linux-kernel - Re: [PATCH 01/10] nbd: Fix timeout detection

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Date:	Thu, 1 Oct 2015 08:04:36 +0200
From:	Markus Pargmann <mpa@...gutronix.de>
To:	Ben Hutchings <ben@...adent.org.uk>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	nbd-general@...ts.sourceforge.net, kernel@...gutronix.de,
	Michal Belczyk <belczyk@....krakow.pl>,
	Hermann Lauer <Hermann.Lauer@....uni-heidelberg.de>
Subject: Re: [PATCH 01/10] nbd: Fix timeout detection

Hi,

On Mon, Sep 28, 2015 at 01:27:44AM +0100, Ben Hutchings wrote:
> On Mon, 2015-08-17 at 08:20 +0200, Markus Pargmann wrote:
> > At the moment the nbd timeout just detects hanging tcp operations. This
> > is not enough to detect a hanging or bad connection as expected of a
> > timeout.
> > 
> > This patch redesigns the timeout detection to include some more cases.
> > The timeout is now in relation to replies from the server. If the server
> > does not send replies within the timeout the connection will be shut
> > down.
> > 
> > The patch adds a continous timer 'timeout_timer' that is setup in one of
> > two cases:
> >  - The request list is empty and we are sending the first request out to
> >    the server. We want to have a reply within the given timeout,
> >    otherwise we consider the connection to be dead.
> >  - A server response was received. This means the server is still
> >    communicating with us. The timer is reset to the timeout value.
> > 
> > The timer is not stopped if the list becomes empty. It will just trigger
> > a timeout which will directly leave the handling routine again as the
> > request list is empty.
> > 
> > The whole patch does not use any additional explicit locking. The
> > list_empty() calls are safe to be used concurrently. The timer is locked
> > internally as we just use mod_timer and del_timer_sync().
> 
> This is crazy.  The timer is locked internally but the tasks are not.
> So it is possible for the timeout handler to kill a task after it
> exited from nbd_do_it()/nbd_thread_recv(), or after it exited entirely
> (use-after-free).

Indeed, thanks. I am working on a patch to fix this issue.

Best Regards,

Markus

> 
> [...]
> > +> 	> task = READ_ONCE(nbd->task_send);
> > +> 	> if (task)
> > +> 	> 	> force_sig(SIGKILL, nbd->task_send);
> [...]
> 
> And this is just... what?  What is the point of using READ_ONCE() if
> you're going to look up nbd->task_send again?
> 
> Ben.
> 
> -- 
> Ben Hutchings
> All extremists should be taken out and shot.



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