linux-kernel - Re: [PATCH 0/2] vfs: ensure that dentries are revalidated on open (try #2)

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Message-ID: <20091118173817.GE25150@elf.ucw.cz>
Date:	Wed, 18 Nov 2009 18:38:17 +0100
From:	Pavel Machek <pavel@....cz>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] vfs: ensure that dentries are revalidated on open
 (try #2)

Hi!

> > > This is the second attempt to fix this problem. The first one attempted
> > > to fix this in procfs, but Eric Biederman pointed out that file bind
> > > mounts have a similar problem. This set attempts to fix the issue at a
> > > higher level, in the generic VFS layer.
> > > 
> > > In certain situations, when it knows that they are valid, the path
> > > walking code will skip revalidating dentries that it finds in the cache.
> > > This causes problems with filesystems such as NFSv4 and CIFS that depend
> > > on the d_revalidate routine to do opens during lookup.
> > 
> > ...and it allows bypassing directory permissions. Could we fix both
> > here?
> 
> Does it? Here's what I just did to check that:

Yes it does, see http://seclists.org/bugtraq/2009/Oct/179


> # cp /bin/sleep /root/sleep
> 
> # ls -l /root /root/sleep
> dr-xr-x---. 19 root root  4096 2009-11-18 07:20 /root
> -rwxr-xr-x.  1 root root 29152 2009-11-18 07:20 /root/sleep
> 
> # /root/sleep 600
> 
> ...then as unprivileged user:
> 
> $ ps -ef | grep sleep 
> (find pid of sleep program that root is running)
> 
> $ /proc/5258/exe 600
> bash: /proc/5258/exe: Permission denied
> 
> ...it looks like directory permissions are respected here. Did I
> misunderstand what you're concerned about?

/proc does not allow you to use /proc/XX/fd of unrelated users; it is
another mechanism disallowing access. (Plus, I did my experiments with
/proc/XX/fd, not /exe).
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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