linux-kernel - Re: [PATCH] cgroup: don't queue css_release

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Message-ID: <20220414164409.GA5404@blackbody.suse.cz>
Date:   Thu, 14 Apr 2022 18:44:09 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Tadeusz Struk <tadeusz.struk@...aro.org>
Cc:     cgroups@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Christian Brauner <brauner@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        syzbot+e42ae441c3b10acf9e9d@...kaller.appspotmail.com
Subject: Re: [PATCH] cgroup: don't queue css_release_work if one already
 pending

Hello Tadeusz.

Thanks for analyzing this syzbot report. Let me provide my understanding
of the test case and explanation why I think your patch fixes it but is
not fully correct.

On Tue, Apr 12, 2022 at 12:24:59PM -0700, Tadeusz Struk <tadeusz.struk@...aro.org> wrote:
> Syzbot found a corrupted list bug scenario that can be triggered from
> cgroup css_create(). The reproduces writes to cgroup.subtree_control
> file, which invokes cgroup_apply_control_enable(), css_create(), and
> css_populate_dir(), which then randomly fails with a fault injected -ENOMEM.

The reproducer code makes it hard for me to understand which function
fails with ENOMEM.
But I can see your patch fixes the reproducer and your additional debug
patch which proves that css->destroy_work is re-queued.

> In such scenario the css_create() error path rcu enqueues css_free_rwork_fn
> work for an css->refcnt initialized with css_release() destructor,

Note that css_free_rwork_fn() utilizes css->destroy_*r*work.
The error path in css_create() open codes relevant parts of
css_release_work_fn() so that css_release() can be skipped and the
refcnt is eventually just percpu_ref_exit()'d.

> and there is a chance that the css_release() function will be invoked
> for a cgroup_subsys_state, for which a destroy_work has already been
> queued via css_create() error path.

But I think the problem is css_populate_dir() failing in
cgroup_apply_control_enable(). (Is this what you actually meant?
css_create() error path is then irrelevant, no?)

The already created csses should then be rolled back via 
	cgroup_restore_control(cgrp);
	cgroup_apply_control_disable(cgrp);
	   ...
	   kill_css(css)

I suspect the double-queuing is a result of the fact that there exists
only the single reference to the css->refcnt. I.e. it's
percpu_ref_kill_and_confirm()'d and released both at the same time.

(Normally (when not killing the last reference), css->destroy_work reuse
is not a problem because of the sequenced chain
css_killed_work_fn()->css_put()->css_release().)

> This can be avoided by adding a check to css_release() that checks
> if it has already been enqueued.

If that's what's happening, then your patch omits the final
css_release_work_fn() in favor of css_killed_work_fn() but both should
be run during the rollback upon css_populate_dir() failure.

So an alternative approach to tackle this situation would be to split
css->destroy_work into two work work_structs (one for killing, one for
releasing) at the cost of inflating cgroup_subsys_state.

Take my hypothesis with a grain of salt maybe the assumption (last
reference == initial reference) is not different from normal operation.

Regards,
Michal