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

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Message-ID: <FC521A3F-4BEB-427E-AE8E-CA54C0128AA3@zytor.com>
Date:	Sat, 09 Apr 2016 17:06:52 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	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 April 9, 2016 5:01:27 PM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Sat, Apr 9, 2016 at 3:37 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> On the flipside, if we were to allow ourselves to break userspace, at
>this point I would suggest making /dev/pts/ptmx have a different device
>number and make the legacy /dev/ptmx print a warning message, after
>which it can at least eventually be deleted.
>
>You don't need a different device number.
>
>The /dev/pts/ptmx file may look like it's the same node as /dev/ptmx,
>but it is trivial to recognize as the pts one:
>
>     if (dentry->d_sb->s_magic == DEVPTS_SUPER_MAGIC)
>
>and you're done.
>
>But nobody actually uses /dev/pts/ptmx, because it has never had sane
>permissions.
>
>So the fact is, /dev/ptmx is what people use, and we're not breaking
>userspace.
>
>But when we fix bad semantics (and always just looking up the initial
>pts mount really is crazy semantics) that doesn't mean that we have to
>bend over backwards to not make the changed semantics visible. We
>don't _break_ user space, but we also don't care about some random
>test-program that checks for particular semantics.
>
>And I can pretty much _guarantee_ that nobody has ever done the "let's
>bind-mount a 'ptmx' node in a /dev directory, and then expect that to
>bind to some _other_ pts thing than the one in /dev/pts/".
>
>Except as a test-program, or possibly as a "why the f*ck doesn't this
>work? Oh, I need to use the single-instance thing because the
>multi-instance pts thing is broken. Damn shitty implementation".
>
>                Linus

Fixing the default permissions is trivial, of course.  The intent from the beginning was to make a ptmx -> pts/ptmx, but user space never did...
-- 
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