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

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Date: Mon, 1 Apr 2024 14:17:18 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, fenghua.yu@...el.com,
 bp@...en8.de, james.morse@....com, tony.luck@...el.com,
 peternewman@...gle.com, tglx@...utronix.de, mingo@...hat.com,
 dave.hansen@...ux.intel.com, x86@...nel.org
Cc: hpa@...or.com, james.greenhalgh@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/resctrl: Fix uninitialized memory read when last CPU
 of domain goes offline



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