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

[Andy Lutomirski]

On Tue, Mar 26, 2019 at 5:31 PM Greg KH <gregkh at linuxfoundation.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 at google.com> wrote:
> > >
> > > From: Matthew Garrett <mjg59 at google.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.