Re: [PATCH 1/1] kthread: introduce kthread_get_run() to fix __nbd_ioctl()

[Christoph Hellwig]

On Sun, Oct 25, 2015 at 03:27:13PM +0100, Oleg Nesterov wrote:
> It is not safe to use the task_struct returned by kthread_run(threadfn)
> if threadfn() can exit before the "owner" does kthread_stop(), nothing
> protects this task_struct.
> 
> So __nbd_ioctl() looks buggy; a killed nbd_thread_send() can exit, free
> its task_struct, and then kthread_stop() can use the freed/reused memory.
> 
> Add the new trivial helper, kthread_get_run(). Hopefully it will have more
> users, this patch changes __nbd_ioctl() as an example.

This looks horrible.  I think the real problem is that nbd is totally
abusing signals for kthreads and that needs to go away.
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