linux-kernel - Re: [PATCH 1/2 v2] pty: fix possible use after free of tty->driver

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Message-ID: <20160113182844.GB8385@cloud>
Date:	Wed, 13 Jan 2016 10:28:44 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	"Herton R. Krzesinski" <herton@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...ux.intel.com>,
	Jiri Slaby <jslaby@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 1/2 v2] pty: fix possible use after free of
 tty->driver_data

On Wed, Jan 13, 2016 at 09:39:29AM -0800, Peter Hurley wrote:
> On 01/11/2016 06:07 AM, Herton R. Krzesinski wrote:
> > This change fixes a bug for a corner case where we have the the last
> > release from a pty master/slave coming from a previously opened /dev/tty
> > file. When this happens, the tty->driver_data can be stale, due to all
> > ptmx or pts/N files having already been closed before (and thus the inode
> > related to these files, which tty->driver_data points to, being already
> > freed/destroyed).
> > 
> > The fix here is to keep a reference on the opened master ptmx inode.
> > We maintain the inode referenced until the final pty_unix98_shutdown,
> > and only pass this inode to devpts_kill_index.
> 
> Ideally, the tty core should be bumping the inode count for the underlying
> controlling tty

That does indeed sound like the right fix.  /dev/tty doesn't act exactly
like opening the underlying device (as it also supports the TIOCNOTTY
ioctl), but it should definitely hold a reference to that underlying
device.