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Date:	Mon, 26 Oct 2009 14:35:36 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Pavel Machek <pavel@....cz>
Cc:	Trond Myklebust <trond.myklebust@....uio.no>,
	Jan Kara <jack@...e.cz>, "Serge E. Hallyn" <serue@...ibm.com>,
	kernel list <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	jamie@...reable.org
Subject: Re: symlinks with permissions

On Sun, Oct 25, 2009 at 10:36:04AM +0100, Pavel Machek wrote:
> On Mon 2009-10-26 13:57:49, Trond Myklebust wrote:
> > On Mon, 2009-10-26 at 18:46 +0100, Jan Kara wrote:
> > >   That's what I'd think as well but it does not as I've just learned and
> > > tested :) proc_pid_follow_link actually directly gives a dentry of the
> > > target file without checking permissions on the way.
> 
> It is weider. That symlink even has permissions. Those are not
> checked, either.
>  
> > I seem to remember that is deliberate, the point being that a symlink
> > in /proc/*/fd/ may contain a path that refers to a private namespace.
> 
> Well, it is unexpected and mild security hole.
> 
> Part of the problem is that even  if you have read-only
> filedescriptor, you can upgrade it to read-write, even if path is
> inaccessible to you.
> 
> So if someone passes you read-only filedescriptor, you can still write
> to it.

By the way, nfs-exporting a filesystem also allows bypassing lookup
permissions: anyone on the network can access an inode directly (using
an nfs filehandle) without necessarily traversing any path to that
inode.  (Assuming they can guess the filehandle--probably doable in most
cases.)

Not arguing for or against, just another data point.

--b.
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