Date:   Thu, 25 Jun 2020 22:23:53 -0700
From:   Cameron Berkenpas <cam@...-zeon.de>
To:     Roman Gushchin <guro@...com>, Cong Wang <xiyou.wangcong@...il.com>
Cc:     Zefan Li <lizefan@...wei.com>,
        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()

Hello,

Somewhere along the way I got the impression that it generally takes 
those affected hours before their systems lock up. I'm (generally) able 
to reproduce this issue much faster than that. Regardless, I can help test.

Are there any patches that need testing or is this all still pending 
discussion around the  best way to resolve the issue?

Thanks!

On 6/23/20 3:21 PM, Roman Gushchin wrote:
> On Fri, Jun 19, 2020 at 08:31:14PM -0700, Cong Wang wrote:
>> 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).
> Yeah, I looked over this code, and I have the same suspicion.
> Especially in sk_alloc(), where cgroup_sk_alloc() is followed by
> sock_update_classid() and sock_update_netprioidx().
>
> I also think your original patch is good, but there are probably
> some other problems which it doesn't fix.
>
> I looked over cgroup bpf code again, and the only difference with cgroup
> refcounting I see (behind the root cgroup, which is a non-issue) is
> here:
>
> void cgroup_sk_alloc(struct sock_cgroup_data *skcd)
> {
> 	...
> 	while (true) {
> 		struct css_set *cset;
>
> 		cset = task_css_set(current);
> 		if (likely(cgroup_tryget(cset->dfl_cgrp))) {
> 			  ^^^^^^^^^^^^^^^
> 			skcd->val = (unsigned long)cset->dfl_cgrp;
> 			cgroup_bpf_get(cset->dfl_cgrp);
> 			^^^^^^^^^^^^^^
> 			break;
> 			...
>
> So, in theory, cgroup_bpf_get() can be called here after cgroup_bpf_release().
> We might wanna introduce something like cgroup_bpf_tryget_live().
> Idk if it can happen in reality, because it would require opening a new socket
> in a deleted cgroup (without any other associated sockets).
>
> Other than that I don't see any differences between cgroup and cgroup bpf
> reference counting.
>
> Thanks!
>
> PS I'll be completely offline till the end of the week. I'll answer all
> e-mails on Monday (Jun 29th).

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