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

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Date:	Mon, 07 Jun 2010 12:18:56 -0400
From:	Valdis.Kletnieks@...edu
To:	Kees Cook <kees.cook@...onical.com>
Cc:	"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>,
	"Serge E. Hallyn" <serue@...ibm.com>, tytso@....edu,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH v6] fs: allow protected cross-uid sticky symlinks

(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..."


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