs_last_event(event, iregs, regs, data, last[bit],
>>>>  					    counts[bit], setup_pebs_adaptive_sample_data);
>