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Date:   Sat, 20 Jun 2020 09:00:40 +0800
From:   Zefan Li <lizefan@...wei.com>
To:     Roman Gushchin <guro@...com>
CC:     Cong Wang <xiyou.wangcong@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Cameron Berkenpas <cam@...-zeon.de>,
        Peter Geis <pgwipeout@...il.com>,
        Lu Fengqi <lufq.fnst@...fujitsu.com>,
        Daniƫl Sonck <dsonck92@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [Patch net] cgroup: fix cgroup_sk_alloc() for sk_clone_lock()

On 2020/6/20 8:51, Roman Gushchin wrote:
> On Fri, Jun 19, 2020 at 02:40:19PM +0800, Zefan Li wrote:
>> On 2020/6/19 5:09, Cong Wang wrote:
>>> On Thu, Jun 18, 2020 at 12:36 PM Roman Gushchin <guro@...com> wrote:
>>>>
>>>> On Thu, Jun 18, 2020 at 12:19:13PM -0700, Cong Wang wrote:
>>>>> On Wed, Jun 17, 2020 at 6:44 PM Zefan Li <lizefan@...wei.com> wrote:
>>>>>>
>>>>>> Cc: Roman Gushchin <guro@...com>
>>>>>>
>>>>>> Thanks for fixing this.
>>>>>>
>>>>>> On 2020/6/17 2:03, Cong Wang wrote:
>>>>>>> When we clone a socket in sk_clone_lock(), its sk_cgrp_data is
>>>>>>> copied, so the cgroup refcnt must be taken too. And, unlike the
>>>>>>> sk_alloc() path, sock_update_netprioidx() is not called here.
>>>>>>> Therefore, it is safe and necessary to grab the cgroup refcnt
>>>>>>> even when cgroup_sk_alloc is disabled.
>>>>>>>
>>>>>>> sk_clone_lock() is in BH context anyway, the in_interrupt()
>>>>>>> would terminate this function if called there. And for sk_alloc()
>>>>>>> skcd->val is always zero. So it's safe to factor out the code
>>>>>>> to make it more readable.
>>>>>>>
>>>>>>> Fixes: 090e28b229af92dc5b ("netprio_cgroup: Fix unlimited memory leak of v2 cgroups")
>>>>>>
>>>>>> but I don't think the bug was introduced by this commit, because there
>>>>>> are already calls to cgroup_sk_alloc_disable() in write_priomap() and
>>>>>> write_classid(), which can be triggered by writing to ifpriomap or
>>>>>> classid in cgroupfs. This commit just made it much easier to happen
>>>>>> with systemd invovled.
>>>>>>
>>>>>> I think it's 4bfc0bb2c60e2f4c ("bpf: decouple the lifetime of cgroup_bpf from cgroup itself"),
>>>>>> which added cgroup_bpf_get() in cgroup_sk_alloc().
>>>>>
>>>>> Good point.
>>>>>
>>>>> I take a deeper look, it looks like commit d979a39d7242e06
>>>>> is the one to blame, because it is the first commit that began to
>>>>> hold cgroup refcnt in cgroup_sk_alloc().
>>>>
>>>> I agree, ut seems that the issue is not related to bpf and probably
>>>> can be reproduced without CONFIG_CGROUP_BPF. d979a39d7242e06 indeed
>>>> seems closer to the origin.
>>>
>>> Yeah, I will update the Fixes tag and send V2.
>>>
>>
>> Commit d979a39d7242e06 looks innocent to me. With this commit when cgroup_sk_alloc
>> is disabled and then a socket is cloned the cgroup refcnt will not be incremented,
>> but this is fine, because when the socket is to be freed:
>>
>>  sk_prot_free()
>>    cgroup_sk_free()
>>      cgroup_put(sock_cgroup_ptr(skcd)) == cgroup_put(&cgrp_dfl_root.cgrp)
>>
>> cgroup_put() does nothing for the default root cgroup, so nothing bad will happen.
>>
>> but cgroup_bpf_put() will decrement the bpf refcnt while this refcnt were not incremented
>> as cgroup_sk_alloc has already been disabled. That's why I think it's 4bfc0bb2c60e2f4c
>> that needs to be fixed.
> 
> Hm, does it mean that the problem always happens with the root cgroup?
> 
>>>From the stacktrace provided by Peter it looks like that the problem
> is bpf-related, but the original patch says nothing about it.
> 
> So from the test above it sounds like the problem is that we're trying
> to release root's cgroup_bpf, which is a bad idea, I totally agree.
> Is this the problem?

I think so, though I'm not familiar with the bfp cgroup code.

> If so, we might wanna fix it in a different way,
> just checking if (!(css->flags & CSS_NO_REF)) in cgroup_bpf_put()
> like in cgroup_put(). It feels more reliable to me.
> 

Yeah I also have this idea in my mind.

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