netdev - Re: [Patch net] cgroup: fix cgroup_sk_alloc() for sk_clone

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Date:   Fri, 19 Jun 2020 14:40:19 +0800
From:   Zefan Li <lizefan@...wei.com>
To:     Cong Wang <xiyou.wangcong@...il.com>, Roman Gushchin <guro@...com>
CC:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "Cameron Berkenpas" <cam@...-zeon.de>,
        Peter Geis <pgwipeout@...il.com>,
        Lu Fengqi <lufq.fnst@...fujitsu.com>,
        Daniël 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 2020/6/19 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.

>>
>> Btw, based on the number of reported-by tags it seems that there was
>> a real issue which the patch is fixing. Maybe you'll a couple of words
>> about how it reveals itself in the real life?
> 
> I still have no idea how exactly this is triggered. According to the
> people who reported this bug, they just need to wait for some hours
> to trigger. So I am not sure what to add here, just the stack trace?
> 
> Thanks.
> .
>