Re: Kernel crash in cgroup_pidlist_destroy_work_fn()

From: Li Zefan
Date: Wed Sep 17 2014 - 05:26:20 EST

On 2014/9/17 13:29, Li Zefan wrote:
> On 2014/9/17 7:56, Cong Wang wrote:
>> Hi, Tejun
>> We saw some kernel null pointer dereference in
>> cgroup_pidlist_destroy_work_fn(), more precisely at
>> __mutex_lock_slowpath(), on 3.14. I can show you the full stack trace
>> on request.
> Yes, please.
>> Looking at the code, it seems flush_workqueue() doesn't care about new
>> incoming works, it only processes currently pending ones, if this is
>> correct, then we could have the following race condition:
>> cgroup_pidlist_destroy_all():
>> //...
>> mutex_lock(&cgrp->pidlist_mutex);
>> list_for_each_entry_safe(l, tmp_l, &cgrp->pidlists, links)
>> mod_delayed_work(cgroup_pidlist_destroy_wq,
>> &l->destroy_dwork, 0);
>> mutex_unlock(&cgrp->pidlist_mutex);
>> // <--- another process calls cgroup_pidlist_start() here
>> since mutex is released
>> flush_workqueue(cgroup_pidlist_destroy_wq); // <--- another
>> process adds new pidlist and queue work in pararell
>> BUG_ON(!list_empty(&cgrp->pidlists)); // <--- This check is
>> passed, list_add() could happen after this
> Did you confirm this is what happened when the bug was triggered?
> I don't think the race condition you described exists. In 3.14 kernel,
> cgroup_diput() won't be called if there is any thread running
> cgroup_pidlist_start(). This is guaranteed by vfs.
> But newer kernels are different. Looks like the bug exists in those
> kernels.

Newer kernels should be also fine.

If cgroup_pidlist_destroy_all() is called, it means kernfs has already
removed the tasks file, and even if you still have it opened, when
you try to read it, it will immediately return an errno.

fd = open(cgrp/tasks)
read(fd of cgrp/tasks)
return -ENODEV

So cgroup_pidlist_destroy_all() won't race with cgroup_pidlist_start().

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