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+55aFw62q5bjNiK7Q9dEQQEDZs5fbUxNPxDq376OZJ6JKT=Rg@mail.gmail.com>
Date:	Sat, 9 Apr 2016 17:16:51 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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 Sat, Apr 9, 2016 at 5:06 PM, H. Peter Anvin <hpa@...or.com> wrote:
>
> 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...

That wasn't my point.

Because the permissions have never been usable, I pretty much
guarantee that no current user space uses /dev/pts/ptmx.

So that node is almost entirely irrelevant. Us fixing the permissions
at this point isn't going to make it any more relevant, we might as
well ignore it.

Which all means that the way forward really is to just make /dev/ptmx
work. It's not going away, and it _is_ fairly easy to fix.

But I don't think the fix should care about permissions - and we might
as well leave the existing pts/ptmx node with broken permissions.
Because we've never been actually interested in looking up
/dev/pts/ptmx - all we actually care about is to look up which devpts
instance it is.

And that's not about the ptmx node, that's really about the
mount-point. So the right thing to do - conceptually - is *literally*
to just say "ok, what is mounted at 'pts'". Note how at no point do we
want to _open_ anything.

That's why I said that conceptually we could just open /proc/mounts.
Because *that* is really the operation we care about. We don't care
about lookup, and we don't care about permissions on the ptmx node.
Those are completely and utterly irrelevant to what we're actually
after.

So I think the permission thing is not just extra code with more
failure points. I think it's conceptually entirely the wrong thing to
do, and just confuses people into thinking that we're doing something
that we aren't.

                Linus