linux-kernel - Re: [PATCH v4 2/2] tty: add TIOCGPTPEER ioctl

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Message-ID: <20170609092659.GA26933@kroah.com>
Date:   Fri, 9 Jun 2017 11:26:59 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Aleksa Sarai <asarai@...e.de>
Cc:     Jiri Slaby <jslaby@...e.com>, Arnd Bergmann <arnd@...db.de>,
        linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-mips@...ux-mips.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linux-arch@...r.kernel.org,
        Christian Brauner <christian.brauner@...ntu.com>,
        Valentin Rothberg <vrothberg@...e.com>
Subject: Re: [PATCH v4 2/2] tty: add TIOCGPTPEER ioctl

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 <christian.brauner@...ntu.com>
> Cc: Valentin Rothberg <vrothberg@...e.com>
> Signed-off-by: Aleksa Sarai <asarai@...e.de>

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