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:	Sun, 20 Dec 2009 20:59:03 +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)

On Wed 2009-12-16 12:31:43, Al Viro wrote:
> On Mon, Nov 23, 2009 at 06:15:45PM -0500, Jeff Layton wrote:
> 
> > The big question with all of this is: Should a task have the ability
> > to follow a /proc/pid symlink to a path that it wouldn't ordinarily be
> > able to resolve with a path lookup. The concensus that I got from the
> > bugtraq discussion was that it should not, and this patch is an attempt
> > to prevent that.
> > 
> > I take it from you and Eric's comments that you disagree? If so, what's
> > your rationale for allowing a task to resolve this symlink when it
> > wouldn't ordinarily be able to do so if it were a "normal" symlink?
> 
> WTF not?  It's convenient and doesn't lose any real security.  If your
> code relies on inaccessibility of <path> since some component of that
> path is inaccessible, you are *already* fscked.  Consider e.g. fchdir()
> and its implications - if you have an opened descriptor for parent,
> having no exec permissions on grandparent won't stop you at all.  Already.
> On all Unices, regardless of openat(), etc.

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.

> I might buy the argument about restricting reopening with wider permissions,
> but
> 	a) we still are looking at possible userland breakage of the worst
> kind - random scripts passing /dev/fd/42 as command line arguments to
> random programs.  Once in a while.  With error checking being... not quite
> sufficient.
> 	b) it's not just open - we have at least chmod/chown/truncate to
> deal with.

That's indeed the sane way to solve that.

> Prohibiting *all* access is a complete non-starter - things like
> cmp foo /dev/stdin || ....
> would bloody better work and nobody cares whether you have redirect
> from something out of your reach at the moment.

Ok.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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