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Date: Mon, 31 May 2010 20:52:30 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Kees Cook <kees.cook@canonical.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, linux-kernel@vger.kernel.org,
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       Tim Gardner <tim.gardner@canonical.com>,
       "Serge E. Hallyn" <serue@us.ibm.com>
Subject: Re: [PATCH v2] fs: block cross-uid sticky symlinks
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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@duke:~/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?