Re: [PATCH bpf-next] bpf: fix cgroup bpf release synchronization

[Roman Gushchin <guro@fb.com>]

On Sun, Jun 23, 2019 at 08:29:21PM -0700, Alexei Starovoitov wrote:
> On 6/23/19 7:30 PM, Roman Gushchin wrote:
> > Since commit 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf
> > from cgroup itself"), cgroup_bpf release occurs asynchronously
> > (from a worker context), and before the release of the cgroup itself.
> > 
> > This introduced a previously non-existing race between the release
> > and update paths. E.g. if a leaf's cgroup_bpf is released and a new
> > bpf program is attached to the one of ancestor cgroups at the same
> > time. The race may result in double-free and other memory corruptions.
> > 
> > To fix the problem, let's protect the body of cgroup_bpf_release()
> > with cgroup_mutex, as it was effectively previously, when all this
> > code was called from the cgroup release path with cgroup mutex held.
> > 
> > Also make sure, that we don't leave already freed pointers to the
> > effective prog arrays. Otherwise, they can be released again by
> > the update path. It wasn't necessary before, because previously
> > the update path couldn't see such a cgroup, as cgroup_bpf and cgroup
> > itself were released together.
> 
> I thought dying cgroup won't have any children cgroups ?

It's not completely true, a dying cgroup can't have living children.

> It should have been empty with no tasks inside it?

Right.

> Only some resources are still held?

Right.

> mutex and zero init are highly suspicious.
> It feels that cgroup_bpf_release is called too early.

An alternative solution is to bump the refcounter on
every update path, and explicitly skip de-bpf'ed cgroups.

> 
> Thinking from another angle... if child cgroups can still attach then
> this bpf_release is broken.

Hm, what do you mean under attach? It's not possible to attach
a new prog, but if a prog is attached to a parent cgroup,
a pointer can spill through "effective" array.

But I agree, it's broken. Update path should ignore such
cgroups (cgroups, which cgroup_bpf was released). I'll take a look.

> The code should be
> calling __cgroup_bpf_detach() one by one to make sure
> update_effective_progs() is called, since descendant are still
> sort-of alive and can attach?

Not sure I get you. Dying cgroup is a leaf cgroup.

> 
> My money is on 'too early'.
> May be cgroup is not dying ?
> Just cgroup_sk_free() is called on the last socket and
> this auto-detach logic got triggered incorrectly?

So, once again, what's my picture:

A
A/B
A/B/C

cpu1:                               cpu2:
rmdir C                             attach new prog to A
C got dying                         update A, update B, update C...
C's cgroup_bpf is released          C's effective progs is replaced with new one
                                    old is double freed

It looks like it can be reproduced without any sockets.

Thanks!