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

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Message-ID: <CAJZ5v0hxoU0nWKMVRfiMU_XSQE9n49CjmhmibEVmEo=+E4kBoA@mail.gmail.com>
Date:   Fri, 22 Apr 2022 17:59:09 +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 Fri, Apr 22, 2022 at 5:10 PM Schspa Shi <schspa@...il.com> wrote:
>
> "Rafael J. Wysocki" <rafael@...nel.org> writes:
>
> > 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.
>
> There is a lock outside of show_cpuinfo_cur_freq(). Please check about
> static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
> {
>         ......
>         down_read(&policy->rwsem);
>         ret = fattr->show(policy, buf);
>         up_read(&policy->rwsem);
>
>     ......
> }

You are right, my mistake.

OK, I'll queue up the patch, then.