Re: [RFC][PATCH-2.4] Prevent mounting on ".."

[Willy TARREAU <willy@w.ods.org>]

On Sun, Jun 29, 2003 at 03:11:03PM +0100, viro@parcelfarce.linux.theplanet.co.uk wrote:
> On Sun, Jun 29, 2003 at 03:09:52PM +0200, Willy TARREAU wrote:
> > chroot("/var/empty") (read-only directory or file-system)
> > chdir("/")
> > listen(), accept(), fork(), whatever...
> > -> external code injection from a cracker :
> >    mount("none", "..", "ramfs")
> >    mkdir("../mydir")
> >    chdir("../mydir")
> >    the cracker now installs whatever he wants here.
> 
> That's a BS.  Same effect would be achieved by replacing ".." with ".".
> Or mounting on any existing subdirectory.

No, it works only with "..", and not with "." ! I don't know why, I believe
it's because the process is still attached to the old FS when mounting on ".".

> If attacker can mount of chroot - you've LOST.  Already.  End of story.

To me, it seems this is the *only* remaining case in an *empty* read-only
directory. The fact is that the attacker needs at least a mount point to mount
something. Not providing him one is efficient, but here he can only exploit
"..".

Please reconsider the question, Al, because I really think that with this, we
can get reliable jails for network daemons which don't need file access at all.

Cheers,
Willy