Date: Tue, 22 Feb 2011 11:22:48 -0800
From: Kees Cook <kees.cook@canonical.com>
To: Greg KH <gregkh@suse.de>
Cc: linux-kernel@vger.kernel.org, Eugene Teo <eugeneteo@kernel.sg>,
        Ralph Campbell <infinipath@qlogic.com>,
        Roland Dreier <roland@kernel.org>, Sean Hefty <sean.hefty@intel.com>,
        Hal Rosenstock <hal.rosenstock@gmail.com>,
        Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Miklos Szeredi <miklos@szeredi.hu>,
        "J. Bruce Fields" <bfields@fieldses.org>, Neil Brown <neilb@suse.de>,
        Matthew Wilcox <matthew@wil.cx>, James Morris <jmorris@namei.org>,
        Stephen Smalley <sds@tycho.nsa.gov>,
        Eric Paris <eparis@parisplace.org>, Nick Piggin <npiggin@kernel.dk>,
        Arnd Bergmann <arnd@arndb.de>, Ian Campbell <ian.campbell@citrix.com>,
        Jarkko Sakkinen <ext-jarkko.2.sakkinen@nokia.com>,
        Tejun Heo <tj@kernel.org>, Casey Schaufler <casey@schaufler-ca.com>
Subject: Re: [PATCH 2/2] debugfs: only allow root access to debugging
 interfaces
Message-ID: <20110222192248.GX4000@outflux.net>
References: <1298398198-18482-1-git-send-email-kees.cook@canonical.com>
 <1298398198-18482-2-git-send-email-kees.cook@canonical.com>
 <20110222181613.GU4000@outflux.net>
 <20110222191333.GA9991@suse.de>
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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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