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

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Date:	Thu, 24 Feb 2011 16:22:14 -0800
From:	Kees Cook <kees.cook@...onical.com>
To:	Greg KH <gregkh@...e.de>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	David Daney <ddaney@...iumnetworks.com>,
	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 12:54:13PM -0800, Kees Cook wrote:
> On Tue, Feb 22, 2011 at 12:37:04PM -0800, Greg KH wrote:
> > On Tue, Feb 22, 2011 at 12:28:56PM -0800, Kees Cook wrote:
> > > On Tue, Feb 22, 2011 at 12:16:10PM -0800, Greg KH wrote:
> > > > On Tue, Feb 22, 2011 at 11:50:18AM -0800, Kees Cook wrote:
> > > > > On Tue, Feb 22, 2011 at 07:34:18PM +0000, Alan Cox wrote:
> > > > > > > What system do you proposed to keep these "stupid mistakes" from
> > > > > > > continuing to happen? If debugfs had already been mode 0700, we could have
> > > > > > > avoided all of these CVEs, including the full-blown local root escalation.
> > > > > > 
> > > > > > And all sorts of features would have put themselves in sysfs instead and
> > > > > > broken no doubt.
> > > > > > 
> > > > > > > The "no rules" approach to debugfs is not a good idea, IMO.
> > > > > > 
> > > > > > It's a debugging fs, it needs to be "no rules" other than the obvious
> > > > > > "don't mount it on production systems"
> > > > > 
> > > > > Okay, so the debugfs is not supposed to be mounted on a production system.
> > > > 
> > > > No, not true at all, the "enterprise" distros all mount debugfs for good
> > > > reason on their systems.
> > > 
> > > What reasons are those? Or better yet, why do you and Alan Cox disagree on
> > > this point?
> > 
> > These distros have made the decision to support the perf interface,
> > which lives in debugfs, for their customers.  I'm not saying that I
> > disagree with Alan about this, just pointing out the reality of the
> > situation here.
> 
> A tool used only by the root user, so the proposed mount mode of 0700
> wouldn't break anything.

The summary is this:
 - debugfs has been demonstrably dangerous to have available
 - Alan Cox says that debugfs should not be used on production systems
 - Greg KH does not disagree
 - however, pref needs it, and this is used by some root users
 - perf will likely move out of debugfs as some point

What is the objection, then, to making the root of debugfs mode 0600? All
the tools I reviewed that need it run as root (e.g. powertop). I've
already written, tested, and sent the patches -- they would not break
the requirements above.

-Kees

-- 
Kees Cook
Ubuntu Security Team
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