Subject: Re: [PATCH v4 2/2] tty: add TIOCGPTPEER ioctl

From: Greg Kroah-Hartman <[email-masked]>
Date: Fri, 9 Jun 2017 11:26:59 +0200

On Sun, Jun 04, 2017 at 12:15:15AM +1000, Aleksa Sarai wrote:
> When opening the slave end of a PTY, it is not possible for userspace to
> safely ensure that /dev/pts/$num is actually a slave (in cases where the
> mount namespace in which devpts was mounted is controlled by an
> untrusted process). In addition, there are several unresolvable
> race conditions if userspace were to attempt to detect attacks through
> stat(2) and other similar methods [in addition it is not clear how
> userspace could detect attacks involving FUSE].
> 
> Resolve this by providing an interface for userpace to safely open the
> "peer" end of a PTY file descriptor by using the dentry cached by
> devpts. Since it is not possible to have an open master PTY without
> having its slave exposed in /dev/pts this interface is safe. This
> interface currently does not provide a way to get the master pty (since
> it is not clear whether such an interface is safe or even useful).
> 
> Cc: Christian Brauner <[email-masked]>
> Cc: Valentin Rothberg <[email-masked]>
> Signed-off-by: Aleksa Sarai <[email-masked]>

Is this going to be documented anywhere?  Is there a man page update
that also goes along with this?  What userspace program wants to use
this?

I'm not objecting to this, I just want to know that people will use
this, and that they can find out information about it if they want to.

thanks,

greg k-h