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

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Date:   Mon, 24 Jun 2019 03:29:21 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Roman Gushchin <guro@...com>, Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Tejun Heo <tj@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
CC:     Kernel Team <Kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: fix cgroup bpf release synchronization

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 should have been empty with no tasks inside it?
Only some resources are still held?
mutex and zero init are highly suspicious.
It feels that cgroup_bpf_release is called too early.

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

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?