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