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

["Liang, Kan" <kan.liang@linux.intel.com>]

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.

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);
>