linux-kernel - Re: pty: fix use after free/oops at pty_unix98

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Message-ID: <56704DBC.4080503@hurleysoftware.com>
Date:	Tue, 15 Dec 2015 09:28:28 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	"Herton R. Krzesinski" <herton@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>
Subject: Re: pty: fix use after free/oops at pty_unix98_shutdown

On 12/15/2015 08:36 AM, Herton R. Krzesinski wrote:
> On Tue, Dec 15, 2015 at 08:17:56AM -0800, Peter Hurley wrote:
>>> I also expect in a rare case where all ptmx references are gone/closed, this also
>>> could happen on final close when the master tty is given to pty_unix98_shutdown.
>>
>> This logic I'm not following. If the pty master is being released, then the inode
>> is valid for the release() operation in-progress.
> 
> Hi Peter,
> 
> yes, you're right if you are eg. closing the /dev/ptmx or /dev/pts/ptmx file
> previously opened. But I thought and refer above to the case where for example
> you are closing /dev/tty and that's the final close and there is no other
> process in the system with the /dev/{,*/}ptmx opened, the inode which referenced
> the previously opened ptmx could be gone. It would be rare though since in a
> running system any logged in user eg. through ssh or with a terminal open in X
> will have at least a ptmx device opened.

/dev/tty can never be an alias for /dev/ptmx in Linux: a master pty cannot be
a controlling terminal. So if the master pty is being released it will always be
with the /dev/ptmx inode.

Regards,
Peter Hurley

PS - for the purpose of this discussion, /dev/pts/ptmx is equivalent.
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