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Date:	Sun, 25 Oct 2009 10:36:04 +0100
From:	Pavel Machek <pavel@....cz>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	Jan Kara <jack@...e.cz>, "J. Bruce Fields" <bfields@...ldses.org>,
	"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 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.
									Pavel

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