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

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Message-ID: <20100601145251.GP4098@outflux.net>
Date:	Tue, 1 Jun 2010 07:52:51 -0700
From:	Kees Cook <kees.cook@...onical.com>
To:	Eric Paris <eparis@...hat.com>
Cc:	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>,
	Dave Young <hidave.darkstar@...il.com>,
	Martin Schwidefsky <schwidefsky@...ibm.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 07:55:02AM -0400, Eric Paris wrote:
> On Tue, 2010-06-01 at 03:55 -0400, Christoph Hellwig wrote:
> > On Mon, May 31, 2010 at 08:24:23PM -0700, Kees Cook wrote:
> 
> > > 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.
> 
> Sounds to me like we should probably follow the same path as
> mmap_min_addr.  We should add these hooks right in the VFS where they
> belong (much like mmap_min_addr hooks into the vm) and control them 2
> ways.
> 
> 1) a Kconfig so distros can choose to turn it on or off by default
> 2) a /proc interface so root can turn it off
> 
> Nothing about that precludes additional similar checks inside an LSM
> (like CONFIG_LSM_MMAP_MIN_ADDR) which can be more finely controlled.  So
> maybe we want to follow up with the core VFS check with new checks in
> SELinux (and maybe apparmour).  This allows the user to disable the
> general check and still be provided with some modicum of protection.
> You might ask why not ONLY do the check in SELinux and drop the generic
> check, but we have seen with mmap_min_addr that the SELinux unconfined
> user can do damn well anything it wants to, so having a non-LSM version
> of appropriate security checks is highly regarded.

Would a CONFIG for this be overkill?  mmap_min_addr is a little different
in that there was desire to control a bottom limit on it, etc.  Given this
is either "on" or "off", I think just a sysctl is needed?

I will send a v3 patch that fixes the sysctl name and default, so that it
is up to the distro and end user how to configure their symlink semantics.

-Kees

-- 
Kees Cook
Ubuntu Security Team
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