Re: [PATCH -next] cgroup: fix uaf when proc_cpuset_show

[Waiman Long <longman@redhat.com>]

On 6/25/24 10:11, chenridong wrote:
>
>
> On 2024/6/25 18:10, Michal Koutný wrote:
>> Hello.
>>
>> On Tue, Jun 25, 2024 at 11:12:20AM GMT, chenridong<chenridong@huawei.com>  wrote:
>>> I am considering whether the cgroup framework has a method to fix this
>>> issue, as other subsystems may also have the same underlying problem.
>>> Since the root css will not be released, but the css->cgrp will be
>>> released.
>> <del>First part is already done in
>> 	d23b5c5777158 ("cgroup: Make operations on the cgroup root_list RCU safe")
>> second part is that</del>
>> you need to take RCU read lock and check for NULL, similar to
>> 	9067d90006df0 ("cgroup: Eliminate the need for cgroup_mutex in proc_cgroup_show()")
>>
>> Does that make sense to you?
>>
>> A Fixes: tag would be nice, it seems at least
>> 	a79a908fd2b08 ("cgroup: introduce cgroup namespaces")
>> played some role. (Here the RCU lock is not for cgroup_roots list but to
>> preserve the root cgrp itself css_free_rwork_fn/cgroup_destroy_root.
>>
>> HTH,
>> Michal
>
> Thank you, Michal, that is a good idea. Do you mean as below?
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>
> index c12b9fdb22a4..2ce0542067f1 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -5051,10 +5051,17 @@ int proc_cpuset_show(struct seq_file *m, 
> struct pid_namespace *ns,
>         if (!buf)
>                 goto out;
>
> +       rcu_read_lock();
> +       spin_lock_irq(&css_set_lock);
>         css = task_get_css(tsk, cpuset_cgrp_id);
> -       retval = cgroup_path_ns(css->cgroup, buf, PATH_MAX,
> - current->nsproxy->cgroup_ns);
> +
> +       retval = cgroup_path_ns_locked(css->cgroup, buf, PATH_MAX,
> +               current->nsproxy->cgroup_ns);
>         css_put(css);
> +
> +       spin_unlock_irq(&css_set_lock);
> +       cgroup_unlock();
> +
>         if (retval == -E2BIG)
>                 retval = -ENAMETOOLONG;
>
>         if (retval < 0)
>
That should work. However, I would suggest that you take task_get_css() 
and css_put() outside of the critical section. The task_get_css() is a 
while loop that may take a while to execute and you don't want run it 
with interrupt disabled.

Cheers,
Longman