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

From: chenqiwu <qiwuchen55@gmail.com>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: kgene@kernel.org, krzk@kernel.org, rjw@rjwysocki.net,
	linux-arm-kernel@lists.infradead.org,
	linux-samsung-soc@vger.kernel.org, linux-pm@vger.kernel.org
Subject: Re: [PATCH] cpufreq: s3c: avoid use after free issue in xxx_cpufreq_reboot_notifier_evt()
Date: Mon, 6 Jan 2020 14:52:49 +0800	[thread overview]
Message-ID: <20200106065249.GA16392@cqw-OptiPlex-7050> (raw)
In-Reply-To: <20200106054811.uduf2qdn5ecvbwrc@vireshk-i7>

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.
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.

> > +		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