linux-kernel - Re: [PATCH v3] fs: allow protected cross-uid sticky symlinks

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Message-ID: <20100601222003.GH4098@outflux.net>
Date:	Tue, 1 Jun 2010 15:20:03 -0700
From:	Kees Cook <kees.cook@...onical.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	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>,
	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 v3] fs: allow protected cross-uid sticky symlinks

On Tue, Jun 01, 2010 at 10:45:27PM +0100, Al Viro wrote:
> On Tue, Jun 01, 2010 at 02:07:34PM -0700, Kees Cook wrote:
> > > I don't buy it.  If we are concerned about the symlinks in the middle of
> > > pathname, your checks are useless (mkdir /tmp/a, ln -s whatever /tmp/a/b,
> > > have victim open /tmp/a/b/something).  If we are not, then your checks are
> > > in the wrong place.
> > 
> > Well, that's not traditionally where the problems happen, but I have no
> > problem strengthening the protection to include a full examination of the
> > entire path looking for sticky/world-writable directories.
> > 
> > If not, what is the right place for the checks?
> 
> Handling of trailing symlink on open().  At most.

What would this look like?  Moving the checks into may_open()?

> And I wouldn't be
> surprised if the real answer turns out to include "... if we have
> O_CREAT in flags", but that needs to be determined.

I think even without O_CREAT the protection is needed (some of the
/tmp-races are things like reading a file pointed to by a symlink and
spewing the contents to stderr, etc).

Thanks,

-Kees

-- 
Kees Cook
Ubuntu Security Team
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