linux-kernel - Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup

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Date:	Fri, 8 Apr 2016 12:05:56 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Greg KH <greg@...ah.com>, Jiri Slaby <jslaby@...e.com>,
	Aurelien Jarno <aurelien@...el32.net>,
	Andy Lutomirski <luto@...capital.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Jann Horn <jann@...jh.net>,
	"security@...nel.org" <security@...nel.org>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	security@...ian.org, Willy Tarreau <w@....eu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated
 devpts via path lookup

On Fri, Apr 8, 2016 at 11:51 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> Given that concern under the rule we don't break userspace we have to
> check the permissions of /dev/pts/ptmx when we are creating a new pty,
> on a instance of devpts that was created with newinstance.

The rule is that we don't break existing installations.

If somebody has root and installs a "ptmx" node in an existing mount
space next to a pts subdirectory, that's not a security issue, nor is
it going to break any existing installation.

The whole point of the patch is that yes, we change semantics. A
change of semantics means that people will see situations where the
behavior is different. But that's not "breaking user space", that's
just "ok, you can see a difference".

                Linus