Re: [PATCH v2 9/9] perf/cxl: Avoid cpumask_of(-1) when no CPU is assigned

[Dave Jiang <dave.jiang@intel.com>]

On 7/29/26 12:14 PM, Jonathan Cameron wrote:
> On Wed, 29 Jul 2026 07:55:55 -0700
> Dave Jiang <dave.jiang@intel.com> wrote:
> 
>> cpumask_show() feeds info->on_cpu straight into cpumask_of() for the
>> world-readable cpumask sysfs attribute. on_cpu is -1 before the first
>> hotplug online callback and transiently in cxl_pmu_offline_cpu() before a
>> new target is chosen. cpumask_of(-1) treats the CPU number as unsigned and
>> does out-of-bounds pointer arithmetic in get_cpu_mask(), so a concurrent
>> read of the attribute dereferences a wild pointer and can fault -- a local
>> denial of service.
>>
> 
> I'm not keen on the solution here. 
> 
> The transient state is ugly anyway. We can just move setting it to -1 into
> the dummy code that deals with that well known case of you have CPUs online
> and code is still running.
> 
> The init case looks like a false positive to me.  But maybe I'm missing stuff.
> The perf registration that surfaces the sysfs happens after hotplug handler is
> added and I believe that synchronously runs it for CPUs that are already up.
> Given we are running code (and CXL stuff isn't super early) something will
> be up so it won't remain -1 by the time of use.
> 
> Can we just use the generic stuff?  Maybe need Robin's stuff to add init / exit
> per driver calls. 
> https://lore.kernel.org/linux-arm-kernel/cover.1784911757.git.robin.murphy@arm.com/

I'll drop this patch for now.

DJ

> 
> In general, I'd like Robin to take a quick look at the more generic perf
> parts of this series given he has clearly been deep in this stuff a lot
> more recently than me :)
> 
> Jonathan
> 
>> Read on_cpu once and emit an empty mask when it is negative.
>>
>> Fixes: 5d7107c72796 ("perf: CXL Performance Monitoring Unit driver")
>> Reported-by: sashiko-bot@kernel.org
>> Closes: https://sashiko.dev/#/patchset/20260715191454.459673-1-dave@stgolabs.net?part=1
>> Assisted-by: Claude:claude-opus-4-8
>> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
>> ---
>>  drivers/perf/cxl_pmu.c | 11 ++++++++++-
>>  1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/perf/cxl_pmu.c b/drivers/perf/cxl_pmu.c
>> index f42238b2b6b0..6aad381c0376 100644
>> --- a/drivers/perf/cxl_pmu.c
>> +++ b/drivers/perf/cxl_pmu.c
>> @@ -501,8 +501,17 @@ static ssize_t cpumask_show(struct device *dev, struct device_attribute *attr,
>>  			    char *buf)
>>  {
>>  	struct cxl_pmu_info *info = dev_get_drvdata(dev);
>> +	int cpu = READ_ONCE(info->on_cpu);
>>  
>> -	return cpumap_print_to_pagebuf(true, buf, cpumask_of(info->on_cpu));
>> +	/*
>> +	 * on_cpu is -1 before the first online callback and transiently during
>> +	 * cxl_pmu_offline_cpu(). cpumask_of(-1) computes an out-of-bounds
>> +	 * pointer, so report an empty mask instead.
>> +	 */
>> +	if (cpu < 0)
>> +		return sysfs_emit(buf, "\n");
>> +
>> +	return cpumap_print_to_pagebuf(true, buf, cpumask_of(cpu));
>>  }
>>  static DEVICE_ATTR_RO(cpumask);
>>  
>