| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <m2wner47ru.fsf@gmail.com>
Date: Thu, 12 May 2022 13:51:53 +0800
From: Schspa Shi <schspa@...il.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Viresh Kumar <viresh.kumar@...aro.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v3] cpufreq: fix race on cpufreq online
"Rafael J. Wysocki" <rafael@...nel.org> writes:
> On Tue, May 10, 2022 at 5:42 PM Schspa Shi <schspa@...il.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@...il.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@...il.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
Powered by blists - more mailing lists