linux-kernel - Re: [PATCH V2 2/3] arch_topology: Avoid use-after-free for scale_freq

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Date:   Wed, 16 Jun 2021 14:40:35 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Qian Cai <quic_qiancai@...cinc.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/3] arch_topology: Avoid use-after-free for
 scale_freq_data

On 16-06-21, 10:31, Greg Kroah-Hartman wrote:
> On Wed, Jun 16, 2021 at 01:48:59PM +0530, Viresh Kumar wrote:
> > On 16-06-21, 09:57, Greg Kroah-Hartman wrote:
> > > On Wed, Jun 16, 2021 at 12:18:08PM +0530, Viresh Kumar wrote:
> > > > Currently topology_scale_freq_tick() may end up using a pointer to
> > > > struct scale_freq_data, which was previously cleared by
> > > > topology_clear_scale_freq_source(), as there is no protection in place
> > > > here. The users of topology_clear_scale_freq_source() though needs a
> > > > guarantee that the previous scale_freq_data isn't used anymore.
> > > > 
> > > > Since topology_scale_freq_tick() is called from scheduler tick, we don't
> > > > want to add locking in there. Use the RCU update mechanism instead
> > > > (which is already used by the scheduler's utilization update path) to
> > > > guarantee race free updates here.
> > > > 
> > > > Cc: Paul E. McKenney <paulmck@...nel.org>
> > > > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> > > 
> > > So this is a bugfix for problems in the current codebase?  What commit
> > > does this fix?  Should it go to the stable kernels?
> > 
> > There is only one user of topology_clear_scale_freq_source()
> > (cppc-cpufreq driver, which is already reverted in pm/linux-next). So
> > in the upcoming 5.13 kernel release, there will be no one using this
> > API and so no one will break.
> > 
> > And so I skipped the fixes tag, I can add it though.
> 
> It would be nice to have to answer this type of question, otherwise you
> will have automated scripts trying to backport this to kernels where it
> does not belong :)

Sure, I will add these to the next version.

-- 
viresh