linux-kernel - Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

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Date:	Sat, 19 Dec 2015 22:11:50 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg KH <greg@...ah.com>, Jiri Slaby <jslaby@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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\@kernel.org" <security@...nel.org>,
	"security\@ubuntu.com \>\> security" <security@...ntu.com>,
	security@...ian.org, Willy Tarreau <w@....eu>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

ebiederm@...ssion.com (Eric W. Biederman) writes:

>>> In that system ptys simply did not work after boot when I tested
>>> associating /dev/ptmx with the first mount of the devpts filesystem.
>>
>> Assuming userspace isn't broken by that patch, is a fixed association
>> with first mount otherwise an acceptable solution for magic /dev/ptmx
>> (where /dev/ptmx is not a symlink to /dev/pts/ptmx)?
>
> I do not believe a fixed association with the first mount is an
> acceptable solution for implementing  /dev/ptmx in association with
> a change to cause mount of devpts to be an independent filesystem.
> Such an association fails to be backwards compatible with existing
> userspace, and it is extremely fragile.

Ugh.  After reviewing the userspace code that mounts devpts we have
to do use a magic /dev/ptmx to solve the issue we are trying to solve.

The fragility of detecting the primary system devpts seems solvable.

CentOS5 and openwrt-15.05 mount devpts, unmount devpts,
then mount devpts again.  So a rule of mouting the internal devpts if it
isn't mounted would work for those.

CentOS6 uses switch_root and moves it's early mount of devpts onto the
primary root, and then because devpts is also in /etc/fstab tries and
fails to mount devpts once more at the same location.  Implying
newinstance will make that mounting devpts twice.  That sounds solvable
but I don't see a clean way of detecting that case yet.

Ugh.

I am going to pound my head up against what is needed to find the
primary system mount of devpts for a bit more and see if I can solve
that.  Otherwise this exercise is pointless.

Eric

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