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

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Date:	Tue, 5 Apr 2016 04:03:33 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	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>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Jann Horn <jann@...jh.net>,
	"security@...nel.org" <security@...nel.org>, 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 Tue, Apr 05, 2016 at 03:54:25AM +0100, Al Viro wrote:

> That, I take it, is a lookup for .. and buggering off if it fails *or* if
> we had been in caller's root or something that overmount it?  Not that the
> latter had been possible - root is a directory and can be overmounted only
> by another such, and we are called from ->open() of a device node.
> 
> > +	/* Remember the result of this permission check for later */
> > +	ret = inode_permission(path.dentry->d_inode, MAY_EXEC);
> > +	if (path_pts(&path))
> > +		goto fail;
> 
> Egads, man - you've just introduced a special function for looking up
> something named "pts" in a given directory!
> 
> The reason not to use kern_path() would be what, the fact that it doesn't
> allow starting at given location?  So let's make a variant that would - and
> rather than bothering with RCU, just go for something like (completely
> untested)

Ah...  Right, that would demand exec permissions on the starting point.
Still, this is incredibly ugly ;-/  I'll try to come up with something
more tolerable, but this "path_pts" thing is too ugly to live.  Seriously.