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

Kees Cook
2011-02-25T01:13:24Z

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