Date:   Fri, 19 Jun 2020 18:14:09 -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 09:00:40AM +0800, Zefan Li wrote:
> On 2020/6/20 8:51, Roman Gushchin wrote:
> > 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?
> 
> I think so, though I'm not familiar with the bfp cgroup code.
> 
> > 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.
> > 
> 
> Yeah I also have this idea in my mind.

I wonder if the following patch will fix the issue?

--

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 4598e4da6b1b..7eb51137d896 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -942,12 +942,14 @@ static inline bool cgroup_task_frozen(struct task_struct *task)
 #ifdef CONFIG_CGROUP_BPF
 static inline void cgroup_bpf_get(struct cgroup *cgrp)
 {
-       percpu_ref_get(&cgrp->bpf.refcnt);
+       if (!(cgrp->self.flags & CSS_NO_REF))
+               percpu_ref_get(&cgrp->bpf.refcnt);
 }
 
 static inline void cgroup_bpf_put(struct cgroup *cgrp)
 {
-       percpu_ref_put(&cgrp->bpf.refcnt);
+       if (!(cgrp->self.flags & CSS_NO_REF))
+               percpu_ref_put(&cgrp->bpf.refcnt);
 }
 
 #else /* CONFIG_CGROUP_BPF */

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