linux-kernel - Re: [PATCH v2] fs: block cross-uid sticky symlinks

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Message-ID: <20100531195230.GS31073@ZenIV.linux.org.uk>
Date:	Mon, 31 May 2010 20:52:30 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Kees Cook <kees.cook@...onical.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
	Randy Dunlap <rdunlap@...otime.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Dave Young <hidave.darkstar@...il.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	James Morris <jmorris@...ei.org>,
	Eric Paris <eparis@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tim Gardner <tim.gardner@...onical.com>,
	"Serge E. Hallyn" <serue@...ibm.com>
Subject: Re: [PATCH v2] fs: block cross-uid sticky symlinks

On Mon, May 31, 2010 at 12:07:54PM -0700, Kees Cook wrote:

> IIRC, screen, when setuid, allows users to share screen sessions (following
> some system-defined ACLs) but it does it via the /tmp directory trees it
> creates.  Per-user /tmp would break this (but yes, it's solvable using some
> kind of /var/lib/screen which maybe even already exists).

screen(1) does *not* put directories in /tmp these days, TYVM.

al@...e:~/linux/trees/vfs-next$ ls -l /var/run/screen/
total 1
drwx------ 2 al al 1024 May 20 21:50 S-al

That's lenny/x86_64; I can't be arsed to install ubuntu, but in case you
have a really ancient screen(1), pulling one from debian -stable would
suffice.  IIRC, -oldstable would work as well, actually, but I could be
wrong on that.

In any case, the suggested "improvement" breaks realistic use cases, AFAICS.
In particular,

cd /tmp
tar jxf foo-2.42.orig.tar.bz2
<...>
tar jxf foo-gtk-wank-wank-wank-2.69.orig.tar.bz2
<...>
ln -s foo-gtk-wank-wank-wank-2.69/docs/GNOME/design/ crap
<...>
lpr crap/taste-is-optional.ps
lpr crap/why-options-are-wrong.ps

is going to break with that, isn't it?
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