netdev - Re: use-after-free in sixpack

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Date:	Fri, 18 Dec 2015 22:02:04 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	David Miller <davem@...emloft.net>
Cc:	dvyukov@...gle.com, ajk@...nets.uni-bremen.de,
	linux-hams@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
	jslaby@...e.com, syzkaller@...glegroups.com, kcc@...gle.com,
	glider@...gle.com, sasha.levin@...cle.com, edumazet@...gle.com
Subject: Re: use-after-free in sixpack_close

> > Also you are at the point the tty is closing so the net device may be
> > active. Don't you need to netif_stop_queue() or defer the buffer
> > kfrees until after the network device is unregistered so you don't pee
> > into free memory if you have a transmit occurring ?
> 
> I'm pretty sure that's what the semaphore down above this sequence is
> accomplishing.  But if we do need the netif_stop_queue() let's do that
> as a separate patch.

Follow the code path for sp_xmit(). If sp_xmit is called it digs out sp
from the ndetdev, locks sp->lock and stops the queue then calls sp_encaps
which touches sp->xbuff.

So if one thread of execution hits sp_xmit and another closes the ldisc
at just the wrong moment then we have no protection.

Alan
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