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Message-ID: <CAJZ5v0guPiGx-ZnC0RcqVgDEp0bh4DcKC7QYjjO0PF_3kHdVGg@mail.gmail.com>
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;
> --
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