linux-kernel - Re: [PATCH v4 1/2] cpufreq: fix race on cpufreq online

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Date:   Fri, 13 May 2022 09:48:33 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Schspa Shi <schspa@...il.com>
Cc:     rafael@...nel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] cpufreq: fix race on cpufreq online

On 12-05-22, 21:52, Schspa Shi wrote:
> When cpufreq online failed, policy->cpus are not empty while
> cpufreq sysfs file available, we may access some data freed.
> 
> Take policy->clk as an example:
> 
> static int cpufreq_online(unsigned int cpu)
> {
>   ...
>   // policy->cpus != 0 at this time
>   down_write(&policy->rwsem);
>   ret = cpufreq_add_dev_interface(policy);
>   up_write(&policy->rwsem);
> 
>   down_write(&policy->rwsem);
>   ...
>   /* cpufreq nitialization fails in some cases */
>   if (cpufreq_driver->get && has_target()) {
>     policy->cur = cpufreq_driver->get(policy->cpu);
>     if (!policy->cur) {
>       ret = -EIO;
>       pr_err("%s: ->get() failed\n", __func__);
>       goto out_destroy_policy;
>     }
>   }
>   ...
>   up_write(&policy->rwsem);
>   ...
> 
>   return 0;
> 
> out_destroy_policy:
> 	for_each_cpu(j, policy->real_cpus)
> 		remove_cpu_dev_symlink(policy, get_cpu_device(j));
>     up_write(&policy->rwsem);
> ...
> out_exit_policy:
>   if (cpufreq_driver->exit)
>     cpufreq_driver->exit(policy);
>       clk_put(policy->clk);
>       // policy->clk is a wild pointer
> ...
>                                     ^
>                                     |
>                             Another process access
>                             __cpufreq_get
>                               cpufreq_verify_current_freq
>                                 cpufreq_generic_get
>                                   // acces wild pointer of policy->clk;
>                                     |
>                                     |
> out_offline_policy:                 |
>   cpufreq_policy_free(policy);      |
>     // deleted here, and will wait for no body reference
>     cpufreq_policy_put_kobj(policy);
> }
> We can fix it by clear the policy->cpus mask.
> Both show_scaling_cur_freq and show_cpuinfo_cur_freq will return an
> error by checking this mask, thus avoiding UAF.

Instead of all above, what about this.

Subject: Abort show/store for half initialized policy

If policy initialization fails after the sysfs files are created,
there is a possibility that we may end up running show()/store()
callbacks for half initialized policies, which may have unpredictable
outcomes.

Abort show/store in such a case by making sure the policy is active.
Also inactivate the policy on such failures.

> Signed-off-by: Schspa Shi <schspa@...il.com>
> 
> ---
> 
> Changelog:
> v1 -> v2:
>         - Fix bad critical region enlarge which causes uninitialized
>           unlock.
>         - Move cpumask_clear(policy->cpus); before out_offline_policy
> v2 -> v3:
>         - Remove the missed down_write() before
>           cpumask_and(policy->cpus, policy->cpus, cpu_online_mask);
> v3 -> v4:
>         - Seprate to two patchs.
>         - Add policy_is_inactive check before sysfs access
> ---
>  drivers/cpufreq/cpufreq.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 80f535cc8a75..35dffd738580 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -953,7 +953,10 @@ static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
>  		return -EIO;
>  
>  	down_read(&policy->rwsem);
> -	ret = fattr->show(policy, buf);
> +	if (unlikely(policy_is_inactive(policy)))
> +		ret = -EBUSY;
> +	else
> +		ret = fattr->show(policy, buf);

I like it the way I have done earlier, initialize ret to -EBUSY and
get rid of the else part and call show/store in if itself. Same for
below.

>  	up_read(&policy->rwsem);
>  
>  	return ret;
> @@ -978,7 +981,10 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
>  
>  	if (cpu_online(policy->cpu)) {
>  		down_write(&policy->rwsem);
> -		ret = fattr->store(policy, buf, count);
> +		if (unlikely(policy_is_inactive(policy)))
> +			ret = -EBUSY;
> +		else
> +			ret = fattr->store(policy, buf, count);
>  		up_write(&policy->rwsem);
>  	}
>  
> @@ -1533,6 +1539,7 @@ static int cpufreq_online(unsigned int cpu)
>  	for_each_cpu(j, policy->real_cpus)
>  		remove_cpu_dev_symlink(policy, get_cpu_device(j));
>  
> +	cpumask_clear(policy->cpus);
>  	up_write(&policy->rwsem);
>  
>  out_offline_policy:
> -- 
> 2.29.0

-- 
viresh