linux-kernel - Re: [PATCH 21/21] x86/intel_rdt/mbm: Handle counter overflow

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Date:   Thu, 6 Jul 2017 16:53:40 -0700 (PDT)
From:   Shivappa Vikas <vikas.shivappa@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Vikas Shivappa <vikas.shivappa@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, hpa@...or.com, peterz@...radead.org,
        ravi.v.shankar@...el.com, vikas.shivappa@...el.com,
        tony.luck@...el.com, fenghua.yu@...el.com, andi.kleen@...el.com
Subject: Re: [PATCH 21/21] x86/intel_rdt/mbm: Handle counter overflow



On Sun, 2 Jul 2017, Thomas Gleixner wrote:

> On Mon, 26 Jun 2017, Vikas Shivappa wrote:
>> +static void mbm_update(struct rdt_domain *d, int rmid)
>> +{
>> +	struct rmid_read rr;
>> +
>> +	rr.first = false;
>> +	rr.d = d;
>> +
>> +	if (is_mbm_total_enabled()) {
>> +		rr.evtid = QOS_L3_MBM_TOTAL_EVENT_ID;
>> +		__mon_event_count(rmid, &rr);
>
> This is broken as it is not protected against a concurrent read from user
> space which comes in via a smp function call.

The read from user also has the rdtgroup_mutex.

Thanks,
Vikas

>
> This means both the internal state and __rmid_read() are unprotected.
>
> I'm not sure whether it's enough to disable interrupts around
> __mon_event_count(), but that's the minimal protection required. It's
> definitely good enough for __rmid_read(), but it might not be sufficient
> for protecting domain->mbm_[local|total]. I leave the exercise of figuring
> that out to you.
>
> Thanks,
>
> 	tglx
>