Re: [PATCH] cgroup: serialize css kill and release paths

[Tadeusz Struk <tadeusz.struk@linaro.org>]

On 6/3/22 10:34, Tadeusz Struk wrote:
> Syzbot found a corrupted list bug scenario that can be triggered from
> cgroup_subtree_control_write(cgrp). The reproduces writes to
> cgroup.subtree_control file, which invokes:
> cgroup_apply_control_enable()->css_create()->css_populate_dir(), which
> then fails with a fault injected -ENOMEM.
> In such scenario the css_killed_work_fn will be en-queued via
> cgroup_apply_control_disable(cgrp)->kill_css(css), and bail out to
> cgroup_kn_unlock(). Then cgroup_kn_unlock() will call:
> cgroup_put(cgrp)->css_put(&cgrp->self), which will try to enqueue
> css_release_work_fn for the same css instance, causing a list_add
> corruption bug, as can be seen in the syzkaller report [1].
> 
> Fix this by synchronizing the css ref_kill and css_release jobs.
> css_release() function will check if the css_killed_work_fn() has been
> scheduled for the css and only en-queue the css_release_work_fn()
> if css_killed_work_fn wasn't already en-queued. Otherwise css_release() will
> set the CSS_REL_LATER flag for that css. This will cause the css_release_work_fn()
> work to be executed after css_killed_work_fn() is finished.
> 
> Two scc flags have been introduced to implement this serialization mechanizm:
> 
>   * CSS_KILL_ENQED, which will be set when css_killed_work_fn() is en-queued, and
>   * CSS_REL_LATER, which, if set, will cause the css_release_work_fn() to be
>     scheduled after the css_killed_work_fn is finished.
> 
> There is also a new lock, which will protect the integrity of the css flags.
> 
> [1]https://syzkaller.appspot.com/bug?id=e26e54d6eac9d9fb50b221ec3e4627b327465dbd
> 
> Cc: Tejun Heo<tj@kernel.org>
> Cc: Michal Koutny<mkoutny@suse.com>
> Cc: Zefan Li<lizefan.x@bytedance.com>
> Cc: Johannes Weiner<hannes@cmpxchg.org>
> Cc: Christian Brauner<brauner@kernel.org>
> Cc: Alexei Starovoitov<ast@kernel.org>
> Cc: Daniel Borkmann<daniel@iogearbox.net>
> Cc: Andrii Nakryiko<andrii@kernel.org>
> Cc: Martin KaFai Lau<kafai@fb.com>
> Cc: Song Liu<songliubraving@fb.com>
> Cc: Yonghong Song<yhs@fb.com>
> Cc: John Fastabend<john.fastabend@gmail.com>
> Cc: KP Singh<kpsingh@kernel.org>
> Cc:<cgroups@vger.kernel.org>
> Cc:<netdev@vger.kernel.org>
> Cc:<bpf@vger.kernel.org>
> Cc:<stable@vger.kernel.org>
> Cc:<linux-kernel@vger.kernel.org>
> 
> Reported-and-tested-by:syzbot+e42ae441c3b10acf9e9d@syzkaller.appspotmail.com
> Fixes: 8f36aaec9c92 ("cgroup: Use rcu_work instead of explicit rcu and work item")
> Signed-off-by: Tadeusz Struk<tadeusz.struk@linaro.org>

I just spotted an issue with this. I'm holding invalid lock in css_killed_work_fn().
I will follow up with a v2 of the patch soon.

-- 
Thanks,
Tadeusz