linux-kernel - Re: [PATCH] cpufreq: fix race on cpufreq online

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Date:   Fri, 22 Apr 2022 16:38:23 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Schspa Shi <schspa@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: fix race on cpufreq online

On Wed, Apr 20, 2022 at 9:16 PM Schspa Shi <schspa@...il.com> 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);
>
>   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@...il.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 change is correct AFAICS, but it doesn't really fix the race,
because show_cpuinfo_cur_freq() uses __cpufreq_get() directly without
locking.

It should use cpufreq_get() instead.

> +
>  out_free_policy:
>         cpufreq_policy_free(policy);
>         return ret;
> --