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

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Message-ID: <20091102195325.GE22624@elf.ucw.cz>
Date: Mon, 2 Nov 2009 20:53:26 +0100
From: Pavel Machek <pavel@....cz>
To: mrex@....com
Cc: Jim Paris <jim@...n.com>, marco@....nl, dan@...htwave.net.ru,
	bugtraq@...urityfocus.com
Subject: Re: /proc filesystem allows bypassing directory permissions on

On Mon 2009-11-02 18:53:19, Martin Rex wrote:
> Jim Paris wrote:
> > 
> > > Therefor it's totally of no influence what you do with the original
> > > directory permission. File access has nothing to do with directory
> > > permissions...!
> > 
> > Right.  However the whole point of this discussion is that that is a
> > non-obvious point, there was no other way that the user could have
> > opened that file without the use of /proc.
> 
> The actual fallacy of the "problem report" is the flawed assumption
> about what a link count of 1 tells you.
> 
> The link count of a files tells you the number of hard links that
> are persisted within the same filesystem.  It is _NOT_ a promise
> that there are no other means to access the inode of the file.

It used to be promise before /proc was mounted.

> /proc creates a virtual reference to an inode, and since it is
> virtual (and in a different filesystem) and not persisted in the
> original filesystem, you will not see it in the link count of
> the original filesystem.

Well, there _may_ be other filesystems with similar features, but they
are neither common nor mounted by default. 

Normally, mounting filesystems does not change security properties of
rest of the system; and it should be possible to fix in this case.

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