linux-kernel - Re: [PATCH v6] fs: allow protected cross-uid sticky symlinks

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Message-ID: <20100607191043.GA8836@hallyn.com>
Date:	Mon, 7 Jun 2010 14:10:43 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Valdis.Kletnieks@...edu
Cc:	Kees Cook <kees.cook@...onical.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Dave Young <hidave.darkstar@...il.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Eric Paris <eparis@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	James Morris <jmorris@...ei.org>, 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>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	David Howells <dhowells@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Tim Gardner <tim.gardner@...onical.com>, tytso@....edu,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH v6] fs: allow protected cross-uid sticky symlinks

Quoting Valdis.Kletnieks@...edu (Valdis.Kletnieks@...edu):
> (Sorry for the late reply, didn't have time last few days to drink from the
> lkml firehose)
> 
> On Thu, 03 Jun 2010 14:00:51 PDT, Kees Cook said:
> > On Thu, Jun 03, 2010 at 01:02:48PM -0700, Eric W. Biederman wrote:
> > > Kees Cook <kees.cook@...onical.com> writes:
> > > > A long-standing class of security issues is the symlink-based
> > > > time-of-check-time-of-use race, most commonly seen in world-writable
> > > > directories like /tmp. The common method of exploitation of this flaw
> > > 
> > > Nacked-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> > > 
> > > This approach to fix the problem to of /tmp looks to me like it
> > > will have the opposite effect.  I think this patch will encourage
> > > more badly written applications.
> > 
> > How to safely deal with /tmp has been well understood for well over
> > a decade.  I don't think this change would "encourage" poor code.
> 
> The fact that you're proposing this patch a decade after we "well understood"
> the problem should suggest that it *will* encourage poor code, as the same
> programmers who don't currently get it right (and are thus the targets of your
> patch) will quite likely just say "Oh, I saw a patch for that, I don't have to
> try to do it right..."

Come on, now, that's a leap, really...

I'm all for doing both this patch AND pushing for per-user /tmp.

-serge
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