linux-kernel - Re: symlinks with permissions

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Message-ID: <20091028081653.GA18290@elf.ucw.cz>
Date:	Wed, 28 Oct 2009 09:16:53 +0100
From:	Pavel Machek <pavel@....cz>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Trond Myklebust <trond.myklebust@....uio.no>,
	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 Tue 2009-10-27 21:15:54, Eric W. Biederman wrote:
> Pavel Machek <pavel@....cz> writes:
> 
> > 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.
> 
> /proc/<pid>/fd is only viewable by the owner of the process or by
> someone with CAP_DAC_OVERRIDE.  So there appears to be no security
> hole exploitable by people who don't have the file open.

Please see bugtraq discussion at
http://seclists.org/bugtraq/2009/Oct/179 .

(In short, you get read-only fd, and you can upgrade it to read-write
fd. Yes, you are the owner of the process, but you are not owner of
the file the fd refers to.)

> > 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.
> 
> Openly if you actually have permission to open the file again.  The actual
> permissions on the file should not be ignored.

The actual permissions of the file are not ignored, but permissions of
the containing directory _are_. If there's 666 file in 700 directory,
you can reopen it read-write, in violation of directory's 700
permissions.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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