Re: [Patch net] cgroup: fix cgroup_sk_alloc() for sk_clone_lock()

[Roman Gushchin <guro@fb.com>]

On Fri, Jun 19, 2020 at 08:31:14PM -0700, Cong Wang wrote:
> On Fri, Jun 19, 2020 at 5:51 PM Zefan Li <lizefan@huawei.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@huawei.com> wrote:
> > >>>
> > >>> On 2020/6/19 5:09, Cong Wang wrote:
> > >>>> On Thu, Jun 18, 2020 at 12:36 PM Roman Gushchin <guro@fb.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@huawei.com> wrote:
> > >>>>>>>
> > >>>>>>> Cc: Roman Gushchin <guro@fb.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).

Yeah, I looked over this code, and I have the same suspicion.
Especially in sk_alloc(), where cgroup_sk_alloc() is followed by
sock_update_classid() and sock_update_netprioidx().

I also think your original patch is good, but there are probably
some other problems which it doesn't fix.

I looked over cgroup bpf code again, and the only difference with cgroup
refcounting I see (behind the root cgroup, which is a non-issue) is
here:

void cgroup_sk_alloc(struct sock_cgroup_data *skcd)
{
	...
	while (true) {
		struct css_set *cset;

		cset = task_css_set(current);
		if (likely(cgroup_tryget(cset->dfl_cgrp))) {
			  ^^^^^^^^^^^^^^^
			skcd->val = (unsigned long)cset->dfl_cgrp;
			cgroup_bpf_get(cset->dfl_cgrp);
			^^^^^^^^^^^^^^
			break;
			...

So, in theory, cgroup_bpf_get() can be called here after cgroup_bpf_release().
We might wanna introduce something like cgroup_bpf_tryget_live().
Idk if it can happen in reality, because it would require opening a new socket
in a deleted cgroup (without any other associated sockets).

Other than that I don't see any differences between cgroup and cgroup bpf
reference counting.

Thanks!

PS I'll be completely offline till the end of the week. I'll answer all
e-mails on Monday (Jun 29th).