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

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Message-ID: <CA+55aFzyo=pxBAt0FutjPQ4EkgAddE19DyQe7jD5Fz_WxcE-ug@mail.gmail.com>
Date:	Mon, 11 Apr 2016 17:50:49 -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 5:22 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> I meant the one where I conceded that the only think that it could
> possible protect against was a denial of service attack, from which we
> probably don't care.

Yeah, that's the same email I was talking about, I was just quoting
another part.

> As I agreed with you that it was unnecessary I was just puzzled why you
> called what was essentially agreement with you deafening silence.

The "deafening silence" was about _why_ this all would be a problem,
and why the security checks would be needed.

Basically, I think that if /dev/pts/ is accessible, we should just say
"ok, you can open a pty on it".

The fact that you could open a pty by bind-mounting it somewhere else,
and then adding a "ptmx" node to the same directory is not a security
issue: it's simply how devpts works.

In no actual sane and relevant situation is that a problem, for the
simple fact that there will *already* be a ptmx node that is
world-accessible in the same directory that has that /dev/pts/ mount.

Anything else is insane and irrelevant. This is *literally* what POSIX
says. Sure, POSIX also has that whole language about "posix_openpt()",
but that's just BS and irrelevant. The very page that mentions
"posix_openpt()" also says

  "On implementations supporting the /dev/ptmx clone device, opening
the master device of a pseudo-terminal is simply:

        mfdp = open("/dev/ptmx", oflag );
        if (mfdp < 0)
            return -1;"

and Linux unquestionably falls in that "supports /dev/ptmx" camp.

So I claim that the _only_ sane use of devpts is to already have a
world-accessible ptmx node there, and nothing else makes sense.

And if you want to be private, you had better make the whole /dev/
subdirectory private (which also takes care of any bind mount issues)

            Linus