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SubjectRe: [PATCH v4 1/2] cpufreq: fix race on cpufreq online
On 12-05-22, 21:52, Schspa Shi 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.

Instead of all above, what about this.

Subject: Abort show/store for half initialized policy

If policy initialization fails after the sysfs files are created,
there is a possibility that we may end up running show()/store()
callbacks for half initialized policies, which may have unpredictable
outcomes.

Abort show/store in such a case by making sure the policy is active.
Also inactivate the policy on such failures.

> Signed-off-by: Schspa Shi <schspa@gmail.com>
>
> ---
>
> Changelog:
> v1 -> v2:
> - Fix bad critical region enlarge which causes uninitialized
> unlock.
> - Move cpumask_clear(policy->cpus); before out_offline_policy
> v2 -> v3:
> - Remove the missed down_write() before
> cpumask_and(policy->cpus, policy->cpus, cpu_online_mask);
> v3 -> v4:
> - Seprate to two patchs.
> - Add policy_is_inactive check before sysfs access
> ---
> drivers/cpufreq/cpufreq.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 80f535cc8a75..35dffd738580 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -953,7 +953,10 @@ static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
> return -EIO;
>
> down_read(&policy->rwsem);
> - ret = fattr->show(policy, buf);
> + if (unlikely(policy_is_inactive(policy)))
> + ret = -EBUSY;
> + else
> + ret = fattr->show(policy, buf);

I like it the way I have done earlier, initialize ret to -EBUSY and
get rid of the else part and call show/store in if itself. Same for
below.

> up_read(&policy->rwsem);
>
> return ret;
> @@ -978,7 +981,10 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
>
> if (cpu_online(policy->cpu)) {
> down_write(&policy->rwsem);
> - ret = fattr->store(policy, buf, count);
> + if (unlikely(policy_is_inactive(policy)))
> + ret = -EBUSY;
> + else
> + ret = fattr->store(policy, buf, count);
> up_write(&policy->rwsem);
> }
>
> @@ -1533,6 +1539,7 @@ static int cpufreq_online(unsigned int cpu)
> for_each_cpu(j, policy->real_cpus)
> remove_cpu_dev_symlink(policy, get_cpu_device(j));
>
> + cpumask_clear(policy->cpus);
> up_write(&policy->rwsem);
>
> out_offline_policy:
> --
> 2.29.0

--
viresh

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