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

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Date:   Fri, 19 Jun 2020 17:51:15 -0700
From:   Roman Gushchin <guro@...com>
To:     Zefan Li <lizefan@...wei.com>
CC:     Cong Wang <xiyou.wangcong@...il.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()

On Fri, Jun 19, 2020 at 02:40:19PM +0800, Zefan Li 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 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.

Hm, does it mean that the problem always happens with the root cgroup?

>From the stacktrace provided by Peter it looks like that the problem
is bpf-related, but the original patch says nothing about it.

So from the test above it sounds like the problem is that we're trying
to release root's cgroup_bpf, which is a bad idea, I totally agree.
Is this the problem? If so, we might wanna fix it in a different way,
just checking if (!(css->flags & CSS_NO_REF)) in cgroup_bpf_put()
like in cgroup_put(). It feels more reliable to me.

Thanks!