linux-kernel - Re: [PATCH 01/11] fs: add O_BENEATH

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Message-ID: <20140701095356.GC2242@google.com>
Date:	Tue, 1 Jul 2014 10:53:56 +0100
From:	David Drysdale <drysdale@...gle.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Meredydd Luff <meredydd@...atehouse.org>,
	Kees Cook <keescook@...omium.org>,
	James Morris <james.l.morris@...cle.com>,
	linux-api@...r.kernel.org
Subject: Re: [PATCH 01/11] fs: add O_BENEATH_ONLY flag to openat(2)

On Mon, Jun 30, 2014 at 01:40:40PM -0700, Andi Kleen wrote:
> David Drysdale <drysdale@...gle.com> writes:
> 
> > Add a new O_BENEATH_ONLY flag for openat(2) which restricts the
> > provided path, rejecting (with -EACCES) paths that are not beneath
> > the provided dfd.  In particular, reject:
> >  - paths that contain .. components
> >  - paths that begin with /
> >  - symlinks that have paths as above.
> 
> How about bind mounts?
> 
> -Andi
> 
> -- 
> ak@...ux.intel.com -- Speaking for myself only

Bind mounts won't get rejected because they just look like normal
path components.  In other words, if dir/subdir is a bind mount to
/root/dir then:
  fd = openat(AT_FDCWD, "dir/subdir", O_RDONLY|O_BENEATH_ONLY);
will work fine.
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