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

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Message-ID: <2c70eff8-c79a-4c99-b8db-491ce25745a0@redhat.com>
Date: Sat, 22 Jun 2024 16:12:57 -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>
Subject: Re: [PATCH] cgroup/cpuset: Prevent UAF in proc_cpuset_show()

On 6/22/24 16:04, 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. Otherwise, it is more straight forward and 
easier to understand with the simple lock/unlock.

Cheers,
Longman