On Tue, Mar 26, 2019 at 10:33 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx> wrote: > > >>>>> > > >>>>> From: Matthew Garrett <mjg59@xxxxxxxxxx> > > >>>>> > > >>>>> 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.
