Re: [PATCH v3] vfs: new O_NODE open flag

[Miklos Szeredi <miklos@szeredi.hu>]

On Mon, 7 Dec 2009, Andrew Lutomirski wrote:
> On Mon, Dec 7, 2009 at 9:13 AM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
> > First obvious attack: get an O_NODE handle to a device you have assigned
> > to your ownership
> >
> >        while(1)
> >                fchmod(fd, 0666);
> >
> > wait for device to unload, reload and be intended for another user
> > Race udev to a real open. You have a similar problem with vhangup() and
> > ttys.
> 
> Huh?  I would've thought that udev would (and already does?), on
> device unload, chown to 0:0, then chmod to 0000, then unlink, in which
> case that attack doesn't work.

Git version of udev does:

	chmod(filename, 0000);
	chown(filename, 0, 0);
	err = unlink(filename);

It should probably do it the other way round, which is how it was
originally, until this commit messed it up:

  commit 39087d3bdd0b5195c2570a4f858b88a82d42a066
  Author: Kay Sievers <kay.sievers@vrfy.org>
  Date:   Sat Aug 29 16:10:24 2009 +0200
  
      util_unlink_secure(): chmod() before chown()
  
      Suggested by Florian Zumbiehl <florz@florz.de>.

And the thread where it came from:

  http://markmail.org/thread/ozwcbju52yb3qs5d

where the poster actually warned Kay that it was wrong...

> Would you be okay with a patch that prevented opening
> /proc/self/fd/xxx on O_NODE handles?

We can't sanely do that.

Thanks,
Miklos