[PATCH v3] cpufreq: fix race on cpufreq online

[Schspa Shi <schspa@gmail.com>]

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);

  down_write(&policy->rwsem);
  ...
  /* cpufreq nitialization fails in some cases */
  if (cpufreq_driver->get && has_target()) {
    policy->cur = cpufreq_driver->get(policy->cpu);
    if (!policy->cur) {
      ret = -EIO;
      pr_err("%s: ->get() failed\n", __func__);
      goto out_destroy_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);
}

We can fix it by clear the policy->cpus mask.
Both show_scaling_cur_freq and show_cpuinfo_cur_freq will return an
error by checking this mask, thus avoiding UAF.

Signed-off-by: Schspa Shi <schspa@gmail.com>

---

Changelog:
v1 -> v2:
        - Fix bad critical region enlarge which causes uninitialized
          unlock.
v2 -> v3:
        - Remove the missed down_write() before
          cpumask_and(policy->cpus, policy->cpus, cpu_online_mask);

Signed-off-by: Schspa Shi <schspa@gmail.com>
---
 drivers/cpufreq/cpufreq.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 80f535cc8a75..d93958dbdab8 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1337,12 +1337,12 @@ static int cpufreq_online(unsigned int cpu)
 		down_write(&policy->rwsem);
 		policy->cpu = cpu;
 		policy->governor = NULL;
-		up_write(&policy->rwsem);
 	} else {
 		new_policy = true;
 		policy = cpufreq_policy_alloc(cpu);
 		if (!policy)
 			return -ENOMEM;
+		down_write(&policy->rwsem);
 	}
 
 	if (!new_policy && cpufreq_driver->online) {
@@ -1382,7 +1382,6 @@ static int cpufreq_online(unsigned int cpu)
 		cpumask_copy(policy->related_cpus, policy->cpus);
 	}
 
-	down_write(&policy->rwsem);
 	/*
 	 * affected cpus must always be the one, which are online. We aren't
 	 * managing offline cpus here.
@@ -1533,7 +1532,7 @@ 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);
+	cpumask_clear(policy->cpus);
 
 out_offline_policy:
 	if (cpufreq_driver->offline)
@@ -1542,6 +1541,7 @@ static int cpufreq_online(unsigned int cpu)
 out_exit_policy:
 	if (cpufreq_driver->exit)
 		cpufreq_driver->exit(policy);
+	up_write(&policy->rwsem);
 
 out_free_policy:
 	cpufreq_policy_free(policy);
-- 
2.24.3 (Apple Git-128)