bugtraq - Re: /proc filesystem allows bypassing directory permissions on Linux

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Message-ID: <20091028213037.GB8027@elf.ucw.cz>
Date: Wed, 28 Oct 2009 22:30:37 +0100
From: Pavel Machek <pavel@....cz>
To: CaT <cat@....com.au>
Cc: Dan Yefimov <dan@...htwave.net.ru>, nomail@...ail.com,
	bugtraq@...urityfocus.com
Subject: Re: /proc filesystem allows bypassing directory permissions on
 Linux

On Tue 2009-10-27 11:49:32, CaT wrote:
> On Tue, Oct 27, 2009 at 12:29:09AM +0300, Dan Yefimov wrote:
> > and testing them. Remember the scenario from the original mail and try 
> > finding a window, during which creating a hardlink would still work thus 
> > evading directory permissions check.
> 
> The main thing this does is allow a hardlink-like attack to work across
> mountpoints afaics.

Yes, plus it allows "hardlinks" on deleted files, and this "strange
hard links" can not be seen on link count.

> You can't actually use /proc/*/fd to gain access to files opened by
> processes you do not own. Only ones you do (at least in a mainline kernel)
> which is fair enough. This means that you can't have user a open a file
> owned by user b and then let user c have access to it via
> /proc/$pid/fd.

No, but you can upgrade file from read-only to read-write using /proc.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html