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

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Date:   Fri, 3 Jul 2020 09:17:09 +0800
From:   Zefan Li <lizefan@...wei.com>
To:     Roman Gushchin <guro@...com>, Cong Wang <xiyou.wangcong@...il.com>
CC:     Cameron Berkenpas <cam@...-zeon.de>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        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 2020/7/3 0:02, Roman Gushchin wrote:
> On Wed, Jul 01, 2020 at 09:48:48PM -0700, Cong Wang wrote:
>> On Tue, Jun 30, 2020 at 3:48 PM Roman Gushchin <guro@...com> wrote:
>>>
>>> Btw if we want to backport the problem but can't blame a specific commit,
>>> we can always use something like "Cc: <stable@...r.kernel.org>    [3.1+]".
>>
>> Sure, but if we don't know which is the right commit to blame, then how
>> do we know which stable version should the patch target? :)
>>
>> I am open to all options here, including not backporting to stable at all.
> 
> It seems to be that the issue was there from bd1060a1d671 ("sock, cgroup: add sock->sk_cgroup"),
> so I'd go with it. Otherwise we can go with 5.4+, as I understand before that it was
> hard to reproduce it.
> 

Actually I think it should be very easy to reproduce the bug.

suppose default cgroup and netcls cgroup are mounted in /cgroup/default and
/cgroup/netcls respectively, and then:

1. mkdir /cgroup/default/sub1
2. mkdir /cgroup/default/sub2
3. attach some tasks into sub1/ and sub2/
4. attach bpf program to sub1/ and sub2/ # get bpf refcnt for those cgroups
5. echo 1 > /cgroup/netcls/classid       # this will disable cgroup_sk_alloc
6. kill all tasks in sub1/ and sub2/
7. rmdir sub1/ sub2/

The last step will deref bpf for the default root cgroup instead of sub1/
and sub2/, and should trigger the bug.

FYI I never use bpf, so I might be wrong.