Re: [PATCH] x86/resctrl: Fix uninitialized memory read when last CPU of domain goes offline

Moger, Babu
2024-04-01T19:17:24Z

On 4/1/24 13:12, Reinette Chatre wrote:
> Hi Babu,
>
> On 4/1/2024 10:57 AM, Moger, Babu wrote:
>> On 3/28/24 16:12, Reinette Chatre wrote:
>
>>> --- a/arch/x86/kernel/cpu/resctrl/internal.h
>>> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
>>> @@ -85,6 +85,10 @@ cpumask_any_housekeeping(const struct cpumask *mask, int exclude_cpu)
>>> if (cpu < nr_cpu_ids && !tick_nohz_full_cpu(cpu))
>>> return cpu;
>>>
>>> + /* Only continue if tick_nohz_full_mask has been initialized. */
>>> + if (!tick_nohz_full_enabled())
>>> + return cpu;
>>> +
>>
>> I am curious why this below check didn't fail?
>>
>> if (cpu < nr_cpu_ids && !tick_nohz_full_cpu(cpu))
>> return cpu;
>>
>> The tick_nohz_full_cpu() already checks tick_nohz_full_enabled().
>>
>> It should returned 'false' and returned cpu already.
>>
>> Did i miss something?
>>
>
> The scenario occurs when the last CPU of a domain goes offline and the cpu itself
> is the cpu to be excluded. In this scenario cpu >= nr_cpu_ids in the check you
> quote.
>
> You may, as did I, wonder why continue the check on a smaller set of CPUs
> if the first check already failed? James addressed that in:
> https://lore.kernel.org/lkml/bd8a64fa-86d3-4417-a570-36469330508f@arm.com/
>

Got it.
--
Thanks
Babu Moger