linux-kernel - Re: [PATCH] cgroup/cpuset: Prevent UAF in proc_cpuset

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Message-ID: <dc0b2650-4c0d-4f52-8c60-b9afbe728103@redhat.com>
Date: Sun, 23 Jun 2024 12:28:21 -0400
From: Waiman Long <longman@...hat.com>
To: Markus Elfring <Markus.Elfring@....de>,
 Chen Ridong <chenridong@...wei.com>, cgroups@...r.kernel.org,
 bpf@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
 Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Julia Lawall
 <julia.lawall@...ia.fr>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] cgroup/cpuset: Prevent UAF in proc_cpuset_show()


On 6/23/24 02:18, Markus Elfring wrote:
>>> …
>>>> +++ b/kernel/cgroup/cpuset.c
>>> …
>>>> @@ -5051,10 +5066,12 @@ int proc_cpuset_show(struct seq_file *m, struct pid_namespace *ns,
>>>>        if (!buf)
>>>>            goto out;
>>>>
>>>> +    mutex_lock(&cpuset_mutex);
>>>>        css = task_get_css(tsk, cpuset_cgrp_id);
>>>>        retval = cgroup_path_ns(css->cgroup, buf, PATH_MAX,
>>>>                    current->nsproxy->cgroup_ns);
>>>>        css_put(css);
>>>> +    mutex_unlock(&cpuset_mutex);
>>> …
>>>
>>> Under which circumstances would you become interested to apply a statement
>>> like “guard(mutex)(&cpuset_mutex);”?
>>> https://elixir.bootlin.com/linux/v6.10-rc4/source/include/linux/mutex.h#L196
>> A mutex guard will be more appropriate if there is an error exit case that needs to be handled.
> Lock guards can help to reduce and improve source code another bit,
> can't they?

For simple lock critical section, there isn't too much difference in 
term of readability between using lock guard and normal lock/unlock 
call. If there are multiple exit points in the critical section, lock 
guard can help to simplify the code. For those situations, I will 
certain try to use lock guard.

Another reason that I go with normal lock/unlock is that none of the 
cpuset_mutex lock/unlock sites in cpuset.c has used lock guard yet and 
there is no good reason in introduce something different from other call 
sites.

Cheers,
Longman