Re: [PATCH 24/24] debugfs: Restrict debugfs when the kernel is locked down

[Andy Lutomirski]

On Wed, Apr 11, 2018 at 1:33 PM, Greg KH <greg@xxxxxxxxx> wrote:
> On Wed, Apr 11, 2018 at 09:09:16PM +0100, David Howells wrote:
>> Greg KH <greg@xxxxxxxxx> wrote:
>>
>> > Why not just disable debugfs entirely?  This half-hearted way to sorta
>> > lock it down is odd, it is meant to not be there at all, nothing in your
>> > normal system should ever depend on it.
>> >
>> > So again just don't allow it to be mounted at all, much simpler and more
>> > obvious as to what is going on.
>>
>> Yeah, I agree - and then I got complaints because it seems that it's been
>> abused to allow drivers and userspace components to communicate.
>
> With in-kernel code?  Please let me know and I'll go fix it up to not
> allow that, as that is not ok.
>
> I do know of some bad examples of out-of-tree code abusing debugfs to do
> crazy things (battery level monitoring?), but that's their own fault...
>
> debugfs is for DEBUGGING!  For anything you all feel should be "secure",
> then just disable it entirely.
>

Debugfs is very, very useful for, ahem, debugging.  I really think
this is an example of why we should split lockdown into the read and
write varieties and allow mounting and reading debugfs when only write
is locked down.