linux-kernel - Re: [PATCH 1/1] kthread: introduce kthread_get_run() to fix __nbd

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Message-ID: <20151027002642.GA32444@infradead.org>
Date:	Mon, 26 Oct 2015 17:26:42 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Markus Pargmann <mpa@...gutronix.de>,
	Tejun Heo <tj@...nel.org>, nbd-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] kthread: introduce kthread_get_run() to fix
 __nbd_ioctl()

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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