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

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Date:   Tue, 26 Mar 2019 21:29:14 -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 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.