Re: [PATCH] cgroup: don't queue css_release_work if one already pending

["Michal Koutný" <mkoutny@suse.com>]

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@linaro.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