Messages in this thread Patch in this message | | | From | Schspa Shi <> | Subject | Re: [PATCH v3] cpufreq: fix race on cpufreq online | Date | Thu, 12 May 2022 13:51:53 +0800 |
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"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
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