Re: [PATCH v3] cpufreq: fix race on cpufreq online

[Schspa Shi <schspa@gmail.com>]

"Rafael J. Wysocki" <rafael@kernel.org> writes:

> On Tue, May 10, 2022 at 5:42 PM Schspa Shi <schspa@gmail.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);
>>
>>   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.
>
> So the UAF only happens if something is freed by ->offline() or
> ->exit() and I'm not sure where the mask is checked in the
> scaling_cur_freq() path.
>

In the current code, it is checked in the following path:
show();
  down_read(&policy->rwsem);
  ret = fattr->show(policy, buf);
    show_cpuinfo_cur_freq
      __cpufreq_get
        if (unlikely(policy_is_inactive(policy)))
          return 0;
  up_read(&policy->rwsem);

> Overall, the patch is really two changes in one IMO.
>
>> 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.
>
> The first change, which could and probably should be a separate 
> patch,
> ends here.
>
> You prevent the rwsem from being dropped in the existing policy 
> case
> and acquire it right after creating a new policy.
>
> This way ->online() always runs under the rwsem, which 
> definitely
> sounds like a good idea, and policy->cpus is manipulated under 
> the
> rwsem which IMV is required.
>
> As a side-effect, ->init() is also run under the rwsem, but that
> shouldn't be a problem as per your discussion with Viresh.
>
> So the above would be patch 1 in a series.
>
> The change below is a separate one and it addresses the 
> particular
> race you've discovered, as long as patch 1 above is present.  It 
> would
> be patch 2 in the series.
>
>> @@ -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);
>
> It is OK to clear policy->cpus here, because ->offline() and 
> ->exit()
> are called with policy->cpus clear from cpufreq_offline() and
> cpufreq_remove_dev(), so they cannot assume policy->cpus to be
> populated when they are invoked.  However, this needs to be 
> stated in
> the changelog of patch 2.
>

OK, I will separate it into two patch.

>>  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);
>
> It is consistent to run ->offline() and ->exit() under the 
> rwsem, so
> this change is OK too.
>
>>  out_free_policy:
>>         cpufreq_policy_free(policy);
>> --
>
> That said, there still are races that are not addressed by the 
> above,
> so I would add patch 3 changing show() to check 
> policy_is_inactive()
> under the rwsem.
>

Yes, let me upload a new patch for this change.

> Thanks!

---
BRs
Schspa Shi