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

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Date:	Mon, 14 Dec 2015 11:55:06 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Hurley <peter@...leysoftware.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Greg KH <greg@...ah.com>, Jiri Slaby <jslaby@...e.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@...nel.org" <security@...nel.org>,
	"security@...ntu.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

On 12/14/15 11:47, Peter Hurley wrote:
> On 12/11/2015 11:40 AM, Eric W. Biederman wrote:
>> Forcing newinstance for every mount of the devpts filesystem actually
>> requires the association between /dev/ptmx and the currently mounted
>> instance of devpts at /dev/pts.  Simply remembering the first mount of
>> the devpts filesystem and associating that with /dev/ptmx is not
>> enough.  I am aware of at least one instance where an initramfs mounts
>> devpts before the main system instance of devpts is mounted.
> 
> Can you point me to that usage please?
> 
> I ask because there's a patch to move devpts init from module initcall
> up to fs initcall (neither devpts nor the pty driver is actually built
> as a module anyway), and I'd like to look at what the consequences
> might be for that userspace configuration.
> 
> 
>> 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)?
> 

The problem is containers, I would think, if they create a new /dev/ptmx
and then mount a separate devpts instance instead of doing a bind mount.

	-hpa


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