linux-kernel - Re: [PATCH 0/3] vfs: plug some holes involving LAST_BIND symlinks and file bind mounts (try #5)

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Date:	Fri, 1 Jan 2010 16:40:27 +0100
From:	Pavel Machek <pavel@....cz>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Jeff Layton <jlayton@...hat.com>,
	Jamie Lokier <jamie@...reable.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	miklos@...redi.hu
Subject: Re: [PATCH 0/3] vfs: plug some holes involving LAST_BIND symlinks and file bind mounts (try #5)

Hi!

> > > > Consider FD passing over unix socket. Passing R/O file descriptor to
> > > > the other task, then having the task write to the file is certainly bad.
> > > 
> > > You've omitted the "R/O file descriptor of a file that is writable for
> > > that other task" part...
> > 
> > That is 666 for the other task. But the other task can't access it due
> > to directory being 700 or something. Your fchdir() argument  does not
> > apply here.
> 
> *snort*
> 
> What you are advocating is a very limited class of setups that might be
> usable for protecting files if not for the existing behaviour on a shitload
> of systems.
> 
> The thing is, that class *is* very limited.  E.g. introduce links and it's
> fallen apart.  Introduce bindings and the same will happen.  Just try to
> extend it one level deeper and fchdir() will bite you, etc.  All of that
> is not dependent on procfs even being there.
> 
> Access rights belong to file, not to a pathname (and there's no such thing
> as _the_ pathname of a file).
> 
> I'd buy that as a minor QoI issue; as a security one - no way.

Ok, so you see it as a (QoI) problem, but not too major. Good; I hope
it gets fixed one day.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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