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

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Date:   Sat, 20 Jun 2020 08:51:31 +0800
From:   Zefan Li <lizefan@...wei.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
CC:     Roman Gushchin <guro@...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>,
        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()

在 2020/6/20 8:45, Zefan Li 写道:
> On 2020/6/20 3:51, Cong Wang wrote:
>> On Thu, Jun 18, 2020 at 11:40 PM Zefan Li <lizefan@...wei.com> 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 skcd->val can be a pointer to a non-root cgroup:
> 
> It returns a non-root cgroup when cgroup_sk_alloc is not disabled. The bug happens
> when cgroup_sk_alloc is disabled.
> 

And please read those recent bug reports, they all happened when bpf cgroup was in use,
and there was no bpf cgroup when d979a39d7242e06 was merged into mainline.