Re: [PATCH] cpufreq: s3c: avoid use after free issue in xxx_cpufreq_reboot_notifier_evt()

[chenqiwu <qiwuchen55@gmail.com>]

On Mon, Jan 06, 2020 at 12:25:02PM +0530, Viresh Kumar wrote:
> On 06-01-20, 14:52, chenqiwu wrote:
> > On Mon, Jan 06, 2020 at 11:18:11AM +0530, Viresh Kumar wrote:
> > > On 28-12-19, 14:43, qiwuchen55@gmail.com wrote:
> > > > From: chenqiwu <chenqiwu@xiaomi.com>
> > > > 
> > > > There is a potential UAF issue in xxx_cpufreq_reboot_notifier_evt() that
> > > > the cpufreq policy of cpu0 has been released before using it. So we should
> > > > make a judgement to avoid it.
> > > 
> > > There is no UAF problem here, but that we do cpufreq_cpu_get() with a
> > > corresponding cpufreq_cpu_put().
> > > 
> > > > Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
> > > > ---
> > > >  drivers/cpufreq/s3c2416-cpufreq.c | 11 ++++++++++-
> > > >  drivers/cpufreq/s5pv210-cpufreq.c | 10 +++++++++-
> > > >  2 files changed, 19 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/cpufreq/s3c2416-cpufreq.c b/drivers/cpufreq/s3c2416-cpufreq.c
> > > > index 1069103..0f576ba 100644
> > > > --- a/drivers/cpufreq/s3c2416-cpufreq.c
> > > > +++ b/drivers/cpufreq/s3c2416-cpufreq.c
> > > > @@ -304,6 +304,7 @@ static int s3c2416_cpufreq_reboot_notifier_evt(struct notifier_block *this,
> > > >  {
> > > >  	struct s3c2416_data *s3c_freq = &s3c2416_cpufreq;
> > > >  	int ret;
> > > > +	struct cpufreq_policy policy;
> > > >  
> > > >  	mutex_lock(&cpufreq_lock);
> > > >  
> > > > @@ -318,7 +319,15 @@ static int s3c2416_cpufreq_reboot_notifier_evt(struct notifier_block *this,
> > > >  	 */
> > > >  	if (s3c_freq->is_dvs) {
> > > >  		pr_debug("cpufreq: leave dvs on reboot\n");
> > > > -		ret = cpufreq_driver_target(cpufreq_cpu_get(0), FREQ_SLEEP, 0);
> > > > +
> > > > +		memset(&policy, 0, sizeof(policy));
> > > > +		ret = cpufreq_get_policy(&policy, 0);
> > > > +		if (ret < 0) {
> > > > +			pr_debug("cpufreq: get no policy for cpu0\n");
> > > > +			return NOTIFY_BAD;
> > > > +		}
> > > > +
> > > 
> > > This doesn't make sense to me, why don't you do cpufreq_cpu_get() and
> > > put() instead ?
> > >
> > Hi viresh,
> > I can't explain which approach is better, but I think both approaches are
> > effective for the situation.
> 
> The second one is better as it doesn't make copy of the policy, but
> rather just increments the counter.
> 
> > By the way, there is a possibility that the cpu0 hotplug thread will call
> > cpufreq_policy_free() to free cpufreq policy if cpu0 hotplug failed.
> > I think there should be a judgement to avoid this UAF risk if necessary,
> > or we just do panic if cpu0's cpufreq policy is free.
> 
> I think there are enough locks in place to avoid such issues and they
> shouldn't happen.
>

Hhh..I don't agree this, since the cpufreq policy of cpu0 may have
been released before such UAF issue happenning.
By the way, why not get invaild cpufreq policy of another cpu but
only cpu0 here?

> > > > +		ret = cpufreq_driver_target(&policy, FREQ_SLEEP, 0);
> > > >  		if (ret < 0)
> > > >  			return NOTIFY_BAD;
> > > >  	}
> > > > diff --git a/drivers/cpufreq/s5pv210-cpufreq.c b/drivers/cpufreq/s5pv210-cpufreq.c
> > > > index 5d10030..d99b4b1 100644
> > > > --- a/drivers/cpufreq/s5pv210-cpufreq.c
> > > > +++ b/drivers/cpufreq/s5pv210-cpufreq.c
> > > > @@ -555,8 +555,16 @@ static int s5pv210_cpufreq_reboot_notifier_event(struct notifier_block *this,
> > > >  						 unsigned long event, void *ptr)
> > > >  {
> > > >  	int ret;
> > > > +	struct cpufreq_policy *policy;
> > > >  
> > > > -	ret = cpufreq_driver_target(cpufreq_cpu_get(0), SLEEP_FREQ, 0);
> > > > +	policy = cpufreq_cpu_get(0);
> > > > +	if (!policy) {
> > > > +		pr_debug("cpufreq: get no policy for cpu0\n");
> > > > +		return NOTIFY_BAD;
> > > > +	}
> > > > +
> > > > +	ret = cpufreq_driver_target(policy, SLEEP_FREQ, 0);
> > > > +	cpufreq_cpu_put(policy);
> > > 
> > > Like what is done here.
> > > 
> > > Also add a blank line here.
> > > 
> > > >  	if (ret < 0)
> > > >  		return NOTIFY_BAD;
> > > >  
> > > > -- 
> > > > 1.9.1
> > > 
> > > -- 
> > > viresh
> > 
> > Thanks for your review!
> > Qiwu
> 
> -- 
> viresh

Thanks!
Qiwu

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