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

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Date:   Sat, 20 Jun 2020 09:04:03 -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 Sat, Jun 20, 2020 at 03:52:49PM +0800, Zefan Li wrote:
> 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.

Hm, so it seems that to disable the refcounting for the root cgroup's cgroup_bpf
is a good idea anyway, as it makes cgroup_bpf refcounting work similar to cgroup
refcounting. But it's not completely clear to me, if it's enough or do we have
something else to fix here?

Thanks!