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

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Date:   Thu, 18 Jun 2020 12:36:11 -0700
From:   Roman Gushchin <guro@...com>
To:     Cong Wang <xiyou.wangcong@...il.com>
CC:     Zefan Li <lizefan@...wei.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 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.

Btw, based on the number of reported-by tags it seems that there was
a real issue which the patch is fixing. Maybe you'll a couple of words
about how it reveals itself in the real life?

Thanks!