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

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Message-ID: <20100601150049.GQ4098@outflux.net>
Date:	Tue, 1 Jun 2010 08:00:49 -0700
From:	Kees Cook <kees.cook@...onical.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	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>,
	Dave Young <hidave.darkstar@...il.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	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 Tue, Jun 01, 2010 at 03:55:29AM -0400, Christoph Hellwig wrote:
> On Mon, May 31, 2010 at 08:24:23PM -0700, Kees Cook wrote:
> > Well, that's what I'm trying to understand.  It sounds like there is some
> > general agreement that the issue needs to be solved, but some folks do not
> > want it in the core VFS.  As in, the objections aren't with how symlink
> > behavior is changed, just that the changes would be in the fs/ directory.
> 
> No, it's not.  It's not a change we can make for the default that
> everyone uses.  If you're keen to mess up installations you control (aka
> ubuntu valuedadd viper) push it into a special LSM or rather a
> non-standard rule for it.  It really doesn't matter if it's in fs/ or
> security/ but it's simplify not going to happen by default.

Okay, thanks; that clarifies some of my confusion.  It sounds like
there are some people that genuinely believe that the symlink-following
logic should not change.  I would pose, then, a question of "what
are legitimate and safe situations that require following cross-user
symlinks in a sticky world-writable directory?"  And if the answers to
that aren't very convincing, then I think it's reasonable to include at
least an option to change the behavior.

> > My rationale is that if it's in commoncaps, it's effective for everyone, so
> > it might as well be in core VFS.  If the VFS objections really do boil down
> > to "not in fs/" then I'm curious if doing this in commoncaps is acceptable.
> 
> If you think the objection is about having things in fs/ you're smoking
> some really bad stuff.

Right, that was my point exactly.  It didn't make sense to object to it
being in fs/.  The objection was to having it in the kernel at all.  So now
I can focus my efforts on convincing people about the value of making this
a setting in the kernel, like turning on or off TCP syn-flood protection.
Some people may demand it, some people may hate it, but the choice it
up to the end user.

-Kees

-- 
Kees Cook
Ubuntu Security Team
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