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

[Schspa Shi]

"Rafael J. Wysocki" <rafael@xxxxxxxxxx> writes:

> On Tue, May 10, 2022 at 5:42 PM Schspa Shi <schspa@xxxxxxxxx> 
> 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@xxxxxxxxx>
> >
> > ---
> >
> > 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@xxxxxxxxx>
> > ---
> >  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