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

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Date:   Fri, 9 Jun 2017 20:24:30 +1000
From:   Aleksa Sarai <asarai@...e.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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

>>>> 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?
>>
>> I will add one, I didn't know where the man-pages project is hosted / where
>> patches get pushed? What is the ML?
> 
>  From the MAINTAINERS file:
>    MAN-PAGES: MANUAL PAGES FOR LINUX -- Sections 2, 3, 4, 5, and 7
>    M:      Michael Kerrisk <mtk.manpages@...il.com>
>    W:      http://www.kernel.org/doc/man-pages
>    L:      linux-man@...r.kernel.org
>    S:      Maintained

Ah, should've looked there first!

Thanks Greg, I'll send it over the weekend.

-- 
Aleksa Sarai
Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/