Re: [PATCH v2] cpufreq: brcmstb-avs-cpufreq: avoid potential stuck and UAF risk

[chenqiwu <qiwuchen55@gmail.com>]

On Mon, Jan 06, 2020 at 11:26:37AM +0530, Viresh Kumar wrote:
> On 05-01-20, 20:50, qiwuchen55@gmail.com wrote:
> > From: chenqiwu <chenqiwu@xiaomi.com>
> > 
> > brcm_avs_cpufreq_get() calls cpufreq_cpu_get() to get cpufreq policy,
> > meanwhile, it also increments the kobject reference count of policy to
> > mark it busy. However, a corresponding call of cpufreq_cpu_put() is
> > ignored to decrement the kobject reference count back, which may lead
> > to a potential stuck risk that percpu cpuhp thread deadly waits for
> > dropping of kobject refcount when percpu cpufreq policy free.
> > 
> > The call trace of stuck risk could be:
> > cpufreq_online()  //If cpufreq online failed, goto out_free_policy.
> >     ->cpufreq_policy_free()     //Do cpufreq_policy free.
> >         ->cpufreq_policy_put_kobj()
> >             ->kobject_put()       //Skip if policy kfref count is not 1.
> >                 ->cpufreq_sysfs_release()
> >                     ->complete()  //Complete policy->kobj_unregister.
> >                 ->wait_for_completion() //Wait for policy->kobj_unregister.
> > 
> > A simple way to avoid this stuck risk is use cpufreq_cpu_get_raw()
> > instead of cpufreq_cpu_get(), since this can be easily exercised by
> > attempting to force an unbind of the CPUfreq driver.
> > 
> > Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
> > ---
> >  drivers/cpufreq/brcmstb-avs-cpufreq.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/cpufreq/brcmstb-avs-cpufreq.c b/drivers/cpufreq/brcmstb-avs-cpufreq.c
> > index 77b0e5d..6d2bf5c 100644
> > --- a/drivers/cpufreq/brcmstb-avs-cpufreq.c
> > +++ b/drivers/cpufreq/brcmstb-avs-cpufreq.c
> > @@ -452,8 +452,15 @@ static bool brcm_avs_is_firmware_loaded(struct private_data *priv)
> >  
> >  static unsigned int brcm_avs_cpufreq_get(unsigned int cpu)
> >  {
> > -	struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
> > -	struct private_data *priv = policy->driver_data;
> > +	struct cpufreq_policy *policy = cpufreq_cpu_get_raw(cpu);
> > +	struct private_data *priv;
> > +
> > +	if (!policy)
> > +		return 0;
> > +
> 
> Since we always reach here after the cpufreq driver is registered, we
> may not need to check the policy pointer at all.
> 
> > +	priv = policy->driver_data;
> > +	if (!priv || !priv->base)
> > +		return 0;
> 
> Can there be a case where priv or priv->base be set to NULL for this
> driver ? I don't think so and so this may not be required.
>

Hi viresh,
There could be a case as the description of this patch besides
brcm_avs_driver unloads. Since cpufreq_policy_free() will free
the mm of cpufreq_policy at the last moment.

Thanks!
Qiwu

> >  
> >  	return brcm_avs_get_frequency(priv->base);
> >  }
> > -- 
> > 1.9.1
> 
> -- 
> viresh