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

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Date:   Thu, 21 Apr 2022 03:15:41 +0800
From:   Schspa Shi <schspa@...il.com>
To:     rafael@...nel.org, viresh.kumar@...aro.org
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        schspa@...il.com
Subject: [PATCH] cpufreq: fix race on cpufreq online

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);
+
 out_free_policy:
 	cpufreq_policy_free(policy);
 	return ret;
-- 
2.24.3 (Apple Git-128)