linux-kernel - Re: [PATCH 2/2] debugfs: only allow root access to debugging interfaces

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Message-ID: <20110222192248.GX4000@outflux.net>
Date:	Tue, 22 Feb 2011 11:22:48 -0800
From:	Kees Cook <kees.cook@...onical.com>
To:	Greg KH <gregkh@...e.de>
Cc:	linux-kernel@...r.kernel.org, Eugene Teo <eugeneteo@...nel.sg>,
	Ralph Campbell <infinipath@...gic.com>,
	Roland Dreier <roland@...nel.org>,
	Sean Hefty <sean.hefty@...el.com>,
	Hal Rosenstock <hal.rosenstock@...il.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Miklos Szeredi <miklos@...redi.hu>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Neil Brown <neilb@...e.de>, Matthew Wilcox <matthew@....cx>,
	James Morris <jmorris@...ei.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	Eric Paris <eparis@...isplace.org>,
	Nick Piggin <npiggin@...nel.dk>, Arnd Bergmann <arnd@...db.de>,
	Ian Campbell <ian.campbell@...rix.com>,
	Jarkko Sakkinen <ext-jarkko.2.sakkinen@...ia.com>,
	Tejun Heo <tj@...nel.org>,
	Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH 2/2] debugfs: only allow root access to debugging
 interfaces

Hi Greg,

On Tue, Feb 22, 2011 at 11:13:33AM -0800, Greg KH wrote:
> On Tue, Feb 22, 2011 at 10:16:13AM -0800, Kees Cook wrote:
> > Har har, I forgot --compose to "git send-email".
> > 
> > Anyway, with the continuing deluge of bugs in the "debug" filesystem, I
> > would like to make that filesystem's root directory mode 0700 by default
> > since it's filled with crazy stuff that regular users do not need to see.
> 
> But that will break existing users of this interface, right?
> 
> > Better to try to just close the door completely on all the stuff in there.
> > It is, after all, supposed to only be used for debugging, right?
> 
> No, not really, people use it for all sorts of crazy things.
> 
> Remember, the only rule in debugfs is:
> 	There are no rules in debugfs.

Right, which seems extremely dangerous to me. I think debugfs should either
be:
 a) used only by the root user (would prefer this was actually CAP_DEBUG or
    something)
or
 b) used by userspace tools

But not both. Creating interfaces with no rules for userspace consumption
is going to lead to disaster. You can't disallow "break[ing] existing
users of this interface" and say there are no rules at the same time. :)

-Kees

-- 
Kees Cook
Ubuntu Security Team
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