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References: <20220510035259.5ep52sgahd2a6rie@vireshk-i7> <20220510154236.88753-1-schspa@gmail.com>
 <CAJZ5v0gcOmd8fXG9_BYxr6rN7ncUWnfki7K9S5wK2Vvh9SxUCA@mail.gmail.com> <m2wner47ru.fsf@gmail.com>
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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Thu, 12 May 2022 12:37:58 +0200
Message-ID: <CAJZ5v0ihy7vP9DbwWXjKwVT3JL+BzPziTERKhfmVCYizZwMuFw@mail.gmail.com>
Subject: Re: [PATCH v3] cpufreq: fix race on cpufreq online
To:     Schspa Shi <schspa@gmail.com>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>
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On Thu, May 12, 2022 at 7:52 AM Schspa Shi <schspa@gmail.com> wrote:
>
>
> "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);

This is cpuinfo_cur_freq and I was talking about scaling_cur_freq.

> > 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.

Cool, thanks!