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Date:   Mon, 9 May 2022 09:27:46 +0530
From:   Viresh Kumar <viresh.kumar@linaro.org>
To:     Schspa Shi <schspa@gmail.com>
Cc:     rafael@kernel.org, linux-pm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] cpufreq: fix race on cpufreq online
Message-ID: <20220509035746.aeggm4cut2ewcmmk@vireshk-i7>
References: <20220420191541.99528-1-schspa@gmail.com>
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I had to dig the old patch first before starting to review what your
next one does.

On 21-04-22, 03:15, 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);

Please keep some code to help understand where we went from here. I am
sure you meant that we will error out in this case, but you removed
the relevant code.

>   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);
> }
> 
> Signed-off-by: Schspa Shi <schspa@gmail.com>
> ---
>  drivers/cpufreq/cpufreq.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 80f535cc8a75..0d58b0f8f3af 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -1533,8 +1533,6 @@ static int cpufreq_online(unsigned int cpu)
>  	for_each_cpu(j, policy->real_cpus)
>  		remove_cpu_dev_symlink(policy, get_cpu_device(j));
>  
> -	up_write(&policy->rwsem);
> -
>  out_offline_policy:
>  	if (cpufreq_driver->offline)
>  		cpufreq_driver->offline(policy);
> @@ -1543,6 +1541,9 @@ static int cpufreq_online(unsigned int cpu)
>  	if (cpufreq_driver->exit)
>  		cpufreq_driver->exit(policy);
>  
> +	cpumask_clear(policy->cpus);
> +	up_write(&policy->rwsem);

This is simply buggy as now an error out to out_offline_policy or
out_exit_policy will try to release a semaphore which was never taken
in the first place. This works fine only if we failed late, i.e. via
out_destroy_policy.

The very first thing we need to do now is revert this patch. Lemme
send a patch for that and you can send a fresh fix over that once you
have a stable fix.

-- 
viresh