linux-kernel - Re: [PATCH V31 25/25] debugfs: Disable open() when kernel is locked down

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Message-ID: <CALCETrUW=r=xyr4pGiGnn9zUEevf=z6PP3111Ds6qguV5SjYUQ@mail.gmail.com>
Date:   Wed, 27 Mar 2019 10:39:53 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        James Morris <jmorris@...ei.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Linux API <linux-api@...r.kernel.org>,
        Matthew Garrett <mjg59@...gle.com>
Subject: Re: [PATCH V31 25/25] debugfs: Disable open() when kernel is locked down

On Tue, Mar 26, 2019 at 10:33 PM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Tue, Mar 26, 2019 at 10:29:41PM -0700, Andy Lutomirski wrote:
> >
> >
> > > On Mar 26, 2019, at 10:06 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
> > >
> > >> On Tue, Mar 26, 2019 at 09:29:14PM -0700, Andy Lutomirski wrote:
> > >>> On Tue, Mar 26, 2019 at 5:31 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> > >>>
> > >>>> On Tue, Mar 26, 2019 at 12:20:24PM -0700, Andy Lutomirski wrote:
> > >>>> On Tue, Mar 26, 2019 at 11:28 AM Matthew Garrett
> > >>>> <matthewgarrett@...gle.com> wrote:
> > >>>>>
> > >>>>> From: Matthew Garrett <mjg59@...gle.com>
> > >>>>>
> > >>>>> debugfs has not been meaningfully audited in terms of ensuring that
> > >>>>> userland cannot trample over the kernel. At Greg's request, disable
> > >>>>> access to it entirely when the kernel is locked down. This is done at
> > >>>>> open() time rather than init time as the kernel lockdown status may be
> > >>>>> made stricter at runtime.
> > >>>>
> > >>>> Ugh.  Some of those files are very useful.  Could this perhaps still
> > >>>> allow O_RDONLY if we're in INTEGRITY mode?
> > >>>
> > >>> Useful for what?  Debugging, sure, but for "normal operation", no kernel
> > >>> functionality should ever require debugfs.  If it does, that's a bug and
> > >>> should be fixed.
> > >>>
> > >>
> > >> I semi-regularly read files in debugfs to diagnose things, and I think
> > >> it would be good for this to work on distro kernels.
> > >
> > > Doing that for debugging is wonderful.  People who want this type of
> > > "lock down" are trading potential security for diagnositic ability.
> > >
> >
> > I think you may be missing the point of splitting lockdown to separate integrity and confidentiality.  Can you actually think of a case where *reading* a debugfs file can take over a kernel?
>
> Reading a debugfs file can expose loads of things that can help take
> over a kernel, or at least make it easier.  Pointer addresses, internal
> system state, loads of other fun things.  And before 4.14 or so, it was
> pretty trivial to use it to oops the kernel as well (not an issue here
> anymore, but people are right to be nervous).
>
> Personally, I think these are all just "confidentiality" type things,
> but who really knows given the wild-west nature of debugfs (which is as
> designed).  And given that I think this patch series just crazy anyway,
> I really don't care :)
>

As far as I'm concerned, preventing root from crashing the system
should not be a design goal of lockdown at all.  And I think that the
"integrity" mode should be as non-annoying as possible, so I think we
should allow reading from debugfs.