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

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Date:   Sat, 20 Jun 2020 15:52:49 +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 11:31, Cong Wang 写道:
> On Fri, Jun 19, 2020 at 5:51 PM Zefan Li <lizefan@...wei.com> wrote:
>>
>> 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.
> 
> I am totally aware of this. My concern is whether cgroup
> has the same refcnt bug as it always pairs with the bpf refcnt.
> 
> But, after a second look, the non-root cgroup refcnt is immediately
> overwritten by sock_update_classid() or sock_update_netprioidx(),
> which effectively turns into a root cgroup again. :-/
> 
> (It seems we leak a refcnt here, but this is not related to my patch).
> 

Indead, but it's well known, see bd1060a1d67128bb8fbe2. But now bpf cgroup comes into play...

Your patch doesn't seem to fix the bug completely. If cgroup_sk_alloc_disable happens after
socket cloning, then we will deref the bpf of the root cgroup while incref-ed the bpf of a
non-root cgroup.