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

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Date:	Mon, 11 Apr 2016 17:08:25 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security@...nel.org" <security@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Willy Tarreau <w@....eu>,
	Aurelien Jarno <aurelien@...el32.net>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jann Horn <jann@...jh.net>, Greg KH <greg@...ah.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.com>,
	Florian Weimer <fw@...eb.enyo.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated
 devpts via path lookup

On Mon, Apr 11, 2016 at 4:49 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> I replied earlier.  Did you not see my reply?

Are you talking about the one where you agreed that the scenario was
made up and insane? The one where you said that you're worried about
breaking out "extension" where ptmx is non-0666?

That was never an extension. It was a simple situation of people (a)
not knowing what the tty group should be in the kernel and (b) then
thinking that using a permission model of "no permission" somehow made
it saner.

What it actually resulted in was that most distros just ignore it
entirely, and just use /dev/ptmx.

Yes, you *can* then chmod it in user space and use a symlink, but so
what? Nobody who actually uses that node uses anythinig but 0666.
Because that would break pretty much everything that uses pty's.

So the whole "we need to worry about permission 0000" is complete and
uttter garbage. We really don't. The situation doesn't come up, and
it's not relevant. The standard part to access ptmx is /dev/ptmx, and
no amount of wishing it were otherwise will make it any different.

Seriously. Just look at the opengroup documentation. It talks about
/dev/ptmx. The whole /dev/pts/ptmx thing was a mistake. WE SHOULD NOT
EXTEND ON THAT MISTAKE.

We should just FIX the mistake. Ignore /dev/pts/ptmx, because that
node is non-standard SHIT.

Really. Really really.

                 Linus