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

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Date:	Thu, 24 Feb 2011 17:12:42 -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

On Thu, Feb 24, 2011 at 04:35:08PM -0800, Greg KH wrote:
> On Thu, Feb 24, 2011 at 04:22:14PM -0800, Kees Cook wrote:
> > 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
> 
> Wait, I do not believe this statement at all.
> 
> It's like saying "sysfs and proc are demonstrably dangerous to have
> available" because there were some bugs with some implementations of
> sysfs and proc files in the past.

Since sysfs and proc have "rules", it discourages bad code more than
debugfs does.

> >  - Alan Cox says that debugfs should not be used on production systems
> >  - Greg KH does not disagree
> 
> I also don't agree, as my day-job entails supporting a wide range of
> production systems with this filesystem mounted and enabled.

I was careful in reproducing your earlier statement about not disagreeing.
:)

> >  - 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.
> 
> There are a wide range of other files that can be safely read as a
> normal user in debugfs.  For example, the usb debugging files which we
> use to help debug hardware controller issues.  Now yes, we could ask the
> user to become root first, but is that really necessary?

If production systems should not have debugfs mounted, and the file is
universally useful to non-root users, it should move like the perf
interfaces, right?

> Again, I feel these were just a few bugs that do not reflect the much
> larger and benificial use of this filesystem.  We now have a set of
> checks in place to prevent this type of error from occuring again, why
> not rely on that instead of just removing the whole filesystem from
> normal users entirely?

I don't feel that a test in checkpatch is sufficient to prevent future
problems. What about Dan Carpenter's patch?

-Kees

-- 
Kees Cook
Ubuntu Security Team
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