Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Thu, 12 May 2022 12:37:58 +0200 | Subject | Re: [PATCH v3] cpufreq: fix race on cpufreq online |
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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!
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