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

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Date:   Fri, 19 Jun 2020 12:51:00 -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 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:

static inline struct cgroup *sock_cgroup_ptr(struct sock_cgroup_data *skcd)
{
#if defined(CONFIG_CGROUP_NET_PRIO) || defined(CONFIG_CGROUP_NET_CLASSID)
        unsigned long v;

        /*
         * @skcd->val is 64bit but the following is safe on 32bit too as we
         * just need the lower ulong to be written and read atomically.
         */
        v = READ_ONCE(skcd->val);

        if (v & 1)
                return &cgrp_dfl_root.cgrp;

        return (struct cgroup *)(unsigned long)v ?: &cgrp_dfl_root.cgrp;
#else
        return (struct cgroup *)(unsigned long)skcd->val;
#endif
}