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

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Date:   Fri, 19 Jun 2020 20:31:14 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Zefan Li <lizefan@...wei.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()

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).

Thanks.